Study On Rainfall-runoff Process In Landuse Types At A Watershed Scale In Western Shanxi, Loess Plateau, China

In this study, we used some statistical methods and analyzed the data of precipitation and runoff measured from 2004 to 2009 in the Caijiachuan watershed in broken loess plateau gully region of western Shanxi Province, China. Firstly, We examined the hydrologic differences of two watersheds with different vegetation covers, one natural forest and one artificial forest in the Caijiachuan watershed. Secondly, we studied on the relationship between rainfall and runoff in different landuse types in the Caijiachuan watershed. Thirdly, we studied on rainfall-runoff process and the rule of water balance in the Caijiachuan watershed. Lastly, With the use of the Soil and Water Assesment Tool (SWAT), The ecological restoration scenarios with different land covers were simulated for the Caijiachuan watershed from 2004 to 2009. This study is meaningful for understanding the regional hydrology mechanism, making good use of water resources, and resolving the relevant ecological and economic problems.The results in this study showed that the proportion of yearly water consumption to yearly rainfall was more than 97%, which suggested that the yearly water consumption was the main expenditure item in the water cycle process in the experimental watershed. But the yearly runoff coefficient was very small, the range of which was just from 1.6% to 2.66%.We examined the hydrologic differences of two watersheds with different landcover types, one natural forest and one artificial forest in the Caijiachuan watershed in the loess hilly region of western Shanxi Province. These two neighboring experimental watersheds (one of which was planted artificial trees such as Robinia, Pinus and Platycladus, and the other has been closed for 26 years) have the similar characters of soil,geology and physiognomy. Based on water observation data and precipitation data collected from 2004a to 2009a in experimental watersheds of loess areas, comparisons of the annual runoff and the rainy-season runoff between natural forest and artificial forest, and analyses of the correlation and regression under different types of rainfall were used to discuss on the relationships between runoff and vegetation covers.The results showed that the annual runoff and the rainy-season runoff (June-September) from the natural forest watershed was significantly lower than that from the artificial forest watershed,58.57% and 48.04%, respectively. Annual and rainy season overland flow from the natural forest watershed was lower by 82.95%,81.12% respectively when compared to that from the artificial forest watershed. Baseflow was negligible in the artificial forest watershed. The types of rainfall had impact on runoff in small watersheds with different vegetation covers. For the artificial forest watershed, the average peak discharge per unit area under A-type rainfall(stonn events with short duration and high intensity) was highest, about 10.08 times of that under B-type rainfall(storm events with long duration and high intensity), about 3.35 times of that under C-type rainfall(storm events with long duration and low intensity). For natural forest watershed, the average peak discharge per unit area under A-type rainfall was also highest, about 6.79 times of that under B-type rainfall, about 1.64 times of that under C-type rainfall. So we concluded that flood events and thus soil erosion would likely to occur under A-type rainfall events. And some measures for water and soil conservation should be taken especially under A-type rainfall events.We also examined the hydrologic differences of three watersheds with different landuse, one watershed with farmlands and grasslands, one watershed with natural forests and artifical forests, one watershed with natural forests of Betula, Populus, Syzygium, and Ostryopsis in the Caijiachuan watershed in the loess hilly region of western Shanxi Province.The results showed that the annual runoff and the rainy-season runoff (June-September) from the farmlands and grasslands watershed was significantly highest, which was about 2.46 times,2.59 times of that from the natural forests and artifical forests watershed respectively, and about 2.67 times,3.14 times of that from the natural forests watershed respectively. The runoff and peak discharge from the natural forests watershed was always lower than that from the natural forests and artifical forests watershed.The runoff was simulated for the Caijiachuan watershed with the use of the Soil and Water Assesment Tool (SWAT). The model was calibrated by adjusting hydrological parameters. In calibration period, Nash-Suttcliffe coefficient is 0.812, relative coefficient is 0.857. In validation period, Nash-Suttcliffe coefficient is 0.794, relative coefficient is 0.826. So it could be concluded that the SWAT model is applicable to the Caijiachuan watershed on the Loess Plateau.With the use of SWAT, The ecological restoration scenarios with different land covers were simulated for the Caijiachuan watershed from 2004 to 2009. The result showed that forest could reduce more runoff amout than grassland in the Caijiachuan watershed.