Simulated Effects Of Land Use Change On Runoff Of Dongchan Watershed, Jinghe River

The Loess Plateau is the area with most serious soil erosion in our country and its ecological environment is very fragile. Long unreasonable land use and vegetation damage, affecting the regional hydrological cycle and water balance, leading to reduction in water resources, limited economic and social sustainable development.Dongchuan watershed of Jinghe river was chosen as research site. The change of runoff was calculated from the runoff data of Jiaqiao station in 1980-2005, which locates at the outlet of Dongchan watershed. Climate change were analyzed based on the meterology data from 15 precipitation stations and Huachi weather station located in Dongchan Watershed during 1980-2005. Then, Soil and Water Integrated Model (SWIM) was employed to simulate the effects of land use change on runoff and evaluate the contribution of land use change and climate change to runoff change during l980-2000. The results showed that as following:(1)Climate change Dongchuan watershed annual rainfall 474.7mm, mainly concentrated in the 7,8,9 month, which accounted for 3 months precipitation 58.7% of annual precipitation.In 1980-2005 years, there was no significant trend in annual precipitation changes, but the average temperature was significantly increased, the increase rate of 0.3℃/ 10a, a small pan evaporation increased significantly increase the rate of 111.8mm/10a.(2)Runoff change Annual runoff 26.7mm, with the main flood season runoff, flood season (May-October) runoff is 20.2mm, total annual runoff of 75.9%, non-flood season (November - April the following year) of 3.2 times. Flood runoff and annual runoff consistent trend, rather than a weak decline in flood runoff.(3)Model calibration and validation The SWIM model can simulate runoff well in the Donghuan watershed, with an efficiency of 0.51 , for the calibration and 0.51 for the validation period. The relative error are - 11% and -9%,respectively.(4)The contribution of land use change to runoff change during last twenty years The mean annual runoff increased by 6.4mm from 1980-1986 to 1993-2000 years. It was estimated that land use change accounted for +141.9% of mean annual runoff increase. The climate change negatively affect on the runoff as -44.0% of mean annual runoff increase.(5) Effect of vegetation change There was no big difference of water yield between forest and the grassland with the cover of 0.3-0.5. When the grassland with the cover of 0.3-0.5, which accounted for 10% of watershed area, was conversed to coniferous forest and broad leaved forest, annual watershed runoff would be reduced by 0.01mm (0.2%) and 0.08mm (1.2% ), respectively. When the grassland with the cover of 0.1-0.2 was conversed to forest or the grassland with the cover of 0.3-0.5, water yield decreased sharply. For example, the annual watershed runoff was reduced by 6.7 - 8.2 mm when the grassland with the cover of 0.1-0.2 was converted to forest and grassland with the cover of 0.3-0.5, which accounted for 10% of watershed area. The reduction amounts more than 25% of current annual watershed runoff.There was no significant difference in the water yield between winter wheat and corn. When the winter wheat, which accounted for 10% of watershed area, was converted to corn, annual watershed runoff would be reduced by 0.2 mm (0.3%).