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Improved Soil and Water Assessment Tool (SWAT) Performance by Removal of the Curve Number Method

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.

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Improved Soil and Water Assessment Tool (SWAT) Performance by Removal of the Curve Number Method

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  1. 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

  2. “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)

  3. Overview • Problems with the Curve Number • Changes we made SWAT-WB • Watersheds modeled • Results • Discussion • Conclusions

  4. 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

  5. SWAT-WB’s Runoff Calculation τ= available storage (mm H2O) EDC = effective depth coefficient ɛ = soil porosity (mm) θ= soil moisture content (mm H2O) Q = surface runoff (mm H2O) P = precipitation (mm H2O) • 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

  6. Test Watersheds • Gumera Basin, Blue Nile, Ethiopia 1270 km2 • Townbrook, Catskills, NY 37 km2

  7. Townbrook Results SWAT-WBNSE = 0.59 R2 = 0.67 SWAT-CN NSE = 0.53 R2 = 0.63 SWAT-VSA NSE = 0.61 R2 = 0.68

  8. 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

  9. 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

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