FAO and UNEP (1984) Provisional methodology for assessment and mapping of desertification.
Rome:FAO
Hammond A (1995) Environmental indicators. Washington: World Resources Institute.
Hunsaker CT, Carpenter DE (1990) Ecological indicators for the environmental monitoring and assessment program. US EPA Office Research and Development, Research Triangle Park, NC.
Li DL, Lu LZ (2002) Climate characters and evolution of agricultural and pasturing interlaced zones in China. J Des Res 22:483-488
Long LQ, Li XR (2003) Effects of soil macrobiotic crust on seedling survival and seedling growth of two annual pants, J Des Res 23:657-660 (in Chinese with English abstract)
Lu JT, Zheng XJ, Li XL (2006) Climatic changes of the desertified regions in China over the past five decades, In: Dynamics of desertification and sand encroachment in China. Beijing:
China Agricultural Press. pp 20-49.
Lu Q, Yang YL, [ia JD (2000) China's desertification. Beijing: Kaiming Press. pp 21-28 Marbutt JA (1986) Desertification indicators. Clim Chang 9:113-122.
Oldeman LR (1988) Guidelines for general assessment to the status of human induced soil degradation. Wageningen: ISRIC.
Reining PH (1978) Handbook on desertification indicators. Washington: AAAS Publication Number 78-7
Rubio JL, Bochet E (1998) Desertification indicators as diagnosis criteria for desertification risk assessment in Europe. J Arid Env 39:113-120.
Shang KZ, Dong GR, Wang SG (2001) Response of Climatic Change in North China Deserted Region to the Warming of the Earth. J Des Res 21:387-392
State Forestry Administration (2005) A bulletin of Status Quo of Desertification and Sand Encroachment in China.
Wang CH (2003) Climate change and desertification. Beijing: China Meteorological Press. pp 206 (in Chinese)
Wei YR, Hao L, HeJJ(2005) Development and application of monitoring and forecasting system of animal husbandry on grassland in North China. Pratac Sci 22:59-65
Wu B, Su ZZ, Yang XH (2005) A frame work of indicator system for desertification monitoring and evaluation. For Res 18:490-496 (in Chinese with English abstract)
XU ZQ, Li WH, Min QW (2005) Experimental research on the anti-wind erosion of typical grasslands. Env Sci 26:164-168
Xue X, Wang T, Wu W, Sun QW, Zhao CY (2005) Desertification development and its cause of agro-pastoral mixed regions in North China, J Des Res 25:320-328
Yang YW, Wang Y, HeJJ(2001) Research of Establishing the Carrying Capacity Model on the Basis of Remote Sensing (RS) Information in Cool season Grassland. Chin J Agromet 22:39- 42
Zhou XD, Zhu QJ, Sun ZP (2002) Preliminary study on regionalization desertification climate in Region. J Nat Disast 11:125-131 (in Chinese with English abstract)
Zhu ZD, Liu S (1984) The Concept of Desertification and the Differentiation ofIts Development.
J Des Res 4:2-8 (in Chinese with English abstract)
Zhu ZD (1998) Concept cause and control of desertification in China. Quart Sci 5:146-155
CHAPTER 19
Coping strategies with agrometeorological risks
and uncertainties for water erosion, runoff and soil loss
P.c. Doraiswamy, E.R. Hunt, Jr., V.R.K. Murthy
19.1
Introduction
The pressure of increasing world population demands for higher crops yields from the finite area of productive agricultural lands. Meeting the needs especially in de- veloping countries through more intensive use of existing agricultural lands and expansion into more marginal lands will substantially increase erosion. There is an urgent need to take preventive and control measures to mitigate the threat to global food security. These concerns are supported by a report by El Swaify (1994) that the annual rates of soil erosion can often range between 20 to over 100 t ha', which results in about 15-30 per cent annual decline in the soil productivity. An estimated loss of about 6 million ha annually is estimated as a result of degradation by erosion and other causes (Pimental et al. 1993). The data for these estimates are often selective from small scale studies conducted over short time periods, howev- er, this does draws attention to the increasing problem of soil loss. In the western world the loss in productivity from erosion may be masked or compensated by in- creased costly and efficient management practices such as improved crop variet- ies, fertilizer, pesticides, and irrigation. Even under these management practices, soil erosion has continued and sediment loss has become a very costly factor in the overall picture.
The processes of water erosion are closely linked to the pathways taken by water in its movement through vegetation cover and over the ground surface. During a rainstorm, part of the water falls directly on the soil, because there is no vegetation or because it passes through gaps in the plant canopy. The rain falling directly on the soil surface can potentially produce rain splash erosion. Rain intercepted by the vegetation may either evaporate or drip down the plant to the soil surface. The rain and intercepted water reaching the soil may infiltrate contributing to the soil moisture storage. However, when the soil is either saturated with water or surface conditions prevent infiltration, the excess contributes to runoff on the surface, re- sulting in erosion by surface flow causing rills and gullies. The infiltration rates are controlled by the soil characteristics such as the water holding capacity and hy- draulic conductivity, surface dryness, the rate of rainfall, land slope and soil man- agement practices. Higher rates of runoff from eroded surfaces wastes valuable moisture-the principal factor limiting productivity in arid lands.
The agrometeorological coping strategies would first require the study and eval- uation of the causes of water erosion, runoff and soil loss for the area of interest.
The erosion process begins at the local level, but requires evaluation at the land-
scape and regional levels to adapt effective measures to minimize and control the erosion and soil loss. Soil runoff models are used to study the temporal and spatial extent of erosion and soil loss. The EPIC model (Williams 1995) is one example used extensively in the U.S. and internationally to study the process and extent of erosion at local and watershed levels. Data from satellite remote sensing can help in defining the channel flow of surface water based on the digital elevations imag- ery maps and also other surface characteristics such as landuse and surface rough- ness.
Soil erosion is a three stage process: (1) detachment, (2) transport, and (3) de- position of soil. Different energy source agents determine different types of ero- sion. There are four principal sources of energy: physical, such as wind and water, gravity, chemical reactions and anthropogenic, such as tillage. Soil erosion begins with detachment, which is caused by break down of aggregates by raindrop im- pact, sheering or drag force of water and wind. Detached particles are transported by flowing water (over-land flow and inter-flow) and wind, and deposited when the velocity of water or wind decreases by the effect of slope or ground cover. Three processes - dispersion, compaction and crusting - accelerate the natural rate of soil erosion. These processes decrease structural stability, reduce soil strength, ex- acerbate erodibility and accentuate susceptibility to transport by overland flow, interflow, wind or gravity. These processes are accentuated by soil disturbance (by tillage, vehicular traffic), lack of ground cover (bare fallow, residue removal or burning) and harsh climate (high rainfall intensity and wind velocity).
The effects of erosion and soil loss on soil properties vary by soil series, manage- ment, landscape position and climate. In general, soil erosion affects the chemical properties by loss of organic matter, plant nutrients, and exposure of subsoil mate- rials with low fertility or high acidity (Olson et al. 1999). The changes in physical properties of soil, include structure, texture, bulk density, infiltration rate, rooting depth, and available water-holding capacity (Frye et aI., 1982). The mineralogical properties of soils are also affected by the thinning of the plow layer (Ap horizon) and subsequent mixing of the subsoil (B horizon) into the Ap horizon by tillage.
Eroded soils are subject to higher temperatures, have lower porosity and microbial activity.
19.2
Agrometeorological coping strategies
The soil erosion process is modified by biophysical environment comprising soil, climate, terrain and ground cover and interactions between them (Figure 19.1).
Soil erodibility - susceptibility of soil to agent of erosion - is determined by in- herent soil properties e.g., texture, structure, soil organic matter content, clay minerals, exchangeable cations and water retention and transmission proper- ties (La12001). Climatic erosivity includes drop size distribution and intensity of rain, amount and frequency of rainfall, run-off amount and velocity, and wind velocity. Important terrain characteristics for studying soil erosion are slope gra- dient, length, aspect and shape. Ground cover strongly reduces the impact of the eroding energy before it has a chance to reach the soil; hence, most strategies to
Chapter 19: Coping strategies withagrometeorological risks anduncertainties 345 limit erosion begin with having some sort of vegetation, live or dead, covering the ground.
The risk of soil erosion begins when natural vegetation, grasslands and forested areas, are either cleared for cultivation or used for grazing. The problem is accel- erated by attempting to farm slopes that are too steep, cultivating up-and-down hills, continuous use of land for the same crop without rotation or fallow, inade- quate use of fertilizers and organic manures, compaction of soil through of heavy machinery and pulverizing of the soil when creating seed-beds. Soil conservation strategies are also aimed at protecting the soil from direct exposure to natural el- ements such as establishing and maintaining good ground cover. Of particular importance in areas of the world where the first rains of a wet season area highly erosive, is the selection of crops that can establish ground cover rapidly. Where climatic conditions permit, early-season and between-season vegetation cover can be provided by off season crops that can be disked in or destroyed by pre-planting applications of herbicide.
Soil conservation relies upon good management of soil combined with agro- nomic practices and the use of mechanical measures playing a supportive role (Fig- ure 19.1). The basics of soil conservation practices to minimize soil erosion and enhance its prevention are shown in Figure 19.1. The initial task in adaptation of conservation practices to minimize the risk of water erosion, run off and soil loss would be to study the specific conditions in the study area or region that is con-
Coping Strategies
Soil conservation strategies for cultivated land (EI-Swaifly etal,1982) Fig.19.1. Soil conservation strategies for cultivated land (El-Swaify et al. 1982)
tributing to the degradation. Some very obvious examples is the use of crop resi- due to increase soil organic matter which may be impractical in some countries as the residue are used for animal feed. Introduction of no-till for crop cultivation is another example that maybe impractical in countries where there is no mecha- nization and most cultivation is done by human labor. Particularly in developing countries, limitation in resources and traditional methods of farming can hinder the adoption of a particular management practice.
19.3
Soil Management Strategies 19.3.1
Organic Matter
The ultimate goals of sound soil management are to maintain the fertility and structure of the soil. Highly fertile soils produce higher crop yields while main- taining good plant cover and minimizing the erosive effects of rainfall, runoff and wind. One of the ways to achieve and maintain soil fertility is to apply organic matter which improves the cohesiveness of the soil of the soil, increases its water retention capacity and promotes a stable aggregate structure. Organic matter may be added as green manures or residue. Under no-till management, the crop resi- due and the roots are left as added organic matter. Green manures, which area nor- mally leguminous crops ploughed in, have a high rate of fermentation and yield a rapid increase in soil stability (Kolenbrander 1974). However, the long-term use of the land for cropping reduces the organic carbon content. Soil erosion accounts for only part of the reduction. Biological mineralization of the carbon from the soil in situis the most important process, particularly in semi-arid areas where bare soil summer fallows are used to conserve moisture (Rasmussen et al. 1998).
19.3.2
Tillage Practices
Tillage is an essential management technique that provides a suitable seed bed for plant growth and in some areas of the world, the only non-chemical method to control weeds. The tillage tools pulled upwards by a tractor are designed to apply an upward force to cut and loosen the compacted soil, sometimes to invert it and mix it. The other main negative effect of driving a tractor across a field is compac- tion of the soil. Compaction may result in an increasing shear strength through an increase in bulk density, followed by low infiltration and increased runoff and ero- sion. The compaction generally extends to the depth of the previous tillage, up to 300 mm for deep ploughing, 180 mm for normal ploughing and 60 mm with zero tillage (Pidgeon and Soane 1978). Spoor et al. (2003) completed a study summariz- ing the risk of soils to compaction in relation to texture and wetness.
Conventional Tillageis the standard system of tillage practice involving plough- ing, secondary cultivation, with disc harrowing, suitable for planting for a wide
Chapter 19: Coping strategies withagrometeorological risks anduncertainties 347 range of soils. The mouldboard plough inverts the soil in the plough furrow and moves all the soil in the plough layer to a depth of 100-200 mm. Secondary disc cultivation helps form seed beds and remove weeds by breaking up the cloddy sur- face produced by ploughing. The increased surface roughness due to tillage can be a successful deterrent to water erosion (Cogo et al. 1984).
Contour Tillagefor planting and cultivation can reduce soil loss from sloping land compared with standard cultivation up-and-down the slope. Itis inadequate as the only conservation practice to reduce soil loss for lengths greater than 180 m at 1° steepness (Troeh et al. 1980). This technique may be effective against soil loss and water erosion only for low rainfall intensity. Protection against more extreme storms is enhanced by supplementing contour farming with strip cropping, dis- cussed later in this paper. Contour ridging and connecting the ridges with cross- ties over the intervening furrows, thereby forming a series of rectangular depres- sions that fill with water during rain can be very effective in controlling soil ero- sion along slopes. This practice known as tied ridging, should be used only in well drained soils to minimize water logging and damage to crops. Tied ridging on sandy soils in Zimbabwe with no till over a period of three years gave soil losses of less that 0.5 t ha" compared with up to 9.5 t ha' for conventional ploughing with mouldboard under maize cultivation (Vogel 1994).
Conservation Tillagecan be defined as any practice that leaves at least 30% cov- er on the soil surface after planting. Numerous studies have examined the effects of different types of conservation tillage as outlined in table 1 (Natural Resources Conservation Service, USDA 1999). The success of various systems is highly soil specific, landscape, climatic pattern and also dependent how well weeds, pests and disease are controlled. The main barriers to adoption are the expense of special- ized equipment for managing cultivation in crop residues, problems of weed con- trol and increases in pest, particularly rodents.
Practice Conventional
No Tillage
Strip Tillage
Mulch Tillage
Description
Standard practice of ploughing with disc or mouldboard plough, one or more disc harrowing, a spike-tooth harrowing and surface planting.
Soil undisturbed prior to planting, which takes place in a nar row, 25-27 mm-wide seed bed. Crop residue covers of 50-100% retained on surface. Weed control by herbicides.
Soil undisturbed prior to planting, which is done in narrow strips using rotary tiller or in-row chisel, plough-plant, wheel track planting.
Intervening areas of soil untilled. Weed control by herbicides and cultivation.
Soil surface disturbed by tillage prior to planting using chisels, field cultivators, discs, or sweeps. At least 30% residue cover left on surface.
Weed control by herbicides and cultivation.
In general the better drained, course- and medium textured soils with low organ- ic content respond best and the systems that are not successful occur on poorly drained soils with high organic content or heavy soils, where the use of the mould-
board plough is essential. The effectiveness of the tillage practice is very depen- dent on the types of crops planted and the use of crop rotations.
No tillagesystem usually restricts tillage only to that necessary for plantings and carried out by drilling directly into the stubble of the previous crop. Weed con- trol by herbicide application is an essential part of this system. This conservation technique has been found to increase the water-stable aggregates in the soil com- pared to disc cultivation and ploughing (Suwardji and Eberbach 1998; Mrabet et al. 2001). Itis not suitable in soils that easily compact and seal because it can lead to lower crop yields and greater runoff.
In the Corn Belt of the USA, where maize, soybean and wheat are cultivated in rotation, no tillage is one ofleading technologies to control erosion in areas. Mold- enhauer (1985) reviewed the various tillage systems in this area and showed that annual soil loss under no till on a range of erodible soils was 5-15% of that from conventional tillage. In the southern regions of the USA, no till is recommended on ultisol soils which have become severely eroded after 150 years of continuous cropping. In this area, Langdale et al. (1992) studied three no tillage systems un- der soybean production as an alternative to the conventional system of growing soybean with a bare fallow over winter: Soybean with winter cover using in-row chisel; soybean with barley as winder cover and using fluted coulters; and soybean with rye as a green manure and using fluted coulters. The respective mean annual soil losses for the four systems (starting with conventional tillage) were 26.2, 0.1, 0.1 and 3.4 t ha" , respectively. One other environmental benefit to no tillage oper- ations is that there is no exposure of the sub-layer soils to oxidation and release of carbon dioxide to the atmosphere.
In strip tillage, the soil is prepared for planning along narrow strips, with the in- tervening areas left undisturbed. In a single plough-planting operation, typically up to one-third of the soil is tilled. The plough-plant systems caused the least soil compaction, conserved most soil moisture and reduced losses of organic matter, nitrogen, phosphorus and potassium (Quansah and Bonnie 1981). In studies con- ducted in Ghana, the technique reduced soil loss in maize plots from multiple rain storms totaling 452 mm to 0.2 t ha' compared with 1.4 t ha' for traditional tillage using hoe and cutlass (Baffoe-Bonnie and Quansah 1985).
Mulch Tillagesystem in general has been successful to reduce water and wind erosion and to promote the conservation of soil moisture in drier wheat-growing areas (Fenster and McCalla 1970). The soil is prepared in such a way that at least 30% of the surface is covered with plant residues, or other mulching materials, are specifically left on or near the surface. Mulch tillage is a broad term and includes practices such as no-till, disk plant systems, chisel plant systems, and strip tillage systems. Sometimes a cover crop, usually a legume, is specifically grown within the cropping cycle to produce mulch material. Another variant of planted fallowing, practiced in North America, is referred to as summer fallow or ecofallow. The latter is a system of fallowing in which weed growth is restricted by shallow cultivation or by using herbicides to conserve soil moisture. Crops are grown every other year or once in 3 years. This type of "cropless" fallow is mostly used in arid climates to conserve soil moisture, without having to resort to irrigation.
Minimum Tillageor reduced tillage is a practice using chiseling or disking to prepare the soil while retaining a 15-25 % residue cover. Minimum tillage is not