Revisiting SWAT as a Saturation-Excess Runoff Model

Tammo S Steenhuis,Emmet M Owens,Rajith Mukundan,Mamaru Moges,Linh Hoang,Elliot M Schneiderman

doi:10.3390/w11071427

Abstract

The Soil Water Assessment Tool (SWAT) is employed throughout the world to simulate watershed processes. A limitation of this model is that locations of saturation excess overland flow in hilly and mountainous regions with an impermeable layer at shallow depth cannot be simulated realistically. The objective of this research is to overcome this limitation with minor changes in the original SWAT code. The new approach is called SWAT-with-impervious-layers (SWAT-wil). Adaptations consisted of redefining the hillslope length, restricting downward percolation from the root zone, and redefining hydrologic response units (HRUs) such that they are associated with the landscape position. Finally, input parameters were chosen such that overland flow from variable saturated areas (VSAs) corresponds to the variable source interpretation of the Soil Conservation Service (SCS) curve number runoff equation. We tested the model for the Town Brook watershed in the Catskill Mountains. The results showed that the discharge calculated with SWAT-wil agreed with observed outflow and results simulated with the original SWAT and SWAT-hillslope (SWAT-HS) models that had a surface aquifer that transferred water between groups of HRUs. The locations of the periodically saturated runoff areas were predicted by SWAT-wil at the right locations. Current users can utilize the SWAT-wil approach for catchments where VSA hydrology predominates.

Highlights

Over the past 25 years, the Soil and Water Assessment Tool (SWAT) [1] has become the most used hydrologic model in part due to its excellent support structure [2]
The SWAT-2012 model can be transposed to the SWAT-wil (SWAT with impermeable layer) model that can simulate saturation overland flow and variable source area (VSA) hydrology
This is accomplished by ranking the hydrologic response units (HRUs) with a hardpan according the topographic index and by (1) controlling the lateral drainage by redefining the slope length according to saturation excess interpretation of the Soil Conservation Service (SCS) curve number; (2) setting the vertical drainage to zero by making the impervious layer at the bottom of the soil profile equal to the root depth vertical; and (3)

Summary

Introduction

Over the past 25 years, the Soil and Water Assessment Tool (SWAT) [1] has become the most used hydrologic model in part due to its excellent support structure [2]. The SWAT model predicts overland flow based on either the Soil Conservation Service (SCS) curve number approach or the Green and. In the SCS method, runoff is calculated as a function of land use, soil, hydrological condition, and some form of antecedent rainfall [3], and inherently generates overland flow when the saturated hydraulic conductivity is less than the rainfall intensity. The SWAT model can be changed from an infiltration to a saturation-excess model without altering the discharge, Q, because the SCS curve number equation can be interpreted as a saturation-excess runoff routine [22] in which the spatially distributed storage in the soil needs to be filled up before overland flow occurs [15].

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Discussion

Conclusion