Land-use induced spatial heterogeneity of soil hydraulic properties on the Loess Plateau in China

On the Loess Plateau in China, soil erosion amounts to between 10 000 and 25 000 tons/km2/year. In 1998, the EROCHINA project was started, with the major objective of developing alternative land-use and soil and water conservation strategies for the Loess Plateau, using the LISEM soil erosion model. In order to provide the model with accurate input on soil hydraulic properties of the catchment, this study tried to quantify these properties for major land-use units and to examine the effects of the statistically identified in-field heterogeneity on model outcome. The study area (Danangou catchment) is located in the middle part of the Loess Plateau in the northern part of Shaanxi Province. The catchment is about 3.5 km2 in size. To determine the hydraulic properties of the soil, a sampling scheme was implemented to measure unsaturated conductivity and water retention characteristics. The saturated conductivity measurements were performed on land-use clusters, based on treatment and plant and soil differences. A 100×100 m sampling grid was set out on 12 fields, with 1×1 m grid squares. On each field, 10 sampling spots were randomly selected, using Simple Random Sampling. The sensitivity of the LISEM model to the measured heterogeneity of saturated conductivity was analysed by using the geometric mean and standard deviation (S.D.) of the Ks-optimized of the various land-use units to calculate discharge and soil loss during a single rain event. This proved that, using the standard deviation of the saturated conductivity, which was 30–50%, the calculated discharge and total sediment losses varied by a factor 2. Using the standard deviation had a minor effect on the calculated sediment concentration. As regards the on-site effects, the use of the geometric mean of Ks minus the S.D. resulted in an increase in the level of erosion, while the use of geometric mean of Ks plus the S.D. value resulted in a significant decrease in erosion level relative to that obtained using the geometric mean of Ks itself. Using randomly selected sampling spots and a calibration procedure as was done in the present study make detailed information on Ks available, which can be used to compare alternative land-use options for their effectiveness in reducing discharge and sediment losses.