Assessment of Erosion Risk with the Rusle and Gis in the Middle and Lower Reaches of Hanjiang River

Abstract

Soil erosion in Hanjing River basin has occurred at an alarming rate. Mapping erosion risk in this region is increasingly needed by environment agencies. A research has, therefore, been carried out to evaluate the soil erosion risk at the middle and lower reaches of Hanjiang River. This research integrates the Revised Universal Soil Loss Equation (RUSLE) with a Geographic Information System (GIS) to quantify erosion risk. Soil data, land use inventory, digital elevation data, and climatic atlases were used as resource data sets to generate RUSLE factor values. Data sets for the RUSLE factors, along with associated maps showing geographic distributions, were established on IDRISI geographic information system (GIS). IDRISI was used to interactively calculate soil loss and map erosion risk. About 31 percent of the study area was predicted to have moderate risk of erosion and its annual soil loss is about10t/ha ~30t/ha; about 5.7% of the study area is extreme risk of erosion, which annual soil loss is more than 80t/ha. The extreme erosion areas are mainly slopelands and downlands in the middle reaches. It is, therefore, necessary to prescribe appropriate soil and water conservation practices to control the volume of soil loss within acceptable limits. The middle and lower reaches of Hanjiang River is one of most important base for agricultural production in China. However, intensive cultivation and socioeconomic pressure for more land have accelerated the rate of soil erosion in this area. In most part, erosion rate reaches 20t/ha, which means an annual soil loss of 3mm. Accelerated soil erosion is detrimental both in terms of reduced agricultural productivity and environmental impacts such as nonpoint-source pollution. During the past decade, algae bloom has taken place three times because of water quality deteriorating in Hanjiang River. South-to- North Water Transfer Project (SNWTP) of China is a huge project purported to mitigate chronic drought in northern China, but meanwhile it may inevitably worsen those ecological environment problems. With the aim of providing a valuable aid to conservationist for planning soil conservation strategies at the regional level, an important aspect is identifying and targeting specific high-priority areas for the implementation of best management practices. Thus, mapping regional erosion risk is increasingly needed by national and local environment agencies. Given the unique physical and biological conditions within middle and lower reaches of Hanjiang River, this research develops a soil erosion-predicting model based on the Universal Soil Loss Equation (USLE) and its subsequent Revised Universal Soil Loss Equation (RUSLE) in a geographic information systems (GIS) environment. The Universal Soil Loss Equation (USLE) developed by Wischmeier and Smith (1978) is the most frequently used empirical soil erosion model worldwide. More recently, Renard et al. (1997) has modified the USLE into a revised Universal Soil Loss Equation (RUSLE) by introducing improved means of computing the soil erosion factors. The RUSLE is written as: