In the water source areas of Beijing City and Tianjin City

ecological environment is highly fragile

and soil and water loss often exhibits strong spatial variability and occurs unpredictably.Along with data of climate

soil

terrain

land use and vegetation coverage

soil erosion and its spatial differentiation were studied based on the revised universal soil loss equation (RUSLE)model and geographic information system (GIS)

in Xibeigou watershed of Luanping County

Hebei Province.The results show that the annual average rate of soil erosion was3816.835t/(km 2·a)

classified as middle-grade erosion.Annual average of potential soil erosion was 31583.15t/(km2·a)

about 8.28 times of the former.Meanwhile

soil loss was obviously spatially differentiated.Among different land uses

sporadically distributed farmlands with steep slopes had the strongest soil erosion intensity

and next to it was highly weathered and degraded grassland's

which accounts for 59.38% of the total area of the watershed and 88.48% of the total sediment as the main source of the soil loss.With the increasing slope

the soil erosion intensity increased markedly.Approximately one third of the total area of this region has a slope degree higher than 25°

while this proportion contributed two third of soil loss.Soil losses also differed among lands of various aspects

as in order of sunny slope>half-sunny slope>half-shady slope>shady slope>flatland.