Re-conceptualizing SWAT for Variable Source Area Hydrology

Transcription

1 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

2 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

3 Common Perception of Runoff CN-method based on this theory (Horton 1933, 1940) Rain Infiltration Runoff Infiltration Excess a.k.a. Hortonian Flow

4 % of Area Generating Hortonian Flow Biological and Environmental Engineering Is Hortonian Flow Common? New York September August October April November 1 10 March 100 Return Period (yr) Walter, et al ASCE J. Hydrol. Eng. 8:

5 Variable Source Areas of Saturation Excess Runoff Rain Subsurface water rises Some areas saturate to the surface

6 Variable Source Areas of Saturation Excess Runoff Rain Rain on saturated areas becomes overland flow Dunne and Black Water Resour. Res. 6: Dunne and Black Water Resour. Res. 6:

7 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)

8 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

9 USDA-NRCS Curve Number Model Runoff =P e2 /(Pe+S) S=25400/CN-254 Tables link CN to land use and soil infiltration capacity

10 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

11 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 ASCE Div Drain. & Irr. 121:

12 We know how much area is contributing A f = f(s,p e ) but from where in the landscape?

13 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 s 1 =f(s) High : Low : 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., J. Hydrol. 348: Lyon et al Hydrol. Proc. 18(15): Schneiderman et al Hydrol. Proc. 21:

14 CN-ing Storm Runoff Q i SWAT-VSA (Easton et al., J. Hydrol. 348: ) 0.2s i s i VSLF (Schneiderman et al Hydrol. Proc. 21: ) P

15 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

16 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

17 Revisit the HRU concept Wetness index class Land use w1 w2 forest corn HRU 4 HRU 1 HRU 3 HRU 2

18 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)

19 CN or Storage Biological and Environmental Engineering Distributing CN-values Wetness Index Classes Calibrated SWATs basin CN = 73.1 index CNs = 73.1, just distributed according to a wetness index Local Storage CN Fractional Contributing Area

20 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 clay 100 b s e,i ρ b = soil bulk density (g cm -3 ) clay = soil clay content (cm 3 cm -3 ).

21 Soil Water (mm) Biological and Environmental Engineering 400 SWAT-VSA Soil Moisture Wetness class 10 Wetness class Wetness class 2 Wetness class 1 Time (d)

22 Streamflow (cm) SWAT Streamflow (cm) SWAT Biological and Environmental Engineering Test Results: Streamflow r 2 = 0.76 E = 0.82 Measured SWAT-VSA Measured Oct-98 Mar-99 Aug-99 Jan-00 Jun-00 Nov-00 Apr-01 Sep Feb-02 Jul-02 Dec-02 May-03 Oct-03 Mar-04 Aug Measured 12 SWAT-Standard r 2 = 0.74 E = Measured 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

23 Cumulative Runoff (mm) Cumulative Runoff (mm) Biological and Environmental Engineering Test Results: Runoff from pastures a SWAT-VSA Wetness Index b SWAT-Standard From: Easton et al., J. Hydrol. 348:

24 Test Results: Runoff a SWAT-VSA swat_vsa_hrus SWAT-VSA SURQ Runoff [mm] (mm) b SWAT-Standard swat_hrus SWAT SURQ Runoff [mm] (mm) Easton et al., J. Hydrol. 348:

25 Test Results: Soil water a SWAT-VSA SWAT-VSA SW_END Soil Water [mm] (mm) b SWAT-Standard SWAT SW_END Soil Water [mm] (mm) Easton et al., J. Hydrol. 348:

26 Test Results: Soil water Data curtsey of : Lyon et al Adv. Water Resour. 29(2): Lyon et al HESS. 10: N 0 100m 585 m 1m Cont. Water Level Loggers 600 m

27 Soil Water (mm) Biological and Environmental Engineering SWAT-Standard

28 Soil Water (mm) Biological and Environmental Engineering SWAT-VSA

29 SWAT-VSA Movie courtesy of Steve Lyon

30 SWAT-VSA Water Table (mm) SWAT Water Table (mm) Biological and Environmental Engineering Test Results: Soil water 1500 a 1500 b Index Mixed Forest Pasture Shrub R 2 = R 2 = Measured Water Table (mm) Measured Water Table (mm) From: Easton et al., J. Hydrol. 348:

31 Dissolved P (kg) SWAT Dissolved P (kg) SWAT Biological and Environmental Engineering Test Results: Phosphorus a Measured SWAT-VSA r 2 = 0.76 E = Measured 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 r 2 = 0.68 E = Measured Oct-98 Apr-99 Oct-99 Apr-00 Oct-00 Apr-01 Oct-01 Apr-02 Oct-02 Apr-03 Oct-03 Apr-04

32 Test Results: Phosphorus a SWAT-VSA swat_vsa_hrus SWAT-VSA ORGP Dissolved KG/HA P (kg ha -1 ) SWAT-Standard b swat_hrus SWAT ORGP Dissolved KG/HA P (kg ha -1 )

33 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

34 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?

35 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 Cumulative Precip (mm) Runoff (mm)

36 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

37 Sreamflow (mm) Biological and Environmental Engineering Preliminary Results 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