Soil Loss Tolerance On The Loess Plateau Based On The Healthy Function In The Lower Reaches Of The Yellow River

The serious soil loss on the Loess Plateau results in lots of sediment deposited in the lower reaches of the Yellow River that fomated famous'suspended river'. Rainfall not only scour surface soil but also take away lots of nutrients, which become an important source of NPS (non-point source pllution) that influence water quality in the lower reach of the Yellow River. Therefore, according to two main indicators'safe and clear circulation of stream and sediment'and'nice water quality'in'Healthy Yellow River'that submited by YRCC (Yellow River Conservancy Commission), taking Huayuankou to Gaocun region as typical reaches, referring to the results of previous researches, the critical sediment discharge when reaching the equilibrium between scouring and deposition and demanded water quality criterion was analysed, and then the soil loss tolerance of the Loess Plateau in the middle reaches and the different regions of soil erosion types were obtained. The mail conclusions are including:(1) Based on the previous researches on critical flow discharge of different sediment concentration under the condition of equilibrium between scouring and deposition, and using daily flow of Huayuankou and Gaocun hydrologic station, the different flow amount frequencies in flood season and non-flood season in different hydrological year were analysed, and then the critical sediment discharges in the condition of equilibrium between scouring and deposition in the lower reaches of the Yellow River were obtained, which is 6.54×108t, 8.83×108t and 3.95×108t in average year, high flow year and low flow year respectively,and with an average of 6.44×108t.(2) The NPS pollution load of ammonia nitrogen caused by soil loss in Sanmenxia section, Huayuankou section and Gaocun section takes up 52.8%, 52.7%, and 53.5% of the average proportion of the total pollution load respectively, while total phophorus is 85.9%, 76.8% and 73.6%. Referring to enviornment quality standards of surface water, Sanmenxia section has a severe pollution, and the proportion reaching or exceedingⅣcategory of water quality occupied 92.5%, while the water quality in Huayuankou section and Gaocun section were better, and the proportion that reaching or underⅢcategory of water quanlity reached 70.4% and 59.2%. The critical sediment discharge when the water quality in the standard ofⅢandⅡin the lower reaches of the Yellow River were 7.93×108t and 5.20×108t. (3) Combining the results of reduced sediment discharge by water conservancy and soil and water conservation, average sediment discharge of Toudaoguai hydrologic station, sediment discharge ratio of the Loess Plateau, the soil loss tolerance in the Loess Plateau were obtained, which is 9.41×108t and 8.41×108t in Hekouzhen to Huayuankou region, and 8.43×108t and 7.19×108t in Hekouzhen to Helonghuazhuang region when water quality beingⅢorⅡunder the condition of equilibrium between scouring and deposition in the Lower reaches of the Yellow River.(4) Taking soil erosion sediment discharge, population density, sediment reduction benefit and per capita net income as distribution indicators and refering to the soil erosion intensity in the'Grain for Green'period (from 1998 to 2007), growing rate of sediment reduction benefit (from 1998 to 2007) compared to before'Grain for Green'period (from 1970 to 1989), the soil loss tolerance in the different erosion type regions were determined, which varing from 50.0t/(km2·a) to 5 000.0t/(km2·a), and the minimum appeared in Plateau mountain and rock region, the maximum in Loess residual plateau hilly and gully region.