Biological and Environmental Engineering Soil & Water Research Group Re-conceptualizing SWAT for Variable Source Area Hydrology Zachary Easton 1* , Daniel Fuka 1 , Todd Walter 1 , Dillon Cowan 2 , Elliot Schneiderman 3 , Tammo Steenhuis 1 1 Dept. Biological and Environmental Engineering, Cornell University 2 School of Civil and Environmental * [email protected]
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Biological and Environmental Engineering Soil & Water Research Group Re-conceptualizing SWAT for Variable Source Area Hydrology Zachary Easton 1*, Daniel.
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Biological and Environmental Engineering
Soil & Water Research Group
Re-conceptualizing SWAT for Variable Source Area Hydrology
Walter et al. 2003. ASCE J. Hydrol. Eng. 8: 214-218
Biological and Environmental Engineering
Soil & Water Research Group
Rain
Subsurfacewaterrises
Some areas saturate to the surface
Saturation Excess Runoff
Biological and Environmental Engineering
Soil & Water Research Group
Rain
Rain on saturated areas becomes overland flow
Upland interflow may exfiltrate
Dunne and Black. 1970. Water Resour. Res. 6: 478-490Dunne and Black. 1970. Water Resour. Res. 6: 1296-1311
Saturation Excess Runoff
Biological and Environmental Engineering
Soil & Water Research Group
Variable Source Areas
Current Water Quality Models were not Intended to Capture this Complexity•General Watershed Loading Function (GWLF)•Soil Water Assessment Tool (SWAT)•Agricultural Nonpoint Source Pollution Model (AGNPS)
Flow Path
Biological and Environmental Engineering
Soil & Water Research Group
Re-conceptualizing SWAT
• Variable Source Area (VSA) Hydrology• Curve Number and VSA hydrology• Convincing SWAT it recognizes VSAs
Biological and Environmental Engineering
Soil & Water Research Group
USDA-NRCS Curve Number Model
“Runoff”=Pe2/(Pe+S)
S=25400/CN-254
Tables link CN to land use andsoil infiltration capacity
Biological and Environmental Engineering
Soil & Water Research Group
Curve Numbering VSA hydrologyQ = Pe
2/(Pe+S)Watershed
UnsaturatedSaturated
AfQ
P
Steenhuis et al. 1995. ASCE Div Drain. & Irr. 121:234-238
Q = AfPe
dQ/dPe = Af
Af = f(S,Pe)
Biological and Environmental Engineering
Soil & Water Research Group
We know how much area is contributing…
…but from where in the landscape?
Af = f(S,Pe)
Biological and Environmental Engineering
Soil & Water Research Group
High : 33.103500
Low : 3.520011
Soil Topographic Index
33.10
3.52
Soil Topographic IndexWetness Index Classes
10=f(S)
=f(S)
=f(S)=f(S)
=f(S)
=f(S)
=f(S) =f(S)
f(S)
=f(S)
Easton et al., 2008. J. Hydrol. 348: 279-291.Lyon et al. 2004. Hydrol. Proc. 18(15): 2757-2771.Schneiderman et al. 2007. Hydrol. Proc. 21: 3420-3430.
Wetness IndexClasses
ilocal storage
Biological and Environmental Engineering
Soil & Water Research Group
Re-conceptualizing SWAT
• Variable Source Area (VSA) Hydrology• Curve Number and VSA hydrology• Convincing SWAT it recognizes VSAs
Biological and Environmental Engineering
Soil & Water Research Group
Soils Landuse
HRUs
Revisit the HRU concept• Define HRUs as the
coincidence of soil type and landuse• Hydrological/chemical
properties are defined at the HRU
• So runoff/P loss is the same here (lowland pasture)
• As here (upland pasture)• Is this a good assumption?
Biological and Environmental Engineering
Soil & Water Research Group
LanduseSTI
HRUs
SSURGO
• VSA concept defines HRUs as the coincidence of soil topographic index and landuse
• So runoff/P loss is now not the same here (lowland pasture)
• As here (upland pasture)• Better Assumption?
Biological and Environmental Engineering
Soil & Water Research Group
Distributing CN-values
0102030405060708090
100
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
CN
or
Sto
rage
Fractional Contributing Area
Local Storage
CN
Wetness IndexClasses
• Average of Standard CNs = 73.1 distributed according to landuse/soils
• Average of VSA CNs = 73.1, distributed according to a wetness index
Biological and Environmental Engineering
Soil & Water Research Group
050
100150200250300350400
Jan-99 Mar-99 May-99 Jul-99 Sep-99 Nov-99
So
il W
ater
(mm
)• High runoff prone area = high moisture content (in general)• We relate local soil water storage, e,i, to AWC with the following: