Biological and Environmental Engineering Soil & Water Research Group Improved Soil and Water Assessment Tool (SWAT) Performance by Removal of the Curve Number Method Eric White, Zach Easton, Dan Fuka, and Tammo Steenhuis Cannonsville CEAP Synthesis Visit June 11, 2009

Biological and Environmental Engineering Soil & Water Research Group “Barring appropriate modifications, the [CN] method should not be used to model the long-term hydrologic response of a catchment. Nevertheless, it is recognized that the method has been used in several long-term hydrologic simulation models developed in the past two decades with varying degrees of success.” - Ponce & Hawkins (1996)

Biological and Environmental Engineering Soil & Water Research Group Overview Problems with the Curve Number Changes we made SWAT-WB Watersheds modeled Results Discussion Conclusions

Biological and Environmental Engineering Soil & Water Research Group The Curve Number Many critiques of the CN method Garen & Moore (2005) JAWRA Many improvements have been suggested Spatial modifications (STIs & SWAT-VSA) Temporal modifications In a nutshell… The CN calculates runoff volume from a watershed’s “potential maximum retention” In temporal models, this retention is related to the soil moisture (Antecedent Runoff Conditions) via various equations

Biological and Environmental Engineering Soil & Water Research Group SWAT-WB’s Runoff Calculation τ = available storage (mm H 2 O) EDC = effective depth coefficient ɛ = soil porosity (mm) θ = soil moisture content (mm H 2 O) Q = surface runoff (mm H 2 O) P = precipitation (mm H 2 O) Available storage = volume of infiltrated rain STI method from SWAT-VSA used in SWAT-WB EDC calibrated separately for each STI class DDS algorithm used for autocalibration

Biological and Environmental Engineering Soil & Water Research Group Test Watersheds Gumera Basin, Blue Nile, Ethiopia 1270 km 2 Townbrook, Catskills, NY 37 km 2

Biological and Environmental Engineering Soil & Water Research Group Townbrook Results SWAT-WB NSE = 0.59R 2 = 0.67 SWAT-CN NSE = 0.53 R 2 = 0.63 SWAT-VSA NSE = 0.61R 2 = 0.68

Biological and Environmental Engineering Soil & Water Research Group Discussion of EDC Water balance models have trouble separating surface and subsurface flows EDC adjusts surface runoff to baseflow ratios Analogous to tank models EDC defines depth of the “upper zone soil moisture” level used to calculate surface runoff Still utilizes SWAT’s inherent soil moisture routines – no need to add more parameters

Biological and Environmental Engineering Soil & Water Research Group Conclusions The CN can be successfully removed from SWAT SWAT-WB is more accurate than standard CN-based SWAT At worst, same accuracy as SWAT-VSA Uses pre-existing routines within SWAT Implications for non-CEAP study areas Water balances and tank models applicable to any climate or topography