Abstract: Quantifying the relationship between precipitation and hillslope runoff is of great importance for understanding the rainfall runoff generation mechanism and the temporal and spatial dynamics of soil moisture. However, the hillslope runoff characteristics and its controlling factors in the central Tibetan Plateau remains poorly understood because of limited in situ observations, which impedes the understanding of land surface hydrological processes in this region with complex terrain. Based on a wide range of in situ observations of hillslope runoff, soil water and precipitation from a typical alpine grassland in the Silin Co Basin in the Tibetan Plateau, we investigated the hillslope runoff dynamics and elucidated the precipitation infiltration soil water storage runoff generation processes. By measuring soil water, land cover and precipitation, we quantified the runoff generation under different precipitation levels. The results showed that infiltration excess runoff was the main way for the generation of runoff in Silin Co Basin. For the light rain event of 7.6 mm, the runoff depth was 0.23 mm, and the water infiltration was 2.63 mm. For the medium rain event of 18.8 mm, the runoff depth was 0.68 mm and the water infiltration was14.65 mm. For the heavy rain event of 32.4 mm, the runoff depth was 0.78 mm and the water infiltration was 30.49 mm. For the rainstorm with rainfall of 55.4 mm, the runoff depth was 0.89 mm and the water infiltration was 47.59 mm. For those four sorts of rain events, the water infiltration occurred in the soil depth of 0-5 cm, 0-80 cm, 0-10 cm, and 0-80 cm, respectively. With a low soil water content (about 5%), the minimum rainfall that could generate runoff was 5 mm. However, with the relatively high soil water content (about 8%), the minimum rainfall that could generate runoff became 1.8 mm, and the runoff coefficient was approximately 0.016. The main factors controlling runoff coefficient were the maximum 10-min rainfall intensity and the initial soil water content.

Key words: Silin Co, alpine grassland, hillslope runoff, runoff coefficient, the maximum 10 min rainfall intensity.