A comparison of WEPP and SWAT for modeling soil erosion of the Zhangjiachong Watershed in the Three Gorges Reservoir Area

Abstract

Soil and water conservation is important for the Three Gorges Reservoir Area in China, and quantification of soil loss is a significant issue. In this study, two widely used models – the Water Erosion Prediction Project (WEPP) and the Soil and Water Assessment Tool (SWAT) – were applied to simulate runoff and sediment yield for the Zhangjiachong Watershed in the Three Gorges Reservoir Area. The models were run and the simulated runoff and sediment yield values were compared with the measured runoff and sediment yield values. In the calibration period, the model efficiency (E_NS) values for the WEPP and SWAT were 0.864 and 0.711 for runoff, and 0.847 and 0.678 for sediment yield, respectively. In the validation period, the E_NS values for WEPP and SWAT were 0.835 and 0.690 for runoff, and 0.828 and 0.818 for sediment yield, respectively. The results of E_NS and the other criteria indicate that the results of both models were acceptable. WEPP simulations were better than SWAT in most cases, and could be used with a reasonable confidence for soil loss quantification in the Zhangjiachong Watershed.

Introduction

Soil erosion is a major soil conservation problem for the Three Gorges Reservoir Area in China. The statistics of Chongqing Soil and Water Conservation Ecological Environment Monitoring Station showed that the soil erosion area of Chongqing City, which is the main part of the Three Gorges Reservoir Area, was 40,005.59 km² in 2005, accounting for 48.55% of the city; the total amount of soil loss was 14.57 billion tonnes for that year (CQSBJC, 2006). Computer simulation models are increasingly popular for predicting soil loss to quantify the processes of detachment, transport and deposition of eroded soil. It is necessary to validate soil erosion models for this area, to evaluate the effects of different management practices on soil erosion and select best management practices.

Soil erosion models can be divided into empirical and physically based models. Empirical models usually establish relationships between runoff, sediment yield and precipitation, plants, soil types, land use types, tillage styles, water conservation measures and so on. They are still used because of their simple structure and ease of application. Since they are based on coefficients computed or calibrated from measurements and/or observations, they cannot describe or simulate the erosion process as a set of physical phenomena. The Universal Soil Loss Equation (USLE) is the most widely used empirical erosion model (Wischmeier and Smith, 1965). It is used to estimate soil erosion from an area simply as the product of empirical coefficients, which must therefore be accurately evaluated. Original values of coefficients were derived from field observations in different areas of the eastern U.S., but they have been expanded with time using information from researchers who have applied the USLE (and derived models) in other countries (Romero et al., 2007, Vaezi et al., 2008, Zhang et al., 2008).

Physically based models can describe with detail the physical mechanism of sediment yield, and can simulate the individual components of the entire erosion process by solving the corresponding equations; and so it is argued that they have a wider range of applicability. Such models are also generally better in assessing both spatial and temporal variability of the natural erosion processes. The WEPP model is a physically based model that simulates sediment yield and deposition using a spatially and temporally distributed approach (Foster et al., 1987, Nearing et al., 1989, Flanagan and Nearing, 1995).

The aim of this study was to evaluate the application of two models – WEPP and SWAT – in the Three Gorges Reservoir Area. These models were selected based on their wide usability, reputation and use of most up-to-date technology; they also have been tested on many agricultural watersheds. Linkages between the WEPP model (GeoWEPP) and SWAT have also been studied (Renschler and Lee, 2003, Renschler and Lee, 2005). However, there have been few comparisons of the simulations of soil erosion between these two models; WEPP has been compared to models of the same or relevant module of SWAT sediment simulation. The WEPP model has been compared with USLE, the Erosion Productivity Impact Calculator (EPIC), the Areal Nonpoint Source Watershed Environment Response Simulation (ANSWERS) and other models for runoff and soil erosion (Bhuyan et al., 2002, Croke and Nethery, 2006, Romero et al., 2007). Since the WEPP is a process-based continuous simulation model and SWAT is based on the empirical Modified Universal Soil Loss Equation (MUSLE), this paper will mainly discuss the differences between the empirical and physical modules of the two models.