Tài liệu salinity management for sustainable irrigation integrating science pdf

Effects on Crops 15 Sensitivity to Salinity Specific Element Effects 18 Chaptec'3 Irrigation Water Z3 Water Quality 23 Use of Brackish and Saline Water 24 Reuse of Wastewater 27 Chapte

Rural Decelopment

Salinity Management for

Sustainable Irrigation Integrating Science, Environment, and Economics

By te

Salinity Management for Sustainable Irrigation

Integrating Science, Environment,

Copyright © 2000

‘The International Bank for Reconstruction

and Development/ THE WORLD BANK

1818 H Street, NW

Washington, D.C, 20835, US.A

All sights reserved

‘Marusactared in the United States of America

Fist printing August 2000

123404 03020100,

“This report has been prepare by the staff ofthe World Bank The judgments expressed do not

necessarily reflect the views of the Board of Executive Directors or ofthe governments they represent

“The World Bank does not guarante the accuracy of te data included in this publication andl accepts

"no esponsibility for any consequence of their use, The boundaries, colors, denominations and other in- formation shown on any map in this volume donot imply an the par of the World Bank Group any judg _ment on the legal status of any territory or the endorsement or acceptance of such boundaries

"The material in this publication is copyrighted The World Bank encourages dissemination of ts work and will nonmally grant permission pro

"Permission to photocopy items for intemal or personal use, forthe internal or personal use of specific lent, of for educational classroom uses granted by the World Bank, provided that the appropriate fee

is paid directly to Copyright Clearance Center, In, 22 Rosewood Drive, Danvers, MA GI523, USA,

ne 978-750-8400, fax 978-750-4470, Please contac the Copyright Clearance Center before

‘Photocopying items,

For permission to reprint individual articles or chapters, pleas fax your request with complete

{information to the Republication Departinent, Copyright Clearance Center, fax 978-750-4470

‘Allother queries on rights and lenses should be addressed to the World Bank atthe address above or faxed 10 200.522.2422

Cover photograph: Slination in an irgated field in Swift Current, Saskatchewan, Canada, Courtery of

Daniel Hills Professor Emeritus of Soil, Water and Plant Sciences atthe University of Massachusetts

ie was contracted to do this stay by the Sustainable Land Resource Management thematic group of the Raral Development Department atthe World Bank

Library of Congress Cataloging: in-Publication Data has been applied for

Contents

Acknowledgments vi

Introduction Posing the Question: Is Irrigation Sustainable?

Chapter1 The Nature of Salinity 5

Soll Salinity 5

Seil Sodicity 7

The Salt Balance 10

Chapter’? Effects on Crops 15

Sensitivity to Salinity

Specific Element Effects 18

Chaptec'3 Irrigation Water Z3

Water Quality 23

Use of Brackish and Saline Water 24

Reuse of Wastewater 27

Chapter $Waterlogging and Drainage 30

High Water-table Conditions 30

Chapter 6 arly Warning Systems 50

‘Need for Early Detection 50

Methods of Monitoring 50

Chapter7 Scaling Up from the Field 56

Crop-Water Production Functions 56

Social al Institutional Issues 58

Policy implications 59

w

1

iv Contents

Conclusion Irrigation ia Sustainable—ata Cost 61

‘Appendix: Economic Aspects of Salinity Management 63

‘Representing Physical and Biological Relationships 63

Review of Economic Analyses 65

Bibliography 79

Index 99

List of Tables

1 Relative salt tolerances of various crops 20

2 Classification of water quality according to total salt concentration 24

3 Prevalent depths and spacings of drainage pipes in different sol types 34

‘AL Estimated marginal rate of substitution among waters from various sources 69

‘A2 Nash-Harsanyi solution for three farm cooperative with joint use

‘of brackish water 71

‘A Estimated income losses for various salinity levels of irrigation water 72 List of Boxes

Stand salt in ancient Mesopotamia 2 How ancient Egypt escaped the scourge of salinity 4

Potential contribution of irrigation water to soil salinity 6

Sampling the soil solution 7 Measuring the salt concentration 7 Sodium adsorption ratio and exchangeable sodium percentage 10

Sample calculation ofthe saltbalance 11

Saline seeps in Australia and North America 13,

Models of oot-zone salt concentration 36

10 Weterlogging and salination inthe Indus River Basin 37

ML Sample calculation of the leaching requirement 38,

32 Drainage problems in California” 39

33 Summary of leaching methods 41

14 Factors determining flow rate to drains 43

15 Predicting water-table height in a drained field 44

16 Native gypaum and soll subsidence 46

17 The solubility of ime and gypsum in the soil 47

18 The shrinking and salination ofthe Aral Sea 48

19 Summary of early warning methods 51

20 Policy options to promote water conservation 56

21 Intergenerational issues 60

List of Figures

1 The process of waterlogging and salination 3

2 Distribution of a monovalent vs a divalent cation (Ca vs Na‘) adsorbed

‘to a negatively charged clay particle The divalent cation is held more closely and strongly tothe particle | 8

Contents Influence of ambient solution concentration (n, > n,) on thickness of the

tonic layer surounding ay partie Higher conenraton compres

the layer, promoting flocculation

"Frc of sandy loan as elated o total sal concentzaton

‘of the sol solution and tothe soi’ exchangeable sodium percentage (ESP) 9 Formation ofa saline seep 12

‘Salt accumlation pattems under furrow irrigation 14

Plant responses to salinity 15

(Classification of crop tolerance to salinity 17

Effects of salinity and sodicity on plants 18

Diagram Rraoeeingsinly mdsoidy bưand íimigtionwalet 25 Irrigation return-flow system

Drcnage water reuse ect dopa fr an rigaton gen — 28

Steady upward flow and evaporation from a sandy loam (a = 3) as a function

‘of the suction atthe sil surface, with water table at various depths 31 (Groundwater drainage (a) under steady flow conditions, and (b) under

tunsteady conditions resulting in a falling water table | 34

Depth of water per unit depth of sll required to leach saline soll by

‘continuous of intermittent ponding 42

‘Water content distribution during infiltration under flooding and under

at two intensities Cumulative infiltration equals 60 mm 42 (Observation wells to determine elevation of the water table 52

Set of plezometers to determine vertical pressure gradients under the

‘water table The condition illustrated suggests downward flow 52

Schematic representation ofa water system at a regional level 64

Acknowledgments

thank the following members of the World

Bank's Rural Development Department:

Messrs Johannes ter Vrugt, Eugene Terry, and

Julian Dumaneki, for entrusting me with thechal-

Jenging assignment to compose this manual; Dr

‘Shawki Barghouti, Senior Research Advisor, for

‘isleadership in tigation development; and Ms

‘Melissa Williams for her caring preparation of

the publication Dr John Letey ofthe University

‘of California at Riverside reviewed an early ver-

sion of the manuscript and offered many con- structive suggestions for is improvement Dr Harold Steppuhn of Agriculture and Agri-Food Canada kindly provided the photographs of salinized land Mi Joae Mendoza at GISS con- tributed his expertise in creating the graphic Finally, Vexpress my specal gratitude to Dr Cynthia Rosenzweig of te NABA/ Goddard In- stitute for Space Studies and Columbia Univer- Sty for her help and encouragement

Dedicated tothe irigetors of arid lands wh tl to feed humanity

INTRODUCTION

Posing the Question: Is Irrigation Sustainable?

‘rigation isthe supply of water to agricul-

tural crops by artificial means, designed to

permit farming in arid regions and to off-

{et drought in semi-arid regions Even in areas

Where total seasonal rainfall is adequate on _

average, it may be poorly distributed during

the year and variable from year to year Wher

‘ever traditional rainfed farming isa high-risk

enterprise owing to scarce or uncertain pre-

ipitation irigation can help to ensure stable

production Irrigation has long played key olen hi:

ing expanding populations and is expected to

Play ast greterrole inthe future ttt only

Faises the yields of pecificerops, but also pro-

Tongs the effective crop-growing period in ar

cas with dry seasons, thus permiltng multiple

czopping (two, ues, or even four, Cops per year) where only a single crop could be grown

‘otherwise, With the security provided by itt

gation, additional inputs needed to intensify

Production further (pest control, fertilizers,

Improved varieties and better tillage) become

economically feasible Irrigation reduces the

Fisk of these expensive inputs being wasted by crop failure resulting from lack of water

“Although irigated land amounts to only

some 17 percent ofthe worlds cropland, itcon-

trates well over 30 percent ofthe total agr-

cultural production That vital contebution is

even greater in ard regions where the supply

‘of water by rainfalls least even as the demand

for water imposed by the bright sun and the dey iris greatest

"The practice of izigation consists of apply-

ing water to the part of the soil profile that

serves asthe root zone, forthe immediate and subsequent use of the crop Inevitably, how- ever, irrigation also entails the addition of wa- ter-borne salts Many arid-zone soils contain natural reserves of salts, which are also mobi- lized by irrigation Underlying groundwaterin such zones may further contribute salts to the root 2one by capillary rise Finally the roots of crop plants typically extract water from thesoil while leaving most of the salts behind, thus causing them to accumulate

‘The problem is age-old From its earliest in-

‘ception in the Fertile Crescent, some six or more

‘millennia ago, irrigated agriculture hasinduced processes of degradation that have threatened Its sustainability (box 1) The artificial applica-

‘tion of water tothe land has ipso facto induced

‘the self-destructive twin phenomena of water- logging and salination (figure 1)

“The same processes that evidently brought about the demise of ancient hydraulic civiliza- tons, including those of the Tigrs-Euphrates land the Indus river valleys, continue to plague {rvigation districts today no les than inthe past Indeed, the problem extends far beyond the

‘confines ofthe irrigated lands themselves, as it affects adjacent lands and water resources Processes occurring offsite (upstream as well as downstream of the irrigated area) strongly affect the sustainability of irigation Forexample, denudation of upland watersheds

by forest clearing, cultivation, and overgrazing

‘induces erosion and the consequent silting of reservoirs and canals, thereby reducing the

‘water supply The construction of reservoirs

‘often causes the submergence of natural

habl-2 Salinity Management or Sustainable pm

Box 1 Sift and salt in ancient

Mesopotamia

‘Ancient Mesopotamia owed Hs prominence to Hs

‘agreuturlproducbly The sols of tis alli va

lay are deep and are, tho topography is ovel, thê

‘late wary, and water is proved by te win

fiver, Euphrates and Tig However the dvesion

ver water onto the vale lands ad a serosof

Intrelated probleme

“Tho problam was socimantaon Eryn his

tory, the upland watersheds were deforested and

cvergrazed The resulting erosion was comeyed by

{he vors.as suspanded ei, whch sete along the

booms and sides ofthe rivers, hus raising ther

beds and barks above the edacint plain Dring

periods of feeds the rere orerlowed th banks, FRundatedlarg tacts ofland, and tended change

tats a8 wel as of valuable seenic and cultural

sites Concurrently the downstream disposal

‘of drainage from irrigated land tends to pol-

Ite aquifers, streams, estuaries, and lakes with

‘salts, nutrients, and pesticides (box 2) Finally,

the irrigation system itself may harbor and

spread water-borne diseases, thus endangering

public health So the very future of irrigation is

threatened by land degradation, as well as by

dwindling water supplies and deteriorating

water quality

For some years now, even as great invest-

ments have been made in the development of

new irrigation project, the total area under ir- tigation has hardly expanded That is because large tracts of irrigated land have degenerated

to the point of being rendered uneconomic to cultivate, min extreme cases—have become totally sterile The dilemma of land deteriors- ton is not exclusive to the less-developed na-

‘ons, where ithas caused repeated occurrences

‘of farnine It applies to an equal extent to such technologically advanced countries as Austra- lia, the United States of America, and the cen-

‘ral Asian regions of the former Soviet Union Sopervasiveand inherent are the problems that

Bnưahcio 3) Figure 1 The process of wateriogging and salination

Secure: (1991)

some critics doubt whether irrigation can be

sustained in any one area for very long—and

they have much evidence to support their pes-

simism Herein, we examine the facts in search

cof a more positive approach, albeit one based

‘on carefully conditional optimism

“The concept of sustainability, as pointed out

by Letey (1994) isitself ambiguous dictionary

definition is “being capable of remaining in ex-

fstence, and of continuously and

indefinitely.” In the past, the extent of human

Jnowledge was definitely too limited for people

to foresee, let alone to forestall, the eventual con-

sequences ofthe way they managed the environ-

‘ment Atpresent, we know a great dal about the

processes involved, and we do have the technol-

‘ogy to cope with problems formerly considered

luncontrolabie Athough our knowledge i til

incomplete and much remains to be researched,

what we do know presents us with an opportu rity and a challenge to avoid practices that ane

‘cetain to eause degradation and to promote prac- tices that are ikely to maximize the probability

<alled for selecting and organizing the dispar- fate facts into a unified exposition, combining physico-chemical, agronomic, environmental, land economic principles into practical recom mendations, The ultimate aim of this and other such efforts must be to help ensure the long term viability and productivity of ivigated ag- roulture in aid and semiarid regions around the world

4 “Salinity Management or Sustainable Iristion

Box 2 How enclent Egypt escaped

the scourge of salinity

{noontaat to Mesopotamia, the chlzation of Epyst

‘eve for several milornia, What expiin the per

‘stance of gata farming in Egypt inthe toe of

‘is demise in Mesopotamia? The answer os inthe

<iforent sol and water regimes of th two lands,

Neither logging by ot norpolsoning by sa was as

sevore slong the Nie as in tv Tite-Euphates

plain "The sito Egypte brought bythe Bue le rom

‘ha volar highlands of Ethiopia and it is msod

‘wah he organle mater brought by the White Nie

‘rom is swampy sources was not so excossie

235 to choke the irigaton canal, yet was fertie

‘snough fo ads utints to the elds and nourish

thar crop Whereb in Mesopotamia the nunc

‘on usualy comasin the eping, and summa avapo-

raton nds to maka De so aie, the Nil ses in

the ite summer anc crest in autor So In Egy

the inundation comes ata mre faverabl ine:

tor he summer host has Wied the weeds an aor-

‘Sted ho so justin time forthe pre-wintr parting,

“The nar Hoedpan ofthe Nie (xcept nthe

'Dạla) precluéed the videepreed fEe of te walor

table Over moat o length, the ie ls blow te

level of ha adjacont and Wen th river crested

and nunated th and the seepage natural raised

‘he water tabla As the rer receded and Re walar lovl cropped pulled tne water table down after

‘Tho slinportart annual pulsaton ofthe river and {he sseocatd ftuation othe water able under & freearering Hoodplain ereatod an automaticaly ro- peatngsel-tushing cycle by which the sais were feached rom te irigated land and cared sway by the Nile Rei (tel, 1984) 'Unfotunaleyha oi Egypt—famous for ta di-

‘ably end preductvty in ancient times—is now {treatoned wits degradation The Aswan Fgh Dam {complated in 1070) has blocked the fete sit at hha formarty been delved by the Nile The river

‘sol now nrg claro alt has increased ts oro-

‘shy andhas been seouring ts own banks An along {he estuaries of the Dal hore Is no more depos: tion, so tho coast hus been subject to progressive

‘reson andto intusion of ea waler (a proces kay

to worsen as global warming causes to soa lovel'o oe), Fly, Io aril maintenance of a neatly

‘constant water level the rv, necoesary lo allow Yearround ldealion and euocesatve cropping, hes

‘aid the water table So Egypte now subject tothe

‘maladies of walrioging and salnation io which fad or 2 ong sumed rnune) and must vest the instalaton í edanhe groundwater drainage sjstome to prevent coll degradation,

CHAPTER ]

The Nature of Salinity

The term salinity refers tothe presence in

soil and water of various electrolytic

‘mineral solutes in concentrations that are

harmful to many agricultural crops Most com-

‘mon among these solutes are the dissociated

cations Nar, K*, Ca, and Mg; and the anions

(C1, S02, NO, HICO,, and CO; In addition,

hypersaline waters may contain trace concen-

trations ofthe elements B, Se, SLi, SiO, Rb F

“Mo, Mn, Ba, and Al, some of which may be toxic

to plants and animals (Tani, 1950)

Soll Salinity

“The sources of salts causing soil salinity may

reside in the soil itself, or in the subsoil They

‘may result, in the ist instance, from the chemi-

cal decomposition (termed “weathering” of the

‘minerals that constitute the rocks from which

the sols derived Soils formed in arid regions,

‘where rainfall is scanty, are especialy likely to

contain appreciable quantities of salts, simply

Dbecaute they have not been leached In other

instances salts may be infused into the sil fom

brackish groundwater This takes place espe-

cially where the subsoil contains salts that had

accumulated over prior geologic eras Arising

‘water table may then mobilize such salts and

convey them into the rooting zone of crops

‘Some salts enter the sil with rainwater A

though the initial condensation of vapor pro-

duces pure water, the raindrops that form in clouds and fall earthward tend to pick up soluble constituents during their brief residence {ntheatmosphere One such constituent i car- bbon dioxide, which dissolves in rainwater to form a dilute solution of carbonic acid That acid, though relatively weak, reacts with min- erals in dust, rocks and the soil, and causes certain minerals to dissolve more eadily than, they would otherwise, thus contributing indi- rectly to sol salinity The acidity of rainwater {ncreases significantly in industrialized regions where it mixes with emitted gases such as ox- {des of sulfur and nitrogen

In addition, rainfall that occurs in coastal regions often mixes with sa spray, which may, contribute appreciable quantities of salt to ar-

as that extend some distance (in some cases, scores of kilometers) from the shore Seawater also tends to intrude landward via tidal estu- aries, as well as into groundwater aquifers due

to overdraft The latter process occurs where groundwater is extracted from wells in

‘amounts greater than the annual recharge, thus causing the water table to fall and creating a pressure gradient that draws seawater into the subterranean aquifer

Except along seacoasts, however, saline soils seldom occur in humid regions, thanks to the ret downward percolation of water through the

sl profile brought about by the excess of

rain-Salinity Management or Sustainabergation

{all over evapo-transpiration In arid regions,

fn the other hand, there may be periods of no

net downward percolation and hence no effec-

tive leaching, so salts can—and often do—ac-

ccumulate in the soll Here the combined effect

‘of meager rainfall, high evaporation, the pres-

ence of salt-bearing sediments, and—in

places—the occurrence of shallow brackish

groundwater gives rise to a group of soils

known in classical pedology as Solonchaks

‘Less obvious than the appearance of natu-

zally saline soils, but perhaps more insidious,

is the inadvertent induced salinaton of originally

productive soils, caused by human interven

tion Irrigation waters generally contain ap-

preciable quantities of salts (box 3) An

‘accompanying calculation illustrates the poten

tial contribution of even good-quality iigation

water to soil salination, if there is insufficient

leaching Hence irrigated soils, typically located

insiver valleys ofthe dry zone, are particularly

vulnerable tothe effect of rising groundwater

<dueto inadequate drainage Crop plants extract

‘water from the soil while leaving most of the

salt behind Unless leached away (preferably

‘continuously, but at least periodically) such

salts must sooner or later begin to hinder crop

growth Additional anthropogenic sources of

falts include applications of soll amendments

(g, limeand gypsum), chemical fertilizers and

pesticides, and such saltcontaining organic

materials as manure and sewage sludge

Overall salinity is generally expressed in

terms of the total dissolved solutes (TDS) in mil-

ligrams per liter (mg/l) of solution (roughly

equivalent to parts per million, ppm, or in terms of the total solute concentration of cat-

ions (TSC) or of anions (TSA) in milli-

equivalents per liter (eq/1), Total salinity may alaobe characterized by measuring the electri- cal conductivity expresible in terms of dec-semens per meter ofthe solution (EQ), generally {aS/m, equivalent to theald unite of millomos

per centimeter, mmho/cm) ‘Although no universally exact relationship exats between the total concentration and the electrical conductivity of solution, in of salts practice TDS may be approximated by muli- lying EC (S/n) by a factor of 640 for date {olution Forhighly concentrated solution (in

‘which ionization, affecting electrical conductiv- ity, is somewhat suppressed), the factor in- ceases to about 600 For solutions in the EC fange of 0 105.0 dS/m, the total cation or an ion concentration in terme of meq/1 may be cetimatedby multiplying the value of EC by 10 (McNeal, 1960) ‘Quantitative criteria for diagnosing soil salinity were originally formulated by the

US Salinity Laboratory in its Handbook 60

(Richards, 1954), in terms of the electrical con-

<lctivty of the sols “saturation extrat”— Le; the solution extracted from a soil sample thathad been pre-aturated with water (box),

‘Accordingly saline soll has been defined as having an BC greater than 4 4S/m This value senerally corresponds to about #0 meq/\ of fall In the case of a sodium chloride sohu- tion this equals about 2400 mg/(or parts pet million)

Box 3 Potential contribution of Irrigation

water to sol salinity

‘Asa rough calculation, tus assume thatthe nar-

‘vested lid at crop ls abou 10,000 ky of ry mat

ter per hectare AI %, the sll contained in thai

harvests 20 ig Now compare that wit he amount

of al aplled i irgaion Assuring a seasonal

Figatlon of 1,000 mm (equal to 10,000 bie meters

ar necire) with wale of good quality, containing

500 mg(0 Sho"), te amount eal added woud

toi 0 8:10000 = 3009 vợ par hecre,Thụa, the

‘amount of sal aéed vi [he IlgsMen water lề

‘oughly 40 tes the amount of salt removed a the arvect it appears, hortore, hat uncer most ag

cutura contin sat removal by crops is ony &

‘nor component ofthe everal sal balance

Now consider the mass of salt added to crops in {ertzers The nitogan commonly sppled to e 0p

‘such as malze amouns to abou 250 kgha annul ‘Assume that ti fertizr lei the form of amen num sulfate (21% ntrogon, 8% hyớogon, 73% suk fate, an ta tb trogen sent taken up bythe

‘8p Thon the amount of sulle addod would bo

79250721 =870 kgha Thai leeethan 30% ofDe

‘moun of alt added in irigation (calculated above) Ite irigation water contains twice as much sl (as Itoommenly does), thn the sult added ne for tizer woud constitute about 15% of the sai oad

The Natureef Salinity 7

Box 4 Sampling the soll solution

Đtferel valerlsei volume ratios maybe used to

‘obtain a sample eluton fr characterizing tho sa

‘content of he sol Eat investigators used various

ral, sich a¢ 6:1 of 31 or 1:1 ralos The rsuts

‘hon dered however, asthe volume of water mised

‘wih ho sol was fund to afc the degree of csso-

lion of mineral salts ct varying soy To provide

‘auntorm basis fr comparison, hrsor a standard

‘eracton meted was sought The maod propoeed

for ths purpose by the US Salty Laboratory

(Pichards, 1058) was the so-called saturation ax-

tract M was basa on the notion that oach sol has a Sancti water corer eaturaon tas termined

by 1a parla combination ot totoponent py

al atrivios, euch as txt, specie sac, clay ort, and caon exchange capacry (Mart ta, 1887) in practice, te method consists of ming a Sample of cl wih a recundy easing scunt ot led wator url the samp bacomos a salted asi Th soon is thon exacts by vacuum rae {ion and ted fo sal contort Atheugh higher vate {orol vat mak exeacton ear, amour of water

‘ceeding In sturaton value are baled o ba ls

‘epresentatve of condons lly to eccrine fad

‘The measurement of electrical conductivity

{a affected by the temperature of the solution,

and it increases by about 2 percent per degree

rise in temperature in the range of 15 0 35 Cel-

sius For purposes of comparison, EC data are

referred fo a standard temperature of 25 Cel

sus The results obtained at higher of lower

temperature are normalized by means of the

following empirical relation:

(standard) = [1 ~0.02(7-25)]EC(measured)

‘The day fraction in saline soils is generally

‘well-flocculated As the salts are leached, how-

‘ever, the flocs may tend to disperse and thesoil

‘aggregates to break down (or sll) This occurs

‘especially where an appreciable concentration

‘of sodium ions is adsorbed onto the clay par-

ticles The tendency for floes to disperse and

for aggregates to slake and collapse results in

the deterioration of soil structure by the clog-

ging of large poresin the sol, and consequently

{nthe reduction of soll permeability Ths leads

to the associated phenomenon of so sdicity, also known as alain

Soil Sodicity

‘Soll sodicity is a condition caused by the effect

Of sodium ions adsorbed onto the electrostati- cally charged clay particles Colloidal clay par- ticles generally exhibita negative charge When surrounded by an aqueous solution of electro- lytic salts, such particles attract, or adsorb, cations, while repelling anions So the concen- {tation of cations in the vicinity of clay surfaces Increases, while that of anions decreases That

‘inward attraction is countered by the tendency

of the cations to diffuse toward the less-con-

‘centrated regions of the solution farther away from the particle surfaces As a result, the adsorbed cations do not adhere rigidly to the solid surfaces exceptina completely dry soi), Dut tend to form a loose swarm in the hydra: tion envelope surrounding each clay particle

‘The ionsin that swarm are exchangeable in the

Box 5 Measuring the salt concentration

‘The aart rats for determining the concentra:

tion of dissolved salsa coon wast ovaprate

‘a measured volume of he sion {pre-Btare 12

‘remove suspended soi) and to weigh th rosie

(of als The recut has tradtonally boon expreseed

inter of tefl lasotved eas (TDS), Usually

ian as milgrare per Iter (moi) A much more

omveniont mathe isa measuretne electical con- ctv ofthe elton being ho rectprocal of tho

‘lcticl rosie) The capac of sotaion te

‘hos have been replaced by siemens (8) 1 dS/m =

Ý mnhefen, whare mưnholem (nllinbee par con:

‘mote ete unt that had fomar bàn wee (por {o ho universal adoption of tne Si sydem) le char _cirzing tha leerealconduciMtyelsoulons, and

‘38m desgnatendocllemens per meer (1 oSim =

1 8m)

8 Sanity Management for Sustainable rigation

Figure 2_ Distribution of a monovalent va a divalont cation (Ca ve Na") adeorbed to 8

‘negatively charged clay partici The divalent cation ls held more closaly and strongly

sence that they can be replaced by other cat- agaregues An aggregated col typically contains

‘ons whenever the composition of theambient large voidsbbetween adjacent aggregates, hence solution changes (Fel, 196) ittends tobe porous and permeable

Divalent cations, such as calcium and mag-

‘nesium, are attracted to the clay surfaces more

strongly than are monovalent cations such 38

sodium Ifthe swarm of adsorbed cations con-

sists predominantly of divalent cations, it tends

‘tobe compressed (Figure 2) Consequently, the

pparticles can approach one another closely

‘enough to clump together and to form flocs (8

process called flocculation) Those flocs, in turn

an associate in larger assemblages, called

‘The flocculation of clay particles is enhanced

‘when the ambient solution is highly concen- trated, ie when it is saline In such conditions, the tendency of adsorbed cations to diffuse outwards is repressed (there being a weaker

‘osmotic gradient between the region of adsorp- tion close to the particles and the ambient soli tlon farther from the particles) Hence swelling, {is reduced and the particles are drawn together

0 they can flocculate (gure 3)

Figure 3_Intivence of amblont solution concentration (n, >n,) on thickness of the Ionic

In contrast, ifthe ambient solution is dilute

sand many ofthe cations present are monova

lent (eg, when the sodium adsorption ratio i

thigh), the swarm of adsorbed cations tends to

dfs farther away from the particle surfaces

‘This results ina thickening ofthe hydration

envelope surrounding each particle, causing

pronounced swelling and dispersion of the

foil flocs The process described, called dfoci-

lato, destroys soll aggregate from within AS

the aggregates collapse, the wide inter-agare-

(clogged) by dispersed permeability diminishes markedly (igure) Infiltration and aeration

fe then restricted, and so are such vital plant

functions as germination, seedling emergence,

and root growth “The swelling tendency is strongest i soils

with a high content of smectic lay (type of

Adive day alsolnownas montmosilnite) and

‘when sodium fon constitute a sizable faction

‘ofthe cation exchange comple In practice, he ‘xiterion fora “sod si” isan exchangeable

‘sodium percentage (ESP) exceding 15 percent

ofthe total cation exchange capacity (box 6)

‘Thisisa somewhat arbitra eiterion since in

snany cases no sharp distinction i apparent at

The Nature of Salty 9

a particular value of ESP between “sodie” and

“nonsodic" soils

Tm principle, soils may be saline without be-

ng odie, and in other circumstances soils may bbe sodic without being saline All too often,

‘however, irigated sols in arid regions can be

‘both saline and sodic, Le when the EC exceeds 4.48/mand ESP exceeds 15 percent Such soils, When leached, tend to become strongly alka- line, with their pH value rising above 85 Soll sodicity is especially notable for its ef- fecton the soll surface, where it tends to forma relatively impermeable ayer commonly known

as a surface seu When wet, the sol’stop-layer becomes a slick and sticky mud: when dry, it hardens to form a tough crust with an irregue Iar—roughly hexagonal—pattern of cracks

‘This dense surface condition not only reduces the entry of water and air into the soil but also {forms a barrier to the emergence of germinat- ing seedlings and to the penetration of their roots As infiltration is inhibited, greater run- off, erosion, and silting of downstream water

‘courses and reservoirs ensues

Sodic soils (called solonetz by classical ped-

‘ologists) may appear darkish, the coloration

‘being due to the surface coating of dispersed Figure 4 Hydrauile conductivity of a sandy loam as related to total salt concentration

of the soll solution and tothe soll's exchangeable sodium percentage (ESP)

tt

'Saf eeneanhabon (maafla)

‘Source: Aer McNeal and Coloman (1868),

10° Salinity Management for Susanah rigtion

Box 6 Sodlum adsorption ratioand

‘exchangeable sodium percentage

‘A widely accepted index or characlatfng ha soi

soon wih especialy inluance an th ex

‘Shangeable sock in tho sodium ad

-2pion/zio(SAR ihesnlso hen ueedoaasass

the qulty of irigaion water SAR is defined ss

fotos:

SAR = [NarYLGCa™ + Magee

In words SAR ha rato ofthe som on concan-

tai to the square roo ofthe average conoenia:

ion of the đalen calcum and magnesiom fons 0

this context, at concentrations are exprossed in nilleguigalots per fer SAR Is thus an epprad-

‘ate expression forthe relatve activity of None in

‘xchange reacts in sos A high SAR, parley allow concentrations ofthe sol olin, causes high ESP and le slao aly to cause a decrease of el ppetmeebi "The elaonsh botwoon ESP and SAR othe sl soldion wee measured on numerous soll eagle the Weetern sties by the US Salinity Laboratory

«and reported (Fichard, 1854) be

SP = 100 + XSARIV + 2+ SARI whore a= 0.0126 and ð= 001476

organic matter For that reason sodic soils were

long ago described as “black alkali"—in con-

trast with non-sodic saline soils that were de-

scribed as "white alkali” because ofthe typical

‘appearance of crystalline salts atthe surface

‘The Salt Balance

‘Thesalt balance sa summation and outputs for a defined volume of soil dur- ofall salt inputs

{ng a specie period (box 7) If sats are con-

served (that isto say, if they are neither

{generated nor decomposed chemically in the

Sol), then the difference between the total in

‘Put and output must equal the change in salt

‘content of he soll zane monitored According,

iftotal input exeeeds total output hen salt must beaccumulating, The saltbalance hasbeen used

asan indicator of salinity trends and ofthe need for salinty-contol measures in large-scale ei

gation projects as well ab in single igated

Fields

‘Thefollowing simple equation applisto the

amount of salt inthe liquid phase ofthe root

Zone per unit area of lan

.WK +Ve+Ve,) vM,+M]~

IM,+M,49,¥61=4M,,

Herein V, isthe volume of rainwater entering

the soll witha salt concentration ¢,V, the vol-

‘ume of irrigation with a concentration c, V, the

volume of groundwater with a concentration

3 Ngôn te rot zone by capil Hise, Ve volume of water drained from the soil with concentration cM, and M, are masses of salt dissolved from the soil and from agricultural {inputs (fertilizers, soil amendments), respec- tively; M, and M, are mass of salt precipitated

@œ in situ and mass removed by the crop, respectively; p, is density of water; and, finaly, AM,, i the change in mass of salt in the soil’s liquid phase during the period consid- ered The actual quantitative magnitudes of all

‘hese terms vary from place to place and from timeto time, depending on local conditions, l-

‘mate, and management practices

“The composition of rain varies according to season, direction of wind, and proximity to pos- sible sources of aizborne salts suchas seashores, volcanoes, dust-producing desert areas, and smoky or smoggy industrial centers The an- sual deposit of salt in rainfall has been es mated to range from 10 to 20 kg/ha in continental interiors (Yaalon, 1963) to 100 to

200 kg/ha near seacoasts Although seemingly

‘small, the amount of salt delivered tothe soil

in rainfall may add up, overtime, to a great

‘deal of salt, which may be stored inthe soll oF

‘subsoil However, nirrigated areas the amount

Of salt contributed by irrigation water gener- ally far exceeds that by rainfall

Soils and subsoil layers in arid regions are relatively unweathered The weathering of pri- rary minerals and the dissolution of salts from newly irrigated soils may contribute signifi- cantly to soll salination and to the sal load of drainage waters For example, irigation with

The Nour of alnty 11

Box7 Sample calculation ofthe salt

balance

‘The folowing data wore cbtainedin ald in an arid

1 Ranta cocurod ont in wr and amounted to

200mm, wth ofa eal concanrason of 40 pp

2 Capilary ise from saline groundwater ‘and autumn fala 100 mm a @ concentration in sping

‘9f1000 ppm

‘8 Inigaton was applied during the gure season

‘and amounted to 800 rn, wih 400 ppm sat

4, Drainage rig the irigation season amounted {200 mm, wih 2 soluble salt concentralon of

‘00 ppm

‘Anadtonaincroment 0.12 ky of soluble ss

wa added tho form of ferizars and sl amend

mort, of wich 0.1 kg? was moved by the ha

als and he preciptaton of eats wit hø so cơm,

pute the cnnual salt balance is there @ nat

[cumin or release of sale by te si

‘We begin wih a sighity madd version ofthe

‘sabalance equation ofthe rootzone per unio land

rea (nt hat nhs formation M, includes My:

aM SANG, +V8 +, Ve) +, M,

Heroin aM, i tho change n mass feat inthe rot;

‘1s the dani of walor V, Vp Vy, V8 the vor

Us of rain, gation, grounawateo, ad dưa

ctvely, with corresponding saiL

‘concentrations of, 9, c,;and M,M, ae tha masses

‘of salts added agriculturally and removed by the

‘ep, respectvey tho unit ol” area, calling water volumes intarms of m? and masts in os

tg (water deny being 1000 kg) we can sub- Site the given quantins Sno the previous equa tion to oblan the change In mass contnt of salt the sat

= 10®ginf0 3m)|404104)+ (0.1m (1000z102)+ (0 8m)đo0zf0 (02m (800x101) +0.12gfm°~0.1 kg"

De, = 0322 kgm

The oot zones accumating sal ataate 3220

kg (2.22 matric tons) por hectare par year

water from California's Feather River, which

hasa low salt content of about 60 mg/t, results

inconsiderably moresaltin the drain water due

to dissolution inthe soil and subsoil than due

to the salt content of the irrigation water

(Rhoades etal, 1974),

Solis and subsoil strata of arid regions may

also contain residues of salts deposited during

earlier eons of time by drying lagoons or lakes,

or by sea-spray Such dormant “fossil” depos-

its may be mobilized when irrigation is begun,

‘or when the water balance is otherwise altered

so that excess water percolate into the pre-

salinated subsoil Such occurrences are well,

Jnown in connection with the saline seeps of

‘Australia and the northem Great Plains of the

US.A (box 8) (Hillel, 1951)

‘The concentration of the sol solution may

{n places reach such levels of salinity that cer-

tain species of salts (primarily calcium carbon-

ate and calcium sulfate, but eventually even

sodium chloride) actually begin to precipitate

land become stored in the soil profile in lenses

of crystalline salts Consequently, the amount

cofsalt leached below the rot zone may be less

than the amount applied inthe iigation wa- ter Internal slt deposition by precipitation can bbe a significant component ofthe salt balance when the leaching fraction is low However, such salt deposits tend tobe highly labile, sơ that when the amount of irigation increased the salts so stored can quickly redissolve Salt removal by agronomic crops is gener- ally too low to contribute significantly to the

‘maintenance of the sat balance of irigated sells For example the average amount of salt

‘contained in mature crops of alfalfa, barley, com silage, sudangrass, and sweet clover grown

in Texas Rio Grande area was about 3.6 per- cenofthe dry mas (yey and Longneck,

‘A simplified form of the salt balance equa- tion in relation tothe water balance equation was offered by Bias (1974) fora complete an

‘ual period Assuming thatthe net change in total soil-water content from one year to the next is close to zero, and disregarding surface runoff the water balance per unit area is

V+V,+V,=V,+V,

12 Salvty Management for Sustetnable Irigation

Figure § Formation of a saline seep

Here V, Vj Vy Vqy and V,are total annual vol-

‘umes of irigation, atmospheric precipitation,

capillary rise of groundwater, evapotranspira-

tion, and drainage, respectively Because

‘evapotranspiration removes no salt and crops

‘generally remove only a small amount, and if

‘we disregard agricultural inputs and in situ

‘precipitation and dissolution of salts, the salt

balance corresponding to the above equation,

assuming no accumulation, is

eV Herein ¢,and ¢,are the average concentrations

of salt in the irrigation, rainfall, and drainage

Waters, respectively If the water tbleiskept deep

‘enough 50 that no substantial capillary rise into

the root zone occurs, and since the salt content of

atmospheric precipitation is usually small thelast

equation further simplifies to:

Va=Veeu Such an overall “black box" approach disre-

garde the mechanisms and rates of salt and

Ve+Ve=

‘water interactions in the root zone, as well as the changing pattern of salt distribution throughout the soll profile

“Most treatments ofthe salt balance havecon- sidered the relevant processes tobe one-dimen- sional, ie vertical In the case of furrow irrigation, however, where furrows alternate with ridges, salinity can vary laterally from the bottom ofeach furrow to the crest ofeach inter- vvening ridge As water spreads sideways from the furrows and rises up the ridges by capillary attraction it caries along the salts dissolved in ity and as water is evaporated from the ridges the salts are deposited there (figure 6) The con- centration of salts in the ridges of furrow iri- gated fields is often high enough to inhibit Seedling development during the subsequent

‘The pattern of salt distribution also varies laterally in the case of drip irigation, where

‘widely spaced emitters deliver water to sepa-

‘ate points on the surface (or below the surface)

‘of the sol, from each of which points the water spreads radially An oft cbserved phenomenon

is the appearance of cystalinesaltsin the form

The Natureof Sainty 13

Box® Saline seeps in Australia

‘and North America

Human management ofthe and may la to un-

foreoen enwarocmentl consequences A sing

‘example ete saline seep phonomanon in Austa-

land North America Aten decades go, Asa:

lan farmers ware eared a nots that brine began

to ooze out of ow spats in the fils Subsequent

research rvealed the phenomenen fo be tho de-

layed resto the extensive clearing of end carted ‘utin auham Acstala a cantury evar

Rains fating on and along tho coasts of Austa-

‘iar th to a spray, which omy the splash

‘of ocean waves again ho shore ands weed onto

{tolandby the winds Over eons othistoy hebracke

Ish rang deposto a load of sa ono te sl Na

‘ve forests etraciod maitre from tho el bt et

‘te sak behind Siwy, the saline solution poro-

lated boyend the rect zone, gradual charging th

eBaol reedua sale

Whon European settoment of Atala began in

Iê@ ml 18005 the soars cleared away the orost

long ta souboastrn and goutwostar edges of

‘he content This changod the water regime nthe

natural ovrgreen frst, ar of to rain had been Intercept bythe olage and then evaportec vi

‘ut ever reaching the found, and moisture extrac:

{onby the dep rots wasconruous The eeesona,

shalow-rooid crops plantad by the eaters ex:

‘raced and evaporated loss moisture Th ncroased eclen of rafal hat percolated downward and aly care to ret over an impervious stalum and formed a water abe That water lo mobfzsd De saN hai hedaecunuletadin heeubsolin aone pet, Gradually, te water table rose, and with returned theancient at Altha wh, the farms a ranch

‘ere were abvous io what was happening down be- low, 80 when the brine reached the surface and formod splotches of stare el, I came as total

‘97355, not ees, andthe source of the sas was aot

‘00 spay but underying deposts cf marine shales, 9fgalig in en varfer googie era However, ho

‘roones by which the sbope Became marist was

‘Sinan Boh continents reso rom human os futon ofa preexisting equim

14 Sly Management for Sustainable gation

Figure 6 Salt accumulation patterns under furrow irigation

"hiep beds and nigaton pastes

Snge-

row bed

Satty with sloping bad

fre PQ Ạ “a = he ONG =

_

Source Bernstein a (1055); Bernstein ane Fireman (1987)

of rings around the perimeters of the wetted

spots Asin the case of furrow irrigation, such

localized accumulations of salt may affect the

seedlings ofa crop planted in a subsequent sea-

son, unless the rings of salt are leached away

during the intervening period by rains or by

sprinkling irigation

In any case, knowledge of the spatial distri-

‘bution of salts in various zones of the sil may permit preferential planting where salts are least concentrated End-of season plowing and

‘other modes of tillage tend to obliterate the lo- calized salt concentrations and to redistribute the salts throughout the surface zone of the field

CHAPTER 2

Effects on Crops

ants that ae especially adapted to grow

funder saline conditions are called halo-

phytes Plants that are not so adapted,

‘alld giyeophytes, generally exhibit symptoms

of phys stress when subjected to s3-

linity Most crop plants are of he latter category,

though they vary from one another in the de-

_gree of their sensitivity A condition of stress is

Such that prevents a plant from realizing its full

potential or growth, development, and repro-

Figure 7 Plant responses to salinity

duction in reality, there is no sharp dividing line between the occurrence or absence of

‘stress, Rather, plant response to increasing sa- linity i a continuum, varying from no appar- lent stress to intense stress (Figure 7) Hence there is no clear distinction between salt-toler- ant and saltintolerant crops Even varieties oF genotypes within a given species may differ significantly in their responses to various, or varying, levels of salinity

‘Halopytosincreasa yes allow conesrrations; stttolerant cops reduce yields beyond a teshelt Sal

‘sonsive crops eter even at lw saliny

Sour NAC (1000,

6

16 Sdinúy Memsgenemer SusuingbleIeriaton

Sensitivity to Salinity

Anatomical and physiological diferencesbe-

tween halophyts and glycoptiytes reflec thelr

‘riaiity in salt tolerance, nme, geneticists

may find ways to implant salt tolerance at

tele in cope Chat twat present salts

tive, Thus far however, there do no ser to

tbeany dramatic breakthroughs in plant breed

ing fo salinity tolerance “Among crops that re considered salt cleat

atarley sugar bet, able bet amperage spine

ach, tomato, cotton and Bermada grass Ameng

‘reps that are known tobe serie (ie, to have

low tolerance) osalinty aeradish ery beans,

lovers mui neuiy all fut res

‘altconcentation in theambient solution (in

the root zone) depresses the energy potential of

water inthe solution thus requiring a greater

‘exertion of energy by the plat o extract the

‘water i requires The plant does this by con-

entrating the interal solution of its cells by

thro mechanisme:sbeorption of walt fom the

‘medium or synthesis of organic solutes Tis

process called “osmatic adjustment” requies

{he investment of addtional metabolic energy,

ic, of photooynthates (Eaten, 180), so a8 1

lower the plant's own water potential further

below that ofthe external slution from which

itimust draw water Highly salt-tolerant plants, or halophytes,

tend fo absorb sls fom the medium and ge cquester itn the vacuoles, while organi comme

patible solutes serve the function of ommotic

Adjustment inthe cytoplasm As most crop

plats (Le, plcophytes aresalt sensitive, they

fend to excide sodium and chloride frm the

Shoots and, especialy the leven Hence they

have to aly mere heonly than do halophyice

onthe synthesis of onganicosmolytes In extreme conditions of very high salinity,

the external esmotie potential may be de:

Dressed Below that ofthe cll water potenti

{hos resulting inthe net oatiow of water from

the plant and in osmotic desiccation (a cond

tion called plemolyis Even were conditions

re not s extreme, salinity reduces the avail

Abily of water to the plant

"The sal senatvty ofa pant canbe defined

as the plan's capacity to endure the effects of

‘xcess satin the medium of root growth aa,

1990, Implicit in this definition is the idea that

a plant can withstand a certain concentration

of salt without experiencing adverse effect In fact, the sensitivity of a plant to salinity de- pends on a multiplicity of interacting factors {and conditions including the climate (tempera- ture and potential evaporation), sol fertility (availability of nutrients) and sol physical con- ditions (porosity, aeration, and water regime) Plant sensitivity also varies with the physi- ological stage of crop development Although

a plants capacity to tolerate salinity cannot be determined in absolute terms, the relative re- sponses of various plants may be compared under definable conditions (figure 8)

Reliance on the mean seasonal salinity of the soilasa criterion may be misleading, asthe soil solution typically fluctuates during the grow-

ng season Even occasional or temporary in- creases of salinity that exceed a crop’s tolerance limit may affect crops deleteriously especialy

if they occur during sensitive stages of plant evelopment

“The salt sensitivity of a crop may express it self in three developmental stages: germination, vegetative growth, and reproductive function,

‘Some halophytes, whose vegetative growth is often stimulated by salinity, may not be salt- tolerant during germination On the other hand, some salt-sensitive plants whose vegetative

‘development is inhibited by salinity, may ger-

‘inate readily in the presence of high concen- tations of salt Ifthe plants do succeed in overcoming salinity during the germination

‘and growth stages, some species of non-halo- pphytes become more tolerant during reproduc- tive development However, no absolute rule prevails, as planis differ widely both in the

‘degree and in the stage of their sensitivity to Salt sensitivity changes during the growth

‘cle of a plant Root growth is often less af- fected by salinity than shoot growth (Lauchls and Epstein, 1990), particularly when supple-

‘mental calcium is provided In the shoot, de-

‘crease of leaf area is commonly observed in

‘conditions of salinity, evidently a consequence

‘of the osmotic effect on reduction of turgor The salinity tolerance of many (but not all) plants increases as the plant matures (Maas, 1990; Pasternak, 1987)

fects on Crops 17 Figure 8 Classification of crop tolerance to salinity

100

Sowa: Ato Mass 1060)

‘The terms salinity tolerance and salinity ress-

tance are often used interchangeably Another

cceasionally used term is salt amoidence The ap-

propriate term in each case depends on the

plant’s specific physiological response to salt

stress Tolerance implies thatthe plant is able

to grow more or less normally in the presence

of higher levels of salinity, though often at a

diminished rate Resistance implies that the

plantean develop active biological mechanisms

tocounter the effect of salinity Salt avoidance

{is the ability of plants to curtail or suspend ac-

tivity 50s to endure periods of salt stress, and

torecover growth and development when con-

ditions improve

(Ff those various terms, tolerance is gener-

ally the most apt (Shannon and Noble, 1990)

‘All responses to salinity depend in a complex

‘way on the anatomical and physiological at-

tributes of the plant Since there is no single

attribute that determines the eapacity ofa plant

to survive or even thrive ina saline medium,

‘the task of promoting tolerance is very diff-

‘cult The much easier approach in mast cases

{stopreventsalinty from building up toharm-

fullevels from the outset

Plants suffer water stress sooner when os-

‘matic pressure is high, and salinity can upset

plant nutrition when an imbalance of certain

‘sential nutrients occurs As mentioned, sa-

line conditions force the plant to divert to i- creased maintenance some of the energy that

it could otherwise invest in net growth Selination of the root zone intially raises the rate of maintenance respiration in many spe- cies, but total respiration eventually falls along, with reduced photosynthesis

Many workers have employed the concept

of “threshold salinity” to characterize the de- pendence of relative crop yield (Y,) on salinity:

l00~bŒC, ~a)

where a is the salinity threshold expressed in 45/m; b is the slope expressed in percent

‘change in yield per dS /my and EC, isthe mean

‘electrical conductivity of the root zone (usu- ally measured using the saturated paste tech- nigue) This relationship implies a linear dependence of yield on soil salinity Accord- ingly, farmers need know the levels of soil sa linity that begin to reduce the yields of specific

‘crops and by how much the yield of each crop

is reduced at levels above its threshold

‘Crop response to soll salinity may be de- scribed more accurately by means of sigmoi- dal (rather than linear) relationship, such as offered by van Genuchien (1983):

Y,=Y/1+(/c]

18 Sanity Managemen or Suslamile Irigaio

where Y isthe yild under non-saline condi-

tions, cis the average salinity ofthe rot zone,

Gis the salinity ofthe root zone that reduces

Yield by 50 percent, and p is an empirical con-

Stant The usefulness of this relationship may

be limited by its greater complexity and the

ical of determining its parameters

‘Specific Element Effects

Beyond the general effects of salinity on crops,

there can also be specific ion effects (Lauchh

and Epstein, 1990; Maas, 1990) High concen-

trations of one or another ion commonly asso-

ciated with salinity may cause disorders in

‘mineral nutrition (igure 9) For example, high

sodium (Na) concentrations have been ob-

served to interfere with plant uptake of such

‘essential nutrient elements as potassium orcal-

cium, and thats quite apart from its detrimen

tal effect on the soils physical properties On

the other hand, calcium at elevated concentra-

tions often mitigates some of the adverse of-

fects of moderate levels of salinity However,

certain elements, primarily chloride, may have

specific toxic effects beyond interference with

the uptake of nutrients The element boron (B)

is another example

Sodiem and chloride are particularly toxle

to fruit crops and woody ornamentals When theleaves ofthese plan's accumulate more than about 05 percent chloride oF 0.25 percent so- dium on a dry weight basis they fend to de- velop characteristic lea injury symptoms Sodium while not considered an essential clement for most crops, may nonetheless be

‘beneficial when preset in concentrations be- low the threshold of salt tolerance Above that threshold, sodium becomes harmful especialy for woody species Tolerance to Na" varies widely among species and rootstock In the

‘ase ofsuch crops asavocado, crus and stone- fruttees, injury may occu at sol sluon con- centrations as low as 5 minol/1 According to Bematein etal (1975), sodium is absorbed and retained fora time inthe roots and lower trunk,

‘but afer some years the sapwood is converted

to heartwood and the accumulated sodium ion

‘seleased.Iis then transported where it may cause leaf scorch up thecanopy,

‘Chloride fs an essential micronutrient for most crops albeit in very small amounts Even

at higher concentrations its not particularly toxic, exept to certain crops, eg, soybeans

‘According to Parker etal, 1983, the decisive attribute appears to be the ability of plants to

restrict the transport of CI from the roots 10

the shoots Many woody species are sensitive

toCr toxicity, though the degree of sensitivity,

varies among varieties and rootstocks (Maas,

1990),

‘Boron is an essential element for plants in

minute concentrations, but becomes toxic at

somewhat higher concentrations For some

plans, the threshold of toxicity may be as low

‘sa few parts per million (ppm) in the ambient

solution, Boron is toxic to many plants when it

‘accumulates in susceptible tissue to reach con-

centrations exceeding about 0.1 mg/kg

‘Symptoms of excess boron may include

chlorotic and necrotic patterns of leaves,

some sensitive fruit crops may be suf

fer reduction of yield even without visible in-

jury The concentration of boron in leaves is

‘ormally in the range of 40 to 100 ppm of the

‘dry mass, but may rise to 250 ppm where the

‘soil approaches toxic levels The concentration

may even approach 1000 ppm in extreme con-

ditions of boron toxicity

Citrus and avocado are especially vulner-

able to boron toxicity The typical symptom

is tip burn or marginal bum of mature leaves,

accompanied by chlorosis (yellowing) of

interveinal tiseue, Stone fruits, apples, and

are also sensitive to boron Cotton,

grapes, potatoes, beans, and peas, among

other crops, exhibit marginal burning and

cupping of the leaves as well as general re-

striction of leaf growth

‘Maas (1990) provided alist of threshold tol-

cerances for a large number of crops, based

largely on the early work of Eaton (1944), as

‘well a much more recent work by Bingham et

al (1985) and Francois (1988, 1989) The data

provided do not apply equally toall situations,

as boron tolerance (like salt tolerance in gen-

eral) is known to vary with climate and soil

conditions, as well as withthe crop

Bingham et al (1985) demonstrated that

yield reductions due to boron toxicity can be

[ited toa two-parameter empirical relationship

(Maas and Hoffman, 1977):

Y=100-mox-A)

‘where Y is the relative yield, m isthe reduc-

tion in yield per unit rise in boron concentra-

fect on Crops 19 tion, Ais the maximum concentration ofB that does not reduce yield (threshold value), and x 1s the boron concentration inthe soil solution

‘The effect of boron on crop yield obviously depends on its concentration in the irrigation

‘water, as well as on the physiological sensitiv- ity (o tolerance) ofthe crop to this element It also depends on the irrigation regime (particu- larly the volume and frequency of water sp- plication), inasmuch as that regime determines hhow high the concentration of boron (as well sof other solutes) is allowed to risein the root

‘zone above its evel inthe irrigation water Ide- ally, the leaching fraction should be such as to

‘preventa significant risein boron between suc-

‘cessive iigations (Bingham etal, 1985) Since

‘boron is adsorbed onto and released from the surfaces of clay particles, soil solutions are somewhat buffered against rapid changes in

‘boron concentration In the long run, however,

‘a steady-state is approached, as the amount of boron in the exchange complex equilibrates

‘with the concentration ofthis element in the soil solution, which itself is influenced by the concentration of the irrigation water as well 5 the leaching fraction

TToxic concentrations of boron occur mainly

in arid regions Although most surface waters đảo not generally contain excessive concentra- tions of boron, well waters occasionally do Since different plant species and varieties vary inthelr response to boron, irrigation water that

is unsuitable for particularly sensitive crops

‘may be suitable for more tolerant ones

“The hazard of boron toxicity persists and

‘may even worsen in recycled and desalted wa- ter, Desalination processes such as reverse ò-

‘mosis may not eliminate traces of boron that

‘might still be present in potentially harmful concentrations

Selenium, the most studied trace element for Ite effects on wildlife, may cause damage at

‘concentrations as low as 1 ppb Its bioaccumu- lation has been found to cause birth deformi-

‘ties and deaths in several species of waterfow!

at California's Kesterson Reservoir (Jacobs, 1988)

(Cay Mg, and K are the major cations required for plant nutrition Fe, Mn, Zn, and Cu are also required, but in much smaller quantities In sodic, non-sline soils, deficiencies of Ca and

20° Solty Management for Sustatate tigation

‘Table 1 Relative salt tolerances of various cropa

Grain crops

Wheat (somber T anstum

Bermuda grass {Cyradon dation

low, alexa “iletum tybddum

over leaine -rlelom repens

Gower, ctawbory TT agfeam

‘Cowpea (rap) Vigra unguiculata

Foxtal, meadow ‘Nopecurus pratencis

(Orchard grass Dayle glomerata

Ryegrass, perennial {lum perenne

‘Soden grass ‘Sorghum eudanenee

Wea forge) -TRlcum seetưum,

Wineat, dour (rage) Tugun

Winoat gass, standard crested Agopyronsbiicum

Wheat ase, fway crosied A eratatum

pects om Crops 21 Table 1 (continod)

Pineapple

Pomegranate Pauniea ganalum,

"Bafhg 9x Seo, Tc Tan M Mose

.#ovre-3edosirolateorglndan rdơtncrdy Ma (100)

22 Salinity Management for Sustainable Irigation

‘Mg may occur, and the addition of these ele-

‘ments as fertilizers or soil amendments may

be advisable

‘Affinal consideration isthe possible accums-

lation in plants of elements that may not be di-

rectly harmful to plants but that can be

hazardous fo consumers Such “minor” ele-

‘ments as selenium and molybdenum may be

toxic fo humans and animals! Toxic elements

that re of particular concern are those that can

concentrate as they move up the food chain, a

‘process called biomagnification (Page et al.,

1990)

“Tabulatedsalttolerancesof variuscropsare listed in Table 1, based on the comprehensive listing provided by Maas (1990) The depen- dence of relative crop yield on electrical con- ductivity of the soil solution is shown schematically in Figure 8 for plants of various seraitivity or tolerance levels

Note

|LAlthough selenium i essential to humans and animals in trace amounts, excesive amounts can

‘ae toxicosis

CHAPTER 3

Irrigation Water

Water Quality

‘The quality of irrigation water affects soll sa-

linity and sodicty, cation exchange, sil acd-

ity or alkalinity, nutrient availability, clay

dispersion and flocculation, and soil structure

(the later, in turn, affects soil-water relations

1 well as soi aeration), Clearly therefore, the

‘composition of irrigation water is an important

determinant of crop growth and agricultural-

drainage quality To avoid the accumulation of

salts otoxiclevels their inputs to thesoil must

‘ot exceed the rate of thelr removal from the

‘si or of their conversion to unavailable forms

‘within it.The control of ol salinity must there-

fore include measures to control both the in-

‘puts and the outputs of salts,

‘Solutes are added tothe soil solution in irri-

gation water infiltrated from above, in

‘groundwater rising by capillaity from below,

in the dissolution of salts initially present in

‘solid form within the soil and subsoil Remov-

als of solutes from the soll include uptake by

plants, downward transport by percolation

‘and drainage (leaching), erosion of the soit

‘surface by overland flow and by wind, precipi-

tation or adsorption onto the solid phase and

‘conversion to unavailable forms, and—for

some substances—volatilization of gaseous

compounds

‘The hazard of plant stress and sol salination

posed by irrigation water containing salts of

varying composition and concentration de-

pends on soil conditions, climatic conditions,

‘crop species and variety, and the amount and

frequency of the irrigation applied In general,

lrsigation water of EC lower than 0.7 dS/m_ poses litle orno danger to most crops, whereas TEC values greater than 3.0 dS/m may restrict the growth of most crops (Ayers and Wescot,

1989

‘The salinity of irrigation water is defined as

‘the total sum of dissolved inorganic ions and

‘molecules, The major components of salinity are the cations Ca, Mg, and Na; and the anions

€1,S0, and HCO, The potassium nitrate, and

‘Phosphate ions, However important nutrition- ally, ae usually minor components of soil s2- linity In addition, certain constituents (such as boron) may have an important effect on crop

‘growth even though their concentrations are

‘usualy too low to have any substantial effect

‘on the soil’ total salinity

Irrigation waters of different sources, loca- tions, and seasons vary greatly in quality.Some Irrigation waters contain as little 50, and oth-

‘ers as much 2s 3000 grams of salts per cubic

‘meter Since the volume of water applied in ir- tigation to a crop during its growing season

‘commonly varies between 5,000 and 20,000 cubic meters per hectare, the saltinputtoa crop

"may thus range between 250 kg and as much

as 60,000 kg per hectare That is a very wide range indeed Water quality categories based

fn total dissolved salt are given in Table 2

‘Another important criterion of irrigation

‘water quality is the sodium adsorption ratio (GAR) (figure 10) High alkalinity of irrigation

‘water, manifested when the pH value is above

85, generally indicates the predominant pres-

‘ence of sodium ions in the solution, and poses

1 danger of sol sodifiation Freshly pumped

2A Sanity Menagement cố

‘Table 2 Classitication of water quality according to total salt concentration

Total dissolved sats

‘groundwater may have a high sodium adsorp-

tion ratio even ifthe pH is below 85, owing to

the presence of dissolved CO, (which forms car-

Donic acid, Ht + HCO3) Sampies of such wa-

ter should be aerated to allow the CO, to

ceffervesce prior to measurement ofthe pH

‘With high SAR water irrigation by sprinkling

‘willinreage the sol's tendency to forma surface

seal (crt) under the impact ofthe drops strk-

ing the bare soil Flood irrigation may also cause

the breakdown of soll aggregates by ar

‘lll, 1996) On the other hand, application of

‘water by drip, at spaced points on the surface

‘or below it, may lessen the physical disruption

‘ofsoil structure that would otherwise take place

‘under the influence of high SAR water

‘High pH water may cause nutritional, as well

‘as structural, problems The addition of gyp-

‘sum to the water orto the soil surface may help

in both respects The natural presence of cal-

Cite (calcium carbonate) in finely fragmented

form in the sol may also help to mitigate the

‘effect of high-sodicty irrigation water Its dis

solution by acidic rainwater, especially in the

surface zone, tends to reduce SAR and to in-

‘crease the electrolyte concentration However,

in soils that do not contain lime or gypsum,

rainfall may even do more harm than good to

soil structure Where itis nearly pure (EC be-

low 0.06 d5/m), rainwater reaching thesurface

leaches away the salts present there, and may

cause spontaneous dispersion of clay especialy

ifite ESP is high

Use of Brackish and Saline Water

[Extensive studies have shown that, in certain,

circumstances, and with appropriate precau-

EA (asm)

08 Drinking and irigation

153, Irdg8ton vân caulon

38 a ‘Secondary drnage and ealne Primary drainage

‘roundwater very sain groundwater

tions, available brackish water can be used safely, and even advantageously, forthe iriga- tion of salt-tolerant crops This is especialy the case in arid regions with deep sandy soils,

‘where drainage is unrestricted and there ttle risk of either groundwater rise or of soil salination and sodification

Various strategies have been proposed for the use of saline water, or forthe reuse of brackish

‘drainage water One way is to use the water a8

is, directly for irigation Another way isto

‘ternate the applications of the brackish water

‘with applications of better quality water, where ssuchis available A third way i to blend good {quality water with brackish water so as to ex- tend the water supply, thus in effect gaining

‘quantity at the expense quality It appears that the optimal strategy depends on circumstances (eg, how saline is the brackish water? How

‘500d is the quality of the nonsaline water? How tolerant are the crops to be grown? How sensi tive is the soil? etc) Therefore, no universal principle can be expected to apply equally in all cases Sina etal (1988) provided a theory, design criteria for anetwork blending systems, and methods of automatically diluting saline water sources for irrigation However, in the opinion of Grattan and Rhoades (1950), the cy clic strategy ie generally preferable othe blend ing strategy, as it obviates the need for a blending facility and allows the soil to be

‘lushed out more completely from time to time,

as the need arises

‘To evaluate reuse potential, standard water sampling techniques and analysis can be used (Chapman and Pratt, 1982) The most impor-

‘tant water quality parameters are EC, SAR, and boron concentration, as well asthe concentr

Tem Me 2 Figure 10 Diagram for assessing salinity and sodicty hazards of ligation water

tion of other potentially toxic elements Irriga-

tion water that contains more than 5 mg/1bo-

zon can be detrimental to many’

Irrigation water with EC of 4 dS/m was re-

ported to improve the soluble solids content of

relatively tolerant crops such as melons and

tomatoes (Grattan eta, 1987), to raise the pro-

tein content of wheat and alfalfa, to increase

the total digestible nutrients in alfalfa, and to

improve the color and netting of cantaloupe

(Rhoades et al,, 1988) However, the use of

brackish water for sprinkling irrigation may

‘cause foliar injury The degree of injury depends

‘on the following factors: concentrations of ons

in the water, sensitivity of the crop at various

stages of growth, water stress ofthe plants prior

to irtigation, and frequency of sprinkling, The

‘potential damage also depends on the prev: ing environmental conditions, including the temperature and eative humidity of the atmo- sphere, which affect the rate of evaporation Sprinkiing at night, when atmospheric tem- perature and evaporativity are relatively low,

‘evidently reduces foliar absorption and injury (Pratt and Suarez, 1990)

‘The greater the rate of evapo-transpiration, and the longer the interval between irrigations,

‘the more concentrated the residual solution in the root zone and the more severe the salt stress imposed on the plants, Hence, when the iri- gation water is brackish, a common manage-

‘ment strategy ie to increase the frequency of

25 Salinity Management for Sustaincteirigetion

{irrigation so as to maintain a high level of wa-

ter potential in the root zone Ths strategy can

help to prevent an excessive rise in the con-

centration ofthe soil solution toward the end

cof each interval between irigations Thanks to

its ready adaptability to high frequency irvi-

gation, drip irigation beneath the canopy ap-

ppears fo be the most appropriate method to

tse with brackish water, especially asitavoids

direct foliar exposure to saline water In some

soils, however, increasing irrigation frequency

‘may also result in impedance of soil aeration

and increased rsk of root disease (Grattan and

Rhoades, 1990)

“The use ofbrackish water for irtigation may

‘beenvironmentally beneficial in alarger sense

intercepting drainage effluents before they are

mixed back into the river and using them to

irrigate certain crops in the agronomic rotation,

‘an help to reduce the salinity of river systems

(Rhoades, 1984) ‘When the drainage water quality is so dete-

‘iorated that its potential for reuse is exhausted,

this water can be discharged into evaporation

ponds Athough ponds provide temporary re-

lief they cannot generally be sustained, because

‘evaporation may resultin the deposition of salts

that contain hazardous levels of trace elements

In the later case, strict precautions must be

taken to prevent damage to wildlife, including

aquatic bids The problem is complicated by

the fact that agricultural drainage waters may

contain, in addition to the salt load, an appre-

lable concentration of pesticide residues, fer-

tilizers, and toxic trace elements

‘A technology that is often mentioned as a

possible solution to the problem of water sa-

Linity isthe process of desalination It is used

mainly to convert sea water and brackish wa-

ter for direct human use, but in most cases has

‘ot yet been found to be economical for the i-

‘gation of crops in the field Of the various de-

salination methods, the one that seems most

promising at present for the conversion of

brackish water is reverse osmosis It consists of

forcing water under pressure through selective

membranes, which permit the passage of

‘water molecules but not of salts or suspended

materials

‘The problems associated with this technique

of desalination (as well as with other tech- niques) are the high energy requirements, the frequent need to replace deteriorated mem- branes, the passage of bron through the mens- branes, and the need to dispose ofthe residual brine safely Estimates of the costs of desalina- tion by reverse osmosis vary from $0.50 per

‘cubic meter to over $1.00, depending on the

‘composition and concentration of the salts in the original solution, and on the costs of en- ergy, brine disposal, and water conveyance

‘Alternative processes of desalination include distillation, freezing, and the use of fon ex-

to reduce the volume and expense of needed ddainage in an area Among the trees suitable fortis type of agroforestry are certain species

of eucalyptus, acacia, casuarins, poplar, mes-

‘The harvested wood may be used for fuel, for pulp, of for construction

‘A publication issued by the US National Research Council (1950) lists and deseribes scores of salt-toleant plants, native to various saline areas, that may be capable of utilizing land and water unsuitable for conventional salt-sensitive crops (lyeaphytes) Some of the plants described can perhaps be grown with highly saline waters for the purpose of pro- viding ood, fodder, fuel, and other products

“Halophytes (plants adapted naturally to grow ing in saline environments) can utilize saline

‘water resources that are generally neglected and are usually considered impediments rather than opportunities for development, However, most undomesticated halophytes display poor agronomic qualities, such as wide variation in germination and mature- tion, tendencies to seed-shatiering and lodg-

i a8 well as possible toxicities The possibility of turing some of the many listed plants into economic crops is a worthy goal,

but will require much research in the com-

ing decades

Reuse of Wastewater

Decomposition in the sol is nature's way to

recycle the organic products of terrestrial bio-

logical activity As long as population density

remained sparse, the waste products of human

activity could be similarly accommodated The

growth of cities and of industries, however,

‘produced quantities of solid and liquid wastes,

In excess ofthe soi’s ability to reprocess in the

Immediate domain of human habitation Some

societies (notably in China and other parts of

‘Asia) continued to transport human wastes

from cities to agricultural land forthe purpose

of fertilizing crops and replenishing soll nuti-

‘ents In many other places, the uncontrolled

discharge of garbage and sewage ended up

polluting streams and wels, to the detriment

of the environment and public health

‘With the advent of integrated sewerage sys-

tems in the cities of Europe at the beginning of

the modem age, interest in land application in-

creased (Shuval et a, 1986) A report issued

{in 1865 by the First Royal Commission on

Sewage Disposal in England stated: “The right

way to dispose of town sewage is to apply it

continuously to the land and itis by such ap-

plication that the pollution of rivers can be

avoided.” In 1868, Victor Hugo, in his “Les

Miserables,” expressed an even stronger opin-

fon: “All the human and animal manure which

‘he world loses by of sewage toriv-

rs if returned to the land instead of being

thrown into the sea, would suffice to nourish

the world.”

Inthe late 19th and early 20th centuries, sew-

age-irrigated farms were widespread in the

environs of many cities In addition tothe aims

of preventing pollution of water supplies and

of conserving nutrients for crops, there arose

the impetus to utilize sewage as an additional

water resource

‘Subsequently the growth of cities and their

suburbs encroached upon the sewage farm ar-

eas and many of the early wastewater irriga-

tion projects in Europe and America were

Irigation Water 27 abandoned Concems over the odor problem and over the transmission of disease from veg table crops irigated with raw sewage also contributed to the decline of sewage farming

‘Another dissdvantage in some areas was that, {following heavy rainstorms, runoff from sew=

‘age irrigated Belds periodically conveyed loads

of pollutants into rivers and reservoirs

‘publichealth officials therefore came tobelieve that sewage farming was an unsanitary and hence undesirable practice ofthe past The al- temative was a sewage-treatment technology

‘based on intensive centralized civil engineer- {ng systems in which the organic components

‘of sewage could be digested and from which

‘purified” fluid could be discharge more

‘of less harmlessly to streams

‘Still more recenty, the pendulum seems to

‘ave swung back again Interest has revived in

‘the used of sewage as a resource rather than

‘merely as a problem, based on more rational scientific than before, Research on the fate of pathogens and of various organic and {inorganic components of wastewaters applied tothe soil has shown that in many cases treated

‘or even partially treated sewage can indeed be used safely and advantageously for irigation, provided that necessary precautions are taken

to prevent deleterious effects on public health

‘as wells on soil quality Guidelines have been formulated to allow wastewater irrigation to

‘become socially acceptable and sanitary prac- tice The trend has been led by the State of Cali- fornia and by the States of Terac, in both of

‘which growing urbanization and industializa- tion increased the competitive demand (and

‘hence the price) for scare freshwater while si-

‘multaneously increasing the volume of gener- ated wastewater

‘Wastewater treatment standards such as those established by the California State Health Department apply stret criteria of bacterial

‘counts (¢g,,n0 more than 22 coliform/100 ml)

‘and required the application of chemical dis- Infection (chlorination) for treated sewage wa- ter to be allowed for general use Partially treated sewage should only be used for the ir rigation of industrial crops such as cotton, oF

of tree-grown fruits that do not come in

con-28 Salinity Management for Sustainable Irian

Figure 11 Irigation returr-flow aystem

tact with the imlgadion water, or— at most—

for vegetable crops that are to be cooked be-

fore being consumed Similar guidelines have

been adopted elsewhere ‘in Israel, some two-thirds of domestic sew

age is curently being recycled for use in agrl-

culture, and the projection is for euse of some

{80 percent in the foreseeable future Infact re:

cycled sewage is expected to become the main

source of water for irigation, Wastewate

gation has been shown to contribute sign

cantly to soil fertility (especially to nitrogen,

pPhosphonis and organic matter augmentation)

{n many azeas, and to produce greater crop

vies

Wastewater irrigation is not always a bless-

Jing, however In many places, the concentra-

tion of nitrates can be excestive, and to

contribute to groundwater and surface-water

‘contamination Heavy metals and various

‘other toxic materials can pose a problem They

tend to accumulate in the sol and thence to

Irrigation Water 29

‘enter the biological food chain Especially haz~ ardous are industrial waste products thet may

‘be toxic as well as carcinogenic

‘Wastewaters typically contain increased con- centrations of soluble salts, and therefore pose the danger soll salnation Where salinaton is prevented by leaching, the salts tend to accu

‘mulatein the underlying aquifer and may cơn- tribute to the salinity of well waters (Such is typically the case along the coastal plain of I- acl) The variable pH of the water applied to the soil may cause either acidification or alkalination, with consequent effects on soil structure and fertility Finally, the materials sus- pended in sewage, if not readily decomposed, may cause clogging of sll pores (as well as of irrigation systems) and thereby reduce soil per-

‘meabilty towater and air Naturally, some soils are more vulnerable than others

Irrigation return flow, and drainage reuse and disposal systems are depicted schemati- cally in figures 11 and 12

CHAPTER 4

Waterlogging and Drainage

High Water-Table Conditions

“The presence of a high water table can be ef-

ther a blessing or a curse The blessing occurs

‘when, in periods of low rainfall or deficiency

‘of water for irigation, upward capillary flow

from the water table augments the water sup-

ply to the roots The curse cccurs as the rising,

‘water brings up salts from below and thereby

salinizes the root zone In extreme cases, the

latter phenomenon becomes manifested at the

surface in the appearance of fine crystalline salt

Inthe field, upward capillary ise and down-

‘ward percolation may take place alternately

during the year (Hoffman, 1990) Percolation

‘occurs typically during the rainy and early =

‘gation seasons, when the water requirements

of the crop are relatively low and the water sup-

ply from above ishigh On the other hand, up-

Ward flow takes place later in the irigation

season, when water requirements are high and

‘both rainfall and irrigation are restricted Over

the long term, a net downward flow of salt-

bearing water through the root zone is essen-

tial to sustainable productivity

Ifthe water table remains ata constant depth

and conditions at the surface remain constant

‘as wel, a steady-state flow regime tends to oc-

‘cur The equation describing steady upward

flow is (Hillel, 1998)

4= Kfđh/đz~ 0)

‘where q is ux (equal to the evaporation rate

under steady-state conditions) h suction head,

K hydraulic conductivity, and z height sbove

the water table The equation shows that the

‘upward flow stops (q = 0) when the suction profile is at equilieium (dh/dz = 1), Integra Hon should give the elation between depth and suction:

‘Their work used an empirical relation between the soil’s hydraulic conductivity K and

‘matric suction ft ofthe form:

K /6+h)

‘here a,b, and n are constants For a saturated soll, the suction h is zero, so K assumes the value of af, For an unsaturated soil, K đe- creases as the suction increases The exponen- tial parameter n expresses the steepness of the decrease of K with increasing hi, and is related

to the sil’ texture Inthe case ofa clayey soil,

‘was found to be about two; in a loam, about

‘three; and ina sand, four or more Using this relationship, the flow equation

‘an be solved to obtain suction distributions with height for different uxes, as wells faxes for different surface-suction values The theo- retical solution is shown graphically in figure

133 fora fine sandy loam with an n value of 3

‘The curves show that the steady rate of capil- lary riseand evaporation depends on the depth

of the water table and on the suction atthe soil

Figure 13 Steady upward flow and evapore-

tion from a sandy loam (n =) a8 afunction ot

the auction atthe soll surface, with water table

‘surface This suction is dictated largely by the

‘external conditions The greater the atmo-

spheric evaporativity, the greater the suction at

the soil surface on which the atmosphere is

acting,

the suction atthe surface, even to the extent of making it infinite can increase the

flix through the soil only up to an asymptotic

‘maximal rate, which depends on the depth of

the water table (figure 12) Even the driest and

most evaporative atmosphere cannot steadily

‘extract water from the surface any faster than

thesoil pofilecan transmit from the water table

to that surface, The fact that the sol profile can

limit the rate of evaporation is useful feature

of the unsaturated flow system The maximal

transmitting ability of the profile depends on

the hydraulic conductivity ofthe soll in rela-

tion to suction, Kt)

DDisregarding the constant b of the preced-

‘ng equation, we can obtain the function

Witerogging end Drainage 31

‘The actual steady evaporation rate is deter-

‘mined either by the external evaporativity or

by the water transmitting of the soll, depending on which of the two is lower and

‘therefore limiting, Where the water tableisnesr

‘the surface, the suction at the soll surfaceis low

‘and the evaporation rate is dictated by exter-

‘nal conditions, However, asthe water table be-

‘comes deeper and as the suction at the soil

‘surface increases, the evaporation rate ap- [proaches a limiting value regardless of how high external evaporativity may be

“The equation above suggests that the maxi- smal evaporation rate decreases with water-able depth more steeply in coarse-textured soils (in

‘Which nis greater because conductivity falls off more markedly with increasing suction) than,

in clayey soils Stil, a sandy loam can evapo- rate water at an appreciable rate even when the

‘water table is as deep a8 1.8 m

Subsequent findings of numerous workers have generally accorded with the theory de- scribed Hadas and Hillel (1968), however, found that experimental soil columns deviated, somewhat from predicted behavior, apparently

‘owing to spontaneous changes of soil proper- ties, particularly at the surface, during the course ofthe evaporation process

‘Lowering the water table by drainage can decisively reduce the rate of capillary rise and,

‘evaporation, and hence also of salt accumula tion Drainage isa costly operation, however, and itsis therefore necessary, ahead of time, to

‘determine the optimal depth to which the wa ter table should be lowered Among the impor- tant considerations in this regard is the necessity to limit the rate of capillary rise to the surface in the soil depicted in figure 13, for example the maximal rate of profile trans- mission to the surface is 8 mm/day where the water-table depth is 0.9 m Because potential cevaporativity is seldom greater than this, itfol- lows that a water-table rise above that depth

32 Salinity Management for Sustcnal Irigaton

‘would notbe ikely to increase the evaporation

‘ate substantially On the ther hand, lower Ông of the water table to depth of 1.8 m can

reduce maximal evaporation fo 1 mim/day An

‘additional lowering ofthe water able depth to

‘3.6mwill reduce the maximal evaporation rate

to 0.12mm/day, while any futher lowering of

the water table can cause only a negligible re-

duction of evaporation and might in any case

be prohibitively expensive t accomplish

‘Comprehensive treatments of salt movement

and accumulation as ated thigh watertable

‘conditions and to drainage requirements can

bbe found in van Schilfgaarde al (1962), medema and Rycroft (1983), Tani (197), Bresleret

(0990), and Hel (1998) Their works confirm

the importance of preventing watertable rise

in all eforts to control salination Sols with a

shallow water table also tend to depress yields

‘due fo restricted sll aeration and inhibited root

‘growth Such findings, confirmed by field ob-

servations, have led to recommendations to intall subsurface drainage systems at depths

0f 15 1025 m, wherever groundwater condi-

tions pose the hazards of waterlogging and

salination In examples cited by Hoffman

(0990) subsurface drains spaced 20 m apartand

placed L5m deepin cay sol increased the yield

Of cotton and rice by 100 percent and of wheat land clover by 0 percent

Subirrigation may be a dangerous practice

in arid regions, where the irgation water is

brackish, owing to the tendency toward salt

accumulation as a consequence of evaporation

at the sol surface A practice that may help to

mitigate that tendency is the mulching of the soll surface to reduce the direct evaporation of

soll moisture

Groundwater Drainage

‘The term “drainage” can be used in a general

sense to denote outflow of water from soi!

More specifically, it can serve to describe the

artificial removal of excess water, or the set of

management practices designed to prevent

the occurrence of excess water (Farr and

Henderson, 1986) The removal of free water

tending to accumilate over the soll surface by

appropriately shaping the land is termed sur-

{face drainage and is outside the scope of our

present discussion Finally, groundwater drain-

‘age refers to the outflow of artificial removal of

‘excess water from within the soil, generally by lowering the water table or by preventing is

Sl stration pests not necessarilyharm- fulto plants The root of various plants can, in fact, hove ina saturated medium, provided it 1s fre of toxic substances and contains sufi- clent oxygen tallow normal respiration Asis

‘wellknown plantroots must resize constantly, and most terrestrial plans are unable fo tans fer the required oxygen fx from their cano- pies to their roots The problem is that water in saturated soll seldom can provide suficent

‘oxygen for root respiration Excess water inthe fall tends to lock sll pores and thus retard eration and in effet strangulate In waterlogged sos, gas exchange with the the root stmosphere ie resrited fo the surface zone of the aol while within the profile proper, oxy-

‘gen may be almost totally absent and carbon

‘lonide may accumulate Under anaerobic con

<itions, various substances are reduced from thelr normally oxidized states Toxic concentra~ tions of ferrous sulfide, and manganous fons can develop These, in combination with prod

es ofthe anaerobic decomposition of organic

‘matter (¢ methane) can greatly inbit plant {growth At the same ime, nitration is pre- tented and various plantand roo diseases (=~ pecially fungal) are more prevalent

“The occurence of «high-water‘able condi- tion may not always be clearly evident at the very surface, which may be deceptively dry even while the sols completely waterlogged just below the surface zone Where the efec- five rooting depth a thus restricted plants may suller not only from lack of exygen inthe sl, Dutalo from lack ofnutrents drops periodically sallow-rooted plants grow ifthe water table ing in waterlogged sols may even, paradox- cally, suffer from occasional lack of wate, especially when the tanepirtiona demand is very high, ligh moisture conditions at or near the soil surface make the soll susceptible to compaction and pudaling by animal and machinery traf fic Necessary operations (eg tillage planting, spraying and harvesting) ae thwarted by poot

"ngfiebilly (Le, the ability ofthe ground to

support vehicular traffic and to provide the

necessary traction for locomotion) Tractors are

bogged down and cultivation tols are clogged

by the soft, sticky, wet soil Furthermore, the

surface zone of a wet sol does not warm up

readily at springtime, owing to greater thermal

inertia and downward conduction, and to loss

‘of latent heat by the higher evaporation rate

‘Consequently germination and early seedling

growth are retarded

Plant sensitivity to restricted drainage is it-

selfaffected by temperature Arise in tempera-

ture is associated with a decrease in the

solubility of oxygen in water and with an in-

‘crease in the respiration rate of both plant roots

‘and soil microorganisms The damage caused

by excessive soil moisture is therefore likely to

’be greater in a warm climate than ina cold one

Moreover, in a warm climate, te evaporation

rate and, hence, the consequent hazard of s-

Tình are likely to be greater than ina cool cli-

mate The process of evaporation inevitably

results inthe deposition of salts at or near the

soil surface, and these sats canbe removed and

prevented from accumulating only ifthe water

{able remains deep enough lo permit leaching

without subsequent resaination through cap-

‘lary rise ofthe groundwater

‘Numerous investigations of groundwater

flow and drainage have resulted in avery ex-

tensive body of literature on this subject Ref-

erence is made particularly to the books by

Luthin (1957), van Schilfgaarde (1974), and

SSmedema and Rycroft (198) ‘The artificial drainage of groundwateris gen-

cally caried outby means of drains, which may

be ditches, pipes, or “mole channels,” into

which groundwater flows as a result of the hy-

draulic gradients existing in the soi The drains

themselves are made to direct the water, by

gravity or by pumping, to the drainage outlet,

‘which may be a stream, lake, an evaporation

pond, othe sea In some places, drainage wa-

{er may be recycied, or reused, for agricultural,

industrial, or residential purposes Because

drainage water may contain potentially harm-

ful concentrations of sats fertilizer nutrients,

pesticide residues, and various other toxic

chemicals as well as biological pathogens, itis

‘ot enough to provide means to “get rid” oft;

‘We must be concemed with the quality of the

Weterogging and Dreinage 33

‘water tobe disposed of and withthe long-term down-stream consequences ofits disposal

In principle, water will not flow out of the soil into a large eavity or drain spontancously Luness the pressure of sil waters greater than atmospheric Drains mustbe located below the water table to draw water and the water table cannotbe lowered below the drains (figure 14) Hence the depth and spacing of drainsis of eru- clalimportance Insufficient depth of placement will prevent a set of drains from lowering the water table to the extent necessary Too great a depth might, on the other hand, lower the wa- ter table excessively and thus deprive the plants

‘ofa possibly important source of water during Grought periods

‘Various equations, empirically or theoret- cally based, have been proposed for determin- {ng the desirable depths and spacings of drain pipes or ditches in different soil and ground-

‘water conditions Since field conditions are ten complex and highly variable, these

‘equations are generally based upon assump- tions that idealize and simplify the flow sys- tem The available equations are therefore approximations that should not be applied blindly Rather, the assumptions must be exam- ined in the light of all information obtainable concerning the circumstances at hand,

‘Among the most serious weaknesses ofthe approach described are the assumptions of an impervious layer at some definable shallow depth and the disregard ofthat portion of the total flow that occurs above the watertable (Bouwer, 1959), Corrections to account for that flow were described by van Schilfgaarde (974)

‘Other equations, derived by altemative and

in some cases more rigorous procedures, have

‘been offered by, among others, Kirkham (1958), and the US Bureau of Reclamation (Luthin, 1966) The renges of depth and spacing gener ally used for the placement of drains in Feld

are shown in table 3

{In Folland, the country with the most expe- lence in drainage, common criteria for drain- age are to provide for the removal of about 7 mm/day and to prevent a water-table rise above 0.5m from the sol surface In more arid regions, because of the greater evaporation rate and groundwater salinity the water table must