Keywords: 
• SWAT model /
• hydrological response unit /
• spatial discretization /
• runoff lag /
• Taihu Lake basin

Abstract: The SWAT (Soil and Water Assessment Tools) is a physically based semi-distributed hydrological model. The basic computational unit in the SWAT is the hydrological response unit (HRU) that is usually determined by land uses, soil types, and slope and aspect of an area. Each HRU is assumed to have a homogeneous hydrological response. HRUs obtained with the traditional approaches for delineation of hydrologic response units are often spatially discontinuous and it is difficult to locate them in a catchment. The interaction among HRUs cannot be precisely described and analyzed. In this study, the land use and soil type data from a typical watershed in the Taihu Lake region are processed using the geographic information system (GIS) tools. A new approach for spatial discretization of hydrologic response units is proposed and applied to the processed data. As the result, the location of each HRU in the watershed can be accurately identified. Accordingly, different values of the surface runoff lag time in the SWAT can be applied. The improved version of SWAT is tested on the Xitiaoxi watershed of the Taihu Lake basin. The result shows that a notable improvement is achieved in the hydrological simulation compared to the original SWAT simulation for both model calibration and validation periods. The Nash-Sutcliffe model efficiency coefficients have been improved from 0.64 to 0.67 for calibration and from 0.70 to 0.76 for validation, respectively. The improved SWAT model can better reflect the runoff lag process and achieve a better simulation result. The new approach for spatial discretization of hydrologic response units is an effective solution for the model optimization, and can provide better estimates to the spatial data analysis at finer scales.