Re-conceptualizing SWAT for Variable Source Area Hydrology Zachary Easton *, Daniel Fuka, Eric White, Todd Walter, Tammo Steenhuis Dept. Biological and Environmental Engineering, Cornell University, Elliot Schneiderman New York City Dept. Environmental Protection, Kingston, NY * zme2@cornell.edu
Re-conceptualizing SWAT Variable Source Area (VSA) Hydrology Curve Number and VSA hydrology Convincing SWAT it recognizes VSAs Recoding SWAT for a VSA Water Budget If there is time
Common Perception of Runoff CN-method based on this theory (Horton 1933, 1940) Rain Infiltration Runoff Infiltration Excess a.k.a. Hortonian Flow
% of Area Generating Hortonian Flow Biological and Environmental Engineering Is Hortonian Flow Common? New York 100 90 80 70 60 50 40 30 20 10 0 September August October April November 1 10 March 100 Return Period (yr) Walter, et al. 2003. ASCE J. Hydrol. Eng. 8: 214-218
Variable Source Areas of Saturation Excess Runoff Rain Subsurface water rises Some areas saturate to the surface
Variable Source Areas of Saturation Excess Runoff Rain Rain on saturated areas becomes overland flow Dunne and Black. 1970. Water Resour. Res. 6: 478-490 Dunne and Black. 1970. Water Resour. Res. 6: 1296-1311
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)
Re-conceptualizing SWAT Variable Source Area (VSA) Hydrology Curve Number and VSA hydrology Convincing SWAT it recognizes VSAs Recoding SWAT for a VSA Water Budget If there is time
USDA-NRCS Curve Number Model Runoff =P e2 /(Pe+S) S=25400/CN-254 Tables link CN to land use and soil infiltration capacity
But maybe the tables are misleading Runoff =P e2 /(Pe+S) Victor Mockus justified his model largely on grounds that it produces rainfall runoff curves of a type found on natural watersheds Victor Mockus later concluded that saturation excess was probably the likely runoff mechanism to be simulated by the method
Curve Numbering VSA hydrology Watershed P Q = P e2 /(Pe+S) A f Q Unsaturated Saturated A f = f(s,p e ) DQ = A f DP e dq/dp e = A f Steenhuis et al. 1995. ASCE Div Drain. & Irr. 121:234-238
We know how much area is contributing A f = f(s,p e ) but from where in the landscape?
Soil Topographic Index Wetness Index Classes s 8 =f(s) s 7 =f(s) s 9 =f(s) s 10 =f(s) Soil Topographic Index 33.10 s 1 =f(s) High : 33.103500 Low : 3.520011 3.52 Wetness Index Classes 10 1 s 6 =f(s) s 5 =f(s) s 4 =f(s) s 3 =f(s) s 2 =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.
CN-ing Storm Runoff Q i SWAT-VSA (Easton et al., 2008. J. Hydrol. 348: 279-291) 0.2s i s i VSLF (Schneiderman et al. 2007. Hydrol. Proc. 21: 3420-3430) P
Re-conceptualizing SWAT Variable Source Area (VSA) Hydrology Curve Number and VSA hydrology Convincing SWAT it recognizes VSAs Recoding SWAT for a VSA Water Budget If there is time
Revisit the HRU concept Soils Landuse SWAT defines HRUs as the coincidence of soil type and landuse Hydrological/chemical properties are defined at the HRU HRUs So runoff/p loss is the same here (lowland pasture) As here (upland pasture) We know this is not the case
Revisit the HRU concept Wetness index class Land use w1 w2 forest corn HRU 4 HRU 1 HRU 3 HRU 2
STI Landuse SSURGO SWAT-VSA defines HRUs as the coincidence of soil topographic index (and soil) and landuse Weighted average of soil properties nested with in an area weighted index class HRUs So runoff/p loss is now not the same here (lowland pasture) As here (upland pasture)
CN or Storage Biological and Environmental Engineering Distributing CN-values Wetness Index Classes 10 1 100 90 80 70 60 50 40 30 20 10 0 Calibrated SWATs basin CN = 73.1 index CNs = 73.1, just distributed according to a wetness index Local Storage CN 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 Fractional Contributing Area
Modify the Available Water Content High runoff prone area = high moisture content (in general) We relate local soil water storage, s e,i, to AWC with the following: AWC = 1 b 2.65 0.5 0.4 clay 100 b s e,i 254 24.5 ρ b = soil bulk density (g cm -3 ) clay = soil clay content (cm 3 cm -3 ).
Soil Water (mm) Biological and Environmental Engineering 400 SWAT-VSA Soil Moisture Wetness class 10 Wetness class 9 350 300 250 200 150 100 50 9 8 7 6 5 4 3 2 1 0 1 174 347 520 693 866 1039 1212 1385 1558 1731 1904 2077 2250 2423 2596 2769 2942 3115 3288 Wetness class 2 Wetness class 1 Time (d)
Streamflow (cm) SWAT Streamflow (cm) SWAT Biological and Environmental Engineering Test Results: Streamflow 16 14 12 10 r 2 = 0.76 E = 0.82 Measured SWAT-VSA 15 8 6 0 0 Measured 15 4 2 0 16 Oct-98 Mar-99 Aug-99 Jan-00 Jun-00 Nov-00 Apr-01 Sep-01 15 Feb-02 Jul-02 Dec-02 May-03 Oct-03 Mar-04 Aug-04 14 Measured 12 SWAT-Standard 10 8 6 4 r 2 = 0.74 E = 0.83 0 0 Measured 15 2 0 Oct-98 Mar-99 Aug-99 Jan-00 Jun-00 Nov-00 Apr-01 Sep-01 Feb-02 Jul-02 Dec-02 May-03 Oct-03 Mar-04 Aug-04
Cumulative Runoff (mm) Cumulative Runoff (mm) Biological and Environmental Engineering Test Results: Runoff from pastures 3500 3000 2500 2000 a SWAT-VSA Wetness Index 10 2500 2000 1500 b SWAT-Standard 1500 1000 500 0 1996 1998 2000 2002 2004 6 1 1000 500 0 1996 1998 2000 2002 2004 From: Easton et al., 2008. J. Hydrol. 348: 279-291
Test Results: Runoff a SWAT-VSA swat_vsa_hrus SWAT-VSA SURQ Runoff [mm] (mm) 0-0.930-0.939 0.93-2.20 2.20-4.53 4.53-8.86 4.532-8.868 8.86-10.21 0.939-2.201 2.201-4.532 8.868-10.213 b SWAT-Standard swat_hrus SWAT SURQ Runoff [mm] (mm) 0.66-0.93 1.978-4.014 0.93-2.20 4.014-4.440 2.20-4.53 4.440-4.783 4.53-8.86 4.783-5.214 8.86-10.21 5.214-5.75 Easton et al., 2008. J. Hydrol. 348: 279-291
Test Results: Soil water a SWAT-VSA SWAT-VSA SW_END Soil Water [mm] (mm) 93-163 - 163-242 - 242-277 - 277 277-298 - 298 298-317 - b SWAT-Standard SWAT SW_END Soil Water [mm] (mm) 93-163 65-99 163-242 99-139 242-277 139-196 277-298 196-245 298-317 245-304 Easton et al., 2008. J. Hydrol. 348: 279-291
Test Results: Soil water Data curtsey of : Lyon et al. 2006. Adv. Water Resour. 29(2): 181-193. Lyon et al. 2006. HESS. 10: 113-125. N 0 100m 585 m 1m Cont. Water Level Loggers 600 m
Soil Water (mm) Biological and Environmental Engineering SWAT-Standard 140 120 100 80 60 40 20 0 1 29 57 85 113 141 169 197 225 253 281 309 337 365 393
Soil Water (mm) Biological and Environmental Engineering SWAT-VSA 350 300 250 200 150 100 50 0 1 27 53 79 105 131 157 183 209 235 261 287 313 339 365 391
SWAT-VSA Movie courtesy of Steve Lyon
SWAT-VSA Water Table (mm) SWAT Water Table (mm) Biological and Environmental Engineering Test Results: Soil water 1500 a 1500 b 1300 1100 Index 10 9 8 7 6 5 1300 1100 900 Mixed Forest Pasture Shrub R 2 = 0.57 900 R 2 = 0.79 700 700 700 900 1100 1300 1500 Measured Water Table (mm) 500 500 700 900 1100 1300 1500 Measured Water Table (mm) From: Easton et al., 2008. J. Hydrol. 348: 279-291
Dissolved P (kg) SWAT Dissolved P (kg) SWAT Biological and Environmental Engineering Test Results: Phosphorus 300 250 200 150 100 a Measured SWAT-VSA r 2 = 0.76 E = 0.71 300 0 0 Measured 300 50 0 250 Oct-98 Apr-99 Oct-99 Apr-00 Oct-00 Apr-01 Oct-01300Apr-02 Oct-02 Apr-03 Oct-03 Apr-04 b Measured 200 SWAT-Standard 150 100 50 r 2 = 0.68 E = 0.47 0 0 Measured 300 0 Oct-98 Apr-99 Oct-99 Apr-00 Oct-00 Apr-01 Oct-01 Apr-02 Oct-02 Apr-03 Oct-03 Apr-04
Test Results: Phosphorus a SWAT-VSA swat_vsa_hrus SWAT-VSA ORGP Dissolved KG/HA P (kg ha -1 ) 0-0.04 0-0.044 0.04-0.09 0.09-0.22 0.22-0.69 0.69-0.99 0.044-0.088 0.088-0.223 0.223-0.694 0.694-0.99 SWAT-Standard b swat_hrus SWAT ORGP Dissolved KG/HA P (kg ha -1 ) 0-0.040-0.076 0.04-0.09 0.09-0.22 0.22-0.69 0.69-0.99 0.076-0.154 0.154-0.189 0.189-0.225 0.225-0.287
Take-home Messages The Curve Number Runoff Model is not a critical limitation to developing realistic water quality models SWAT can be convinced to model VSA hydrology Conceptually it appears to correctly capture VSA phenomena We still need to relate S to measurable field parameters for use in un-gauged basins
Re-conceptualizing SWAT Variable Source Area (VSA) Hydrology Curve Number and VSA hydrology Convincing SWAT it recognizes VSAs Recoding SWAT for a VSA Water Budget Is there is time?
Precip (mm) Runoff (mm) Biological and Environmental Engineering SWAT Water Balance One problem with the Curve Number method Runoff volume is calculated before infiltration In VSA hydrology soil water accumulates before runoff 60 4 50 40 Cumulative Precip (mm) Runoff (mm) 4 3 3 30 2 20 10 2 1 1 0 0
SWAT Modifications Runoff generation method changed Calculates volume of water to saturate soil Storage = Soil Depth *(soil porosity-soil moisture) Rainfall exceeding storage capacity = runoff If Precip < Storage, then Q surf = 0 If Precip > Storage, then Q surf = Precip Storage and Infil = Storage
Sreamflow (mm) Biological and Environmental Engineering Preliminary Results 60 50 40 30 20 10 R 2 = 0.67 NSE=0.61 Measured SWAT Water Balance 0 Oct-98 Apr-99 Oct-99 Apr-00 Oct-00 Apr-01 Oct-01 Apr-02 Oct-02 Apr-03 Oct-03 Apr-04 Oct-04