Evaluating the Effects of Watershed Size on SWAT Calibration

Carlington Wallace,Dennis Flanagan,Bernard Engel

doi:10.3390/w10070898

Abstract

The Soil and Water Assessment Tool (SWAT) has been calibrated in many watersheds of various sizes and physiographic features. However, it is still unclear whether SWAT calibration parameters will produce satisfactory results if they are implemented in watersheds of different sizes. Evaluating the transferability of SWAT calibration parameters between watersheds of different sizes will provide insight into whether it is acceptable to calibrate SWAT in one watershed and apply the optimized parameters in different size watersheds by assuming both watersheds have similar physiographic properties. This study investigated the influence of watershed size on the SWAT model calibration parameters transferability between four watersheds (CCW = 680 km2, F34 = 183 km2, AXL = 42 km2, and ALG = 20 km2) located in Northeastern Indiana. The results show that calibrating SWAT at one size and applying the optimized parameters at different watershed sizes of similar physiographic features provided satisfactory simulation results. The size watershed at which SWAT was calibrated had little effect on streamflow predictions. Soluble nitrogen loss estimates were improved when calibration was performed at the larger CCW watershed while calibrating SWAT at the smaller AXL and ALG watersheds produced improved statistical indicator values (NSE, R2, and PBIAS) for soluble P and total P when applied to the larger CCW and F34 watersheds.

Highlights

Growing concerns over water quality in agricultural watersheds continue to be the topic of many discussions
When calibration was performed at the F34 watershed outlet and its optimized parameters were applied to the AXL watershed, the Kling–Gupta efficiency (KGE), Nash-Sutcliffe efficiency (NSE), and PBIAS values were outside the acceptable ranges (Table 11)
Based on the results presented in this paper, calibrating Soil and Water Assessment Tool (SWAT) at one watershed size and applying the optimized parameters at different sizes may produce satisfactory results despite a drop in the model performance when parameters are transferred across watersheds

Summary

Introduction

Growing concerns over water quality in agricultural watersheds continue to be the topic of many discussions. Agricultural runoff is considered a primary cause of nonpoint source pollution in the United States [1] because it often transports pesticides, nutrients, and sediment from agricultural fields and other areas to rivers and streams. This may have serious implications for the chemical, physical, and biological integrity of the nation’s water bodies [2]. Developing an effective watershed management program, requires comprehensive understanding of the hydrologic and chemical processes within the watershed [5] These processes are usually examined at the watershed scale using computer simulation models such as the Soil and Water Assessment Tool (SWAT) [6]

Results

Discussion

Conclusion