The Effects Of Terrace On Runoff And Sediments Variation In Jinghe Watershed

With the increase of population, intensified human activities had led to a obviously decrease of water and sediment in some important rivers, and which became a serious concerns of country and society, and the Yellow River and its tributaries of the Weihe river basin was very serious areas. Terraced fields as the important soil and water conservation measures were widely distributed. But with the increase of terrace area and the runoff of river, people raised doubt on the pros and cons of terraces. Aiming at the research of terraces measures influence on river runoff, ecological base flow and sediment in watershed scale was weak, this study selected Jinghe river basin which was water shortage, high sand content as the research object, through the method of combining indoor simulation test and mathematical model simulating, studied the impact process on the distribution of rainfall and slope surface erosion process of the slope terrace measures, and calculated the slope runoff coefficient curve(CN) and base flow attenuation coefficient() as parameters for the SWAT model. On this basis, based on SWAT model of embedded terraces module, quantitative analyzed the influence of typical measure of soil and water conservation--terraces on river runoff, sediment and safeguard the ecological base flow of Jinghe basin. The main conclusions were as follows:(1) Terrace measures can effectively control the conversion of rainfall runoff, increase groundwater recharge, and intercept sediment. By a designed rainfall-runoff-erosion-groundwater-stream flow indoor simulation system, bare slope, terraces and straw mulching measures were selected to carry out the artificial rainfall experiment and analyzed the impact of terraces to the surface runoff, soil water, groundwater and the influence of sediment. The results showed that in the process of rainfall, the addition of soil storage water was: terrace- straw mulching > terrace > bare land. Compared with the bare slope, the coefficient of average rainfall runoff(alpha) in terraced fields and terrace- straw mulching reduced 27.27% and 27.27% respectively. The water reducing efficiency of terraces and terrace- straw mulching measures were 26.31% and 89.65% respectively, and the corresponding sediment reduction efficiencies were 65.52% and 94.13% respectively. In the 60 mm/h rainfall intensity experiment, compared with the bare slope, the average ground water level of terraces and straw mulching measures increased by 3.1% and 15.1% respectively; The average groundwater flow of terraces and terrace- straw mulching measures were 1.29 and 1.62 times of the bare slope; At the end of rainfall, the groundwater recharge of terrace and terrace- straw mulching were 1.25 and 2.29 times of the bare slope; After the experiment the groundwater recharges of terrace and terrace-straw mulching were 1.50 times and 2.69 times of the bare slope.(2) The parameters in SWAT model applied to Jinghe river watershed were calculated by test. The results showed the runoff curve number of bare slope(CN) was 89.97, the runoff curve number of terraces(CN) was 82.19, and the curve number terraced straw mulch(CN) was 54.18. The base flow attenuation coefficient of bare slope was 0.21, the base flow attenuation coefficient of terraces and terraced straw mulch were 0.36. Other parameters were calibrated with runoff curve number and base flow attenuation coefficient into SWAT model. Nash coefficient(NS) and the square of the correlation coefficient(r2) monthly runoff were more than 0.66 and 0.75. Nash coefficient(NS) and the square of the correlation coefficient(r2) monthly sediments were more than 0.75 and 0.71 respectively in calibration and validation. The Jinghe river SWAT model embedded the terrace module could sumulate the monthly scale whole watershed runoff and sediments changing well.(3) Based on the Jinghe River SWAT model embedded the terrace module, the influence of wide range watershed terrace construction on the runoff, sediments yield and basic ecological flow were carried out, and basic ecological flow was caculated by Tennant. The results showed that terraces in the Jinghe River watershed could delay the flood and add the drought season runoff, decrease river sediment yield, which were helpful for guaranteeing basic ecological flow. Basic ecological flow in Yangjiaping and Jingcun station were 2.55 and 5.2 m3/s respectively. For the normal year in non-flood season, in Yangjiaping hydrologic section, 0.05m3/s of basic ecological flow increased with every 1% area of terraces increasing above the section, which improved the guarantee rate of recent ecological basic flow about 1.49%, the runoff reduced by per area terrace above the section was 1.60(104m3/km2.a), the runoff supplied by per area terrace above the section was 0.84(104m3/km2.a), the river sediments reduced by per area terrace above the section was 0.32(104m3/km2.a). For the normal year in non-flood season, in Jingcun station hydrologic section, 0.04 m3/s of basic ecological flow increased with every 1% area of terraces increasing above the section, which improved the guarantee rate of recent basic ecological flow about 3.48%, the runoff reduced by per area terrace above the section was 1.31(104m3/km2.a), the runoff supplied by per area terrace above the section was 0.24(104m3/km2.a), the river sediments reduced by per area terrace above the section was 0.45(104t/km2.a).