Modeling phosphorus in the environment - Chapter 8 pdf

Equation 8.1 shows that aswater content in the lower zone water storage increases during a storm, the infiltration rate decreases.. The fraction of this water that goes into the upper so

with Hydrologic

Simulation

Program-Fortran

David E Radcliffe

University of Georgia, Athens, GA

Zhulu Lin

University of Georgia, Athens, GA

CONTENTS

8.1 Brief History of Model Development 189

8.2 Modeling of Hydrology 190

8.3 Modeling of Phosphorus 195

8.4 Modeling of Sediment 199

8.5 Calibration 202

8.6 Case Study: Upper Etowah River Watershed 203

8.7 Comparing HSPF and SWAT 210

8.8 Conclusions 211

Acknowledgments 212

References 212

8.1 BRIEF HISTORY OF MODEL DEVELOPMENT

The Hydrologic Simulation Program-Fortran (HSPF) is a watershed-scale, semi-distributed model developed from the original Stanford model (Bicknell et al 2001)

It is one of the two dynamic models intended for modeling watersheds dominated

by nonpoint sources in the U.S Environmental Protection Agency (EPA) Better Assessment Science Integrating Point and Nonpoint Sources (BASINS) package (U.S Environmental Protection Agency 2004a) The other model is the Soil Water Assessment Tool (SWAT) described by Arnold et al (1998) and Neitsch et al (2002) The functions and processes included in HSPF were derived primarily from the following group of predecessor models:

Hydrocomp Simulation Programming (HSP) (Hydrocomp, Inc 1976, 1977) Nonpoint Source (NPS) Model (Donigian and Crawford 1976a)

Agricultural Runoff Management (ARM) Model (Donigian and Crawford,1976b; Donigian et al 1977)

Sediment and Radionuclides Transport (SERATRA) (Onishi and Wise 1979) The original development of HSPF was sponsored by the EPA EnvironmentalResearch Laboratory in Athens, Georgia, during the 1970s HSP was a descendant

of the Stanford Watershed model (Crawford and Linsley 1966) It was first released

in 1980 as Release 5 Later development was sponsored by the U.S GeologicalSurvey (USGS) Water Resources Division in Reston, Virginia

Probably the best-known application of the HSPF model is its use as part of theChesapeake Bay Model (Linker et al 2002) HSPF is one of three linked modelcomponents: a watershed model (HSPF), an airshed model, and a bay model Themodel was used to establish a goal in 1987 of a 40% nutrient — nitrogen (N) andphosphorus (P) — reduction to the bay by the year 2000 In the current version, thebay watershed is divided into 94 sub-basins with an average area of 194,000 ha

A bibliography of articles describing HSPF applications is available athspf.com/hspfbib.html (confirmed February 25, 2006) Aside from the ChesapeakeBay Model, applications modeling phosphorus include Donigian et al (1996),Donigian and Love (2002), and Hummel et al (2003)

8.2 MODELING OF HYDROLOGY

Most of the material in this chapter is taken from the HSPF User’s Manual for Release 12 (Bicknell et al 2001), which can be downloaded from the EPA Web site,

mixture of English and metric units are used in the user’s manual, and thischapter has aimed for consistency with the manual HSPF uses elements, zones,and nodes One type of element is the land segment, which can be a perviousland segment (PLS) or an impervious land segment (ILS) Within the PLSs, thereare snow, surface, upper soil, lower soil, and groundwater zones A segment is

a portion of the land assumed to have uniform (lumped) properties Another type

of element is a reach element Within a reach, water moves through a singlezone from an upstream node to a downstream node A simulation might consist

of a single watershed (completely lumped) or multiple subwatersheds connectedtogether (partially distributed)

U.S Environmental Protection Agency (2004b) describes which processes need

to be simulated for PLSs, ILSs, and reaches to simulate hydrology Basic hydrologymust include PWATER in the PLSs, IWATER in the ILSs, and HYDR in the reacheselements — there is a process for simulating what happens in the snow layer if that

five different sources in the following order until PET is satisfied (ET = PET) oruntil sources are exhausted (ET < PET): from the groundwater zone as seepage,from vegetation interception, from the upper soil zone, from the groundwater zonedirectly, and from the lower soil zone The model recognizes that within a landsegment there is variability in ET due to differences in, for example, rooting density.Rainfall is distributed in PWATER in the following manner Some of the rainfallfirst goes to interception by vegetation (i.e., grass, leaves, stems, and trunks) Thisvegetation has a storage capacity that accepts water until it is filled The interceptedwater is lost through evaporation The water that remains may infiltrate to upperzone storage or interflow storage, may enter surface detention storage, or may run off None of the conventional methods (e.g., curve number, Green-Ampt) or soilparameters (e.g., saturated hydraulic conductivity, field capacity) are used to calcu-

late infiltration and interflow IBAR is the average infiltration rate over the land

segment (in hr−1 or in day−1), depending on the time step in use (Figure 8.1) Themodel recognizes that within a land segment there is variability in infiltration rates

and that the actual infiltration rate can be less than or greater than IBAR, as this chapter shows IBAR is calculated as

(8.1)

where INFILT is a parameter (in hr−1), LZS is the lower zone water storage (in.), LZSN is the nominal (or average) lower zone water zone storage (in.), and INFEXP

is a parameter (unitless)

FIGURE 8.1 Determination of infiltration and interflow (Redrawn from B.R Bicknell, J.C.

Imhoff, J.L Kittle, Jr., T.H Jobs, and A.S Donigian, Jr., Hydrological Simulation

Program-Fortran: HSPF Version 12 User’s Manual, Mountain View, CA, Aqua Terra Consultants,

Inches of water/interval IMAX

IIMAX

IBAR IIBAR

line I (infiltration capacity)

line II (interflow + infiltration capacity)

This represents infiltration into the lower soil zone; infiltration into the uppersoil zone (a relatively thin layer) is described later In the case that frozen ground

occurs for a significant period of time, INFFAC accounts for this effect, but it has

been left out of Equation 8.1 for the sake of simplicity Equation 8.1 shows that aswater content in the lower zone water storage increases during a storm, the infiltration

rate decreases Once this zone is saturated (LZS = LZSN), the infiltration rate reaches the minimum asymptotic rate of INFILT This implies that INFILT is equal to the

field saturated hydraulic conductivity

The parameter IMAX in Figure 8.1 is the product of INFILD and IBAR INFILD

is a unitless parameter with a recommended value of 2, so IMAX is twice IBAR The

water that is available in a time step for infiltration, interflow, or runoff is moisture

supply in inches, MSUPY (Figure 8.1) The total water that infiltrates the lower soil zone is the area beneath line I and the MSUPY line (clear area) (IBAR is the average infiltration rate, and this is used in determining the total) As IBAR increases, so does

the amount of water that infiltrates The water that goes into potential interflow is

the area beneath line II and the MSUPY line and above line I (lightly shaded area

in Figure 8.1)

Interflow is water that moves laterally to a stream due to a restrictive layer inthe unsaturated zone (Fetter 1988) Potential interflow water can become actual

interflow or inflow into the upper soil zone As IBAR increases, so does the amount

of water that goes to potential interflow IIMIN and IIMAX are calculated as follows:

zone infiltration, surface detention, or runoff is the area below the MSUPY line and

above line II (darkly shaded area in Figure 8.1)

The remaining water is potential runoff The fraction of this water that goes into

the upper soil zone (FRAC) is a function of the upper soil zone water content (UZRAT), which is the upper soil zone water storage in inches (UZS) divided by the upper soil zone nominal water storage in inches (UZSN) FRAC decreases as the upper zone water content increases Note that UZRAT is allowed to be greater than unity This is a recognition that UZSN varies from the average (nominal) value over

the pervious land segment

Once infiltration into the upper soil zone is satisfied, the remaining water goesinto surface storage, runoff, and interflow Surface storage depends in part on

Manning’s n for roughness (increases with roughness and n), slope length (increases

with slope length), and slope angle (decreases with slope angle) The interflowcomponent assumes a certain storage capacity for interflow water The rate at which

IIMIN IMIN INTFW IIMAX IMAX INTFW

LZS LZSN

22

L LZS LZSN

water can enter interflow storage depends on the current storage and the rate atwhich water is discharging from interflow storage to the stream The rate of discharge

to the stream is a function of the interflow recession parameter IRC.

Upper soil zone water can percolate into the lower soil zone Percolation onlyoccurs when the upper soil zone water content is greater than the lower soil zonerelative water content according to the following empirical relationship (units arenot consistent on both sides of equation):

(8.3)

where PERC is the percolation rate (in hr-1) The fact that PERC is proportional

to INFILT reinforces the idea that INFILT is related to field-saturated hydraulic

conductivity

Water that percolates into the lower zone from the upper soil zone, plus trating water, can increase the lower zone soil water storage or pass on through togroundwater The fraction that goes to increasing the lower soil zone storage depends

infil-on the lower zinfil-one relative water cinfil-ontent, LZRAT, which is LZS divided by LZSN.

As the water content increases, less percolation water is retained, and more waterpasses through to groundwater

Infiltrating and percolating water that reaches groundwater storage can dischargeinto the stream or can go to deep groundwater storage; this water is essentially lost

from the watershed system DEEPFR is the parameter (unitless) that determines the

fraction that is lost Outflow to the stream from the remaining groundwater depends

on the slope of the water table (gradient), the groundwater storage, and two

param-eters, AGWRC and KVARY:

(8.4)

where AGWO is the outflow rate (in hr−1), GWVS is an index to the water table slope, AGWS is the current groundwater storage (in.), AGWRC is a groundwater

outflow recession parameter (day−1), and KVARY is a recession parameter — nonzero

values cause recession to vary as a function of groundwater levels and will produceseasonal variability in hydrographs

U.S Environmental Protection Agency (2004c) provides guidance on how tochoose hydrological parameters The primary hydrological parameters are as follows

• FOREST: only used in snow processes.

• LZSN: lower zone nominal soil water storage (in.); recommends an initial

estimate of 1/8 mean annual rainfall + 4 in (humid regions); min-max

values are 3 to 8 in.; lower values of LZSN will cause more stream flow

(less water lost to ET); default = 4 to 6.5 in depending on land use

• INFILT: index to mean soil infiltration rate (in hr−1); provides a rangerelated to soil hydrologic groups (A: 0.4 to 1.0; B: 0.1 to 0.4; C: 0.05 to

USZN

LSZ LSZN

AGWO=AGWRC⋅ +(1 KVARY GWVS⋅ )⋅AGWS

0.1; D: 0.01 to 0.05); default is 0.16 in hr−1; suggests that Z *INFILT* INTFNshould approximate the long-term infiltration rate, or permeability, in thesoil survey database (untested approach); higher values cause less runoffand less storm flow in streams.

• LSUR: length of overland flow path (ft) for the pervious land segment; average

length of travel for water to reach a stream; typical values range from 200 ftfor slopes of 15% to 500 ft for slopes of 1%; default = 300 ft; higher valuesshould cause storm hydrograph to spread out (lower peak value)

• SLSUR: slope of pervious land segment; recommends using digital elevation

data to get this — check change in elevation of pixels in a transect dicular to stream, divide by distance between centers of pixels — makemultiple measurements and average; probably has little effect on hydrologybut may affect erosion; default = 0.036 to 0.55, depending on land use

perpen-• KVARY: nonzero values cause seasonal variation in groundwater flow;

increasing the value should cause faster recession during wet months;default is 0; recommends starting with 0 and adjusting if necessary

• AGWRC: groundwater recession rate; default = 0.98; recommends findingthis through calibration; higher value causes slower recession; suggestsusing higher values for forests

• PETMAX: used only in snow processes.

• PETMIN: used only in snow processes.

• INFEXP: exponent in the infiltration equation; default = 2.0 and mends using the default value

recom-• INFILD: ratio of maximum infiltration rate in a pervious land segment, IMAX, to average infiltration rate, IBAR; default = 2.0 and recommendsusing the default value

• DEEPFR: fraction of infiltrating water that goes into deep groundwater

storage and is lost from the watershed; default = 0.10; recommendsfinding value through calibration; higher value causes less stream flowoverall

• BASETP: the fraction of a pervious land segment area that has vegetation

able to transpire water directly from groundwater (i.e., riparian or marshland vegetation); default = 0.02; recommends calculating this based onarea that is riparian or marsh land vegetation

• CEPSC: rainfall (in.) intercepted by vegetation; default = 0.10; mends different values depending on land cover

recom-• UZSN: upper zone nominal soil water storage (in.); recommends different

values depending on slope, vegetation, and depression storage; overall

rule of thumb is 0.10 LZSN; default = 1.128 in

• NSUR: n in Manning’s equation for overland flow; larger values of n

indicate a rougher surface and slower flow; default is 0.20; probably haslittle effect on water flow but may affect erosion

• INTFW: interflow parameter; increasing interflow value delays water

get-ting to the stream (otherwise it would become overland flow), so it lowersthe hydrograph peak and spreads the curve out; default is 0.75; recom-mends using calibration to find value

• IRC: interflow recession rate, analogous to groundwater recession rate;

increasing value causes the hydrograph to spread out and decreases peakvalue; default = 0.50

• LZETP: index to lower zone ET related to root distribution; varies between

0 and 1 with 1 representing maximum potential for plant uptake; givestypical ranges for different types of vegetation; default is monthly varyingvalues from 0.2 in winter months to 0.4 in summer months

There are a few parameters associated with the ILSs Unless the impervious landarea is a large portion of the modeled watershed, these parameters will not havemuch effect on model predictions The most important factor is what percentage ofurban areas is assumed to be impervious; the default is 50% A few parameters arealso associated with the reaches, which will have little effect on stream flow, althoughthey may be important for sediment and P transport

Another source of information on hydrological as well as water-quality eters is HSPF Parameter (HSPFParm) (Donigian et al 1999) This is a database ofparameter values that have been used by experienced users in 45 HSPF model runs

param-in 14 states (available at http://hspf.com/hspfprms.html)

8.3 MODELING OF PHOSPHORUS

HSPF has a specific routine for modeling P The module matrix in U.S mental Protection Agency (2004b) shows which modules need to be activated in thepervious land, impervious land, and reach segments to model P:

Environ-• PERLND: activate PWATER, SEDMNT, MSTLAY, and PHOS

and desorbed using either first-order kinetics (i.e., subroutine FIRORD) or taneous adsorption using a Freundlich isotherm (i.e., subroutine SV) This chaptercovers only the instantaneous approach A Freundlich isotherm from the user’smanual is shown in Figure 8.3 On the y axis, X is the P adsorbed in parts per million

instan-of soil (mg instan-of P per kg instan-of soil), and on the x axis, C is the P in solution in parts

per million of solution (mg of P per L of solution) The y axis intercept of curve 1

and curve 2 is XFIX, the amount of P permanently adsorbed (mg of P per kg of soil) CMAX is the maximum equilibrium concentration of P in soil solution, and XMAX is the corresponding maximum adsorbed concentration of P Adsorbed P is

described by the following equation:

(8.5)

X=K1⋅C N +XFIX

1 1

FIGURE 8.2 Flow diagram for P reactions (Redrawn from B.R Bicknell, J.C Imhoff, J.L.

Kittle, Jr., T.H Jobs, and A.S Donigian, Jr., Hydrological Simulation Program-Fortran:

HSPF Version 12 User’s Manual, Mountain View, CA, Aqua Terra Consultants, 2001 With

permission.)

FIGURE 8.3 Freundlich adsorption isotherm (Redrawn from B.R Bicknell, J.C Imhoff, J.L.

Kittle, Jr., T.H Jobs, and A.S Donigian, Jr., Hydrological Simulation Program-Fortran:

HSPF Version 12 User’s Manual, Mountain View, CA, Aqua Terra Consultants, 2001 With

permission.)

Atmospheric

deposition

Atmospheric deposition

ORGP

Organic

phosphorus

Organic phosphorus mineral- ization

tion of phosphate

Adsorp-P4AD Phosphorus adsorbed

tion of phosphate

Desorp-P4SU Phosphate in solution

PLTP Plant phosphorus

Plant uptake of phosphorus

Phosphate immobili- zation

C, ppm

CMAX

Curve 1 Curve 2

XMAX XJCT

X, ppm

XDIF

XFIX

where N1 is the Fruendlich exponent (N1 = 1 is a linear isotherm) and K1 is the Fruendlich distribution coefficient (units of L per kg when N1 = 1) These parameters

— XFIX, N1, K1, and CMAX — must be supplied for surface, upper, lower, and

(8.6)

where TMP is the temperature (°C) in the zone, and TH is the correction coefficient

(typically 1.06) There is a similar equation for temperature correction of the bilization rate Soil temperature is modeled by HSPF

immo-Plant uptake is based on a first-order rate or a yield approach In the first-orderrate approach, for each zone the plant uptake rate parameter (in units of inverse

time) is _KPLP where the underlined space is S, U, L, or K, representing the surface,

upper, lower, and active groundwater zones After correction for temperature, the

uptake rate takes the form _KPLPK (in units of inverse time) Plant uptake occurs

from the soluble P pool (Figure 8.2) The amount of plant uptake each day iscalculated as the rate times the mass of P in the soluble pool in each zone Thetemperature correction equation takes the same form as Equation 8.6

The yield approach to plant uptake of P is designed to be less sensitive to soilnutrient levels and nutrient application rates than the first-order rate option It allowscrop needs to be satisfied, subject to nutrient and moisture availability, without beingaffected by soil nutrient level In this method, a total annual target is specified bythe user and is then divided into monthly targets during the crop growing season.The target is further divided into the four soil layers

Soluble P can percolate down through the soil zones, which requires use of theMSTLAY module In the PWATER module, which is used for general hydrology,some moisture that infiltrates can reach the groundwater in a single time step —that is, a day or an hour This has little effect on hydrology, but it is not realistic for

P in many cases The MSTLAY module takes the fluxes and storages computed inPWATER and adapts them for runoff, interflow, and percolation through the soillayers The revised storages, in inches of water, are also expressed in units of massper area units for use in the adsorption and desorption calculations Percolationoccurs from the surface layer through each of the underlying layers Percolation of

P from the surface layer to the upper soil zone is described by the following equation:

(8.7)

where SDOWN is the amount of water percolating down (in.), SMST is the amount

of water stored in the surface layer (in.), SLMPF is an arbitrary reduction factor

(<1), and FSP is the fraction of the soluble P in the surface zone that percolates

Percolation of P from the upper zone to the lower soil zone is described by thefollowing equation:

(8.8)

where ULPF is the factor for retarding percolation (since this variable is in the

denominator, it must be > 1 to cause retardation), UDOWN is the amount of water percolating down (in.), UMST is the moisture storage (in.), and FUP is the fraction

of the soluble P in the upper zone that percolates (between 0 and 1) There is asimilar equation for percolation from the lower zone to ground water storages.The surface layer can lose P in surface runoff Soluble P enters runoff directlyand is adsorbed, and organic P can be removed with sediment The concentration

of soluble P in runoff is assumed to be the same as the concentration in the surfacelayer Particulate P is removed from the surface layer in proportion to the fraction

of the surface soil layer removed by erosion, although the mass of soil in the surfacelayer is a parameter value that does not vary even when material is removed Assuch, an enrichment ratio accounting for the fact that most of the P lost in erosion

is adsorbed to the clay-size fraction (Sharpley 1985) is not employed

Phosphorus can be added to the system as organic or adsorbed P throughatmospheric deposition or through the special actions block where fertilizer andmanure applications are described The special actions block is a table of annual ormonthly inputs

Many processes can be modeled for P in reaches Most of these occur in theNUTRX module They include longitudinal advection of dissolved P, benthal release

of dissolved P, adsorption and desorption of P to suspended sediment in the water

column using a linear adsorption coefficient ADPM(J), which varies for different sediment size fractions J, and desorption and scour and longitudinal advection of

adsorbed P In the PLANK module, sources and sinks of P include uptake byphytoplankton or benthic algae and respiration and inorganic excretion by zooplank-ton Atmospheric deposition is also considered

No guidance document exists — such as the one for selecting values for logical parameters — for selecting values for P parameters The primary parametersfor modeling P are as follows:

hydro-• SKPLP, UKPLP: P plant uptake parameter for surface zone, upper zone

• THPLP, THDSP: temperature correction factor for plant uptake, desorption

• KIMP: first-order immobilization rate constant (day−1)

• KMP: first-order mineralization rate constant (day−1)

• CMAX: maximum equilibrium concentration of P in soil solution (mg L−1)

• XFIX: concentration of P permanently adsorbed to soil (mg L−1)

• K1: Freundlich distribution coefficient

• N1: Freundlich exponent

• ORGP: initial P storage in each layer for organic P

• P4AD: initial P storage in each layer for adsorbed P

• P4SU: initial P storage in each layer for solution P

UZSN ULPF

UDOWN UMST

=

• PLTP: initial P storage in each layer for plant P

• IP4SU: initial P storage in upper layer for interflow pool

• SLMPF: factor for reducing P percolation from the surface to the upper

soil zone; values <1 cause a reduction; default = 0.5

• ULPF: factor for reducing P percolation from the upper to the lower soil

zone; values >1 cause a reduction; default = 2.0

• LLPF: factor for reducing P percolation from the lower soil zone to

groundwater; values >1 cause a reduction; default = 2.0

• BRCON(I): benthal release rate of P (mg m−2 per time interval) for aerobicand anaerobic conditions

• ADPM(J): P adsorption Kd for suspended sand, silt, and clay fractions

• BNUT: constant concentration of P on bed sediments (mg P mg−1sediment)

• CMMP: orthophosphorus Michaelis-Menten constant for P-limited algal

growth (mg P L−1); default is 0.015 mg L−1

8.4 MODELING OF SEDIMENT

In most cases, to model P transport the movement of sediment will also have to bemodeled Erosion processes that take place on each PLS are described in the SEDMNTmodule Many of the routines are taken from the Agricultural Research Model

(ARM) (Donigian et al 1977) SLSED represents external lateral input from an upslope land segment that can be input as a time series by the user NSVI is any net external additions or removals of sediment caused by human activities or wind DET

represents soil that is detached by rainfall and enters detached sediment storage

AFFIX represents the opposite process: detached sediment that reattaches to soil,

which occurs on days when it does not rain Once soil is in the detached sediment

storage, it can be washed off in WSSD if the transport capacity is sufficient Also,

soil can be lost through erosion by scouring (i.e., gully erosion) without being

detached by rainfall splash, which is represented by SCRSD The total soil lost by washoff and scouring is SOSED.

The equation for detachment is

(8.9)

where DET is the detached sediment (tons per acre per time interval), DELT60 is the number of hours per interval (dividing by 60 min), CR is the fraction of land covered by snow (SNOCOV) and vegetation (COVER), SMPF is the supporting management factor, KRER is the detachment coefficient dependent on soil properties, RAIN is the rainfall (in per time interval), and JRER is the detachment exponent

dependent on soil properties To simulate reattachment on days when there is norainfall, the detached soil sediment storage is decreased by multiplying it by the

factor (1.0 – AFFIX) where AFFIX is a parameter.

The equation for transport capacity is

(8.10)

where STCAP is the transport capacity (tons acre−1 time−1), KSER is the coefficient for transport of detached sediment, SURS is the surface detention storage of water from water modeling routines (in.), and SURO is the surface runoff (in per time

interval) If the transport capacity is less than the detachment rate, then erosion islimited by the transport capacity and vice versa

The equation for scouring is

(8.12)

where SLDS is the solids in storage at the end of the day (tons acre−1), SLDSS is

the solids in storage at the beginning of the day (tons acre−1), ACCSDP is the

accumulation rate of solids storage (tons acre−1 time−1), and REMSDP is the fraction

of solids removed each day, by wind or street sweeping If no runoff occurs, solids

will build up and approach an asymptote, which is ACCSDP divided by REMSDP.

Wash off of solids is described by the following equation:

capacity in the reach If the transportation capacity is greater than the sand storage,then sand is scoured from the stream bed If the transportation capacity is less thanthe storage, then deposition occurs The transport of cohesive sediment (i.e., silt andclay) is modeled using deposition and scouring based on shear stress exerted on thebed surface The Toffaleti method requires modeling temperature and specifying the

median bed sediment size, D50 The Colby method also uses D50

The power function method is the simplest to describe, so some of the detailsare shown here It uses the following equation for sand transportation capacity:

(8.14)

where PSAND is the potential suspended sand load (mg L−1), KSAND is the load suspension coefficient, AVVELE is the average velocity of stream reach (ft sec−1),

sand-and EXPSND is the exponent Ssand-and is scoured or deposited depending on whether

the capacity to transport is less than or greater than the current sand suspended load.Silt and clay scouring are treated differently because they resist scouring due tocohesion The rate of scour of silt and clay sediment is described by the followingequation:

(8.15)

where S is the rate at which sediment is scoured from the bed (mass area−1 time−1),

M is the erodibility coefficient (kg−2 time−1), TAU is shear stress, (lb ft−2 or kg m−2),

and TAUCS is critical shear stress for detachment of bed sediment (lb ft−2 or kg m−2)

There is no scouring as long as TAU < TAUCS The rate of deposition of suspended

silt and clay is described by the following equation:

(8.16)

where D is the rate at which sediment settles out of suspension (mass area-1 time−1), W

is settling velocity in still water (in sec−1), CONC is the concentration of suspended

silt or clay (mass volume−1), and TAUCD is critical shear stress below which sediment

deposits (lb ft−2 or kg m−2)

There is a new technical note selecting values for sediment parameters (U.S.Environmental Protection Agency 2006) Donigian and Love (2003) can also beused as a guidance document The primary parameters for modeling sediment are

as follows:

• SMPF: supporting management practice factor; a table shows values for

alternate practices and slopes in U.S Environmental Protection Agency(2006)

• KRER: splash detachment soil coefficient; default = 0.14; ARM manual(Donnigian et al 1977) says that this is equal to the product of the USLE

factors K*P where K is soil erodibility and P is the practice factor (usually 1)

PSAND=KSAND AVVELE⋅ EXPSND