In southern China, most of the forests consist of Pinus massoniana, but they are usually poor, thin, and deteriorated due to human intervention such as cutting, removal of litter for fuel and over-grazing. Therefore, exposed bare soil is found in many devastated hills. As a consequence, both the direct runoff percentage and the sediment yield are very high. In order to understand the surface erosion processes in this environment, we measured sediment yield and surface erosion depth in terms of catchment scale, and carried out in situ sprinkling experiments, focusing on the spatial variability of surface conditions and sediment sources. The results of sediment yield measurements at the end of headwater catchment suggest the following characteristic. (1) The surface water is capable of transporting more material than is supplied by detachment. That is, the sediment discharge process is detachment-limited. In situ sprinkling experiments indicate that: (2) In the lower part of the hillslope where the surface is covered by microbiotic crust, the ratio of surface runoff to rainfall is high, but sediment yield is small. (3) In the upper part of the hillslope where there is no microbiotic crust, the ratio of surface runoff to rainfall is low, but sediment yield is large. Measurements of surface erosion depths indicate that: (4) Microbiotic crusts provide a high degree of protection against soil erosion, even though the surface runoff ratio is large at the lower hillslope. Consequently, the surface erosion depth is almost zero at the lower part of hillslope. (5) The main source of sediment was the upper part of hillslope. Based on these indications, it can be concluded that when the sediment discharge process is detachment-limited, the effect of the surface crust and/or the vegetation cover on the sediment discharge process is great. That is, the spatial variation of the surface crust and/or the vegetation cover controls the source of sediment. (C) 2000 Elsevier Science B.V. All rights reserved.