Study Of The Processes Of Soil Detachment And Sediment Transport By Rill Flow On The Loess Region

The loess region of China is the most heavily eroded region in the world.The severe soil erosion in this region has caused serious impact and threats to the local sustainable development and the safety of the lower reaches of the Yellow River.Rill erosion is one of the most important soil erosion type and source of sediment yield.Soil detachment and sediment transport are the key processes of rill erosion,and their intensity directly affects the rill erosion rate.Study the process of soil detachment and sediment transport by rill flow on the loess region can deeply reveal the rill erosion process mechanism of this region,deepen the understanding of the rill erosion process,effectively realize the scientific establishment of the rill erosion process model,and provide important scientific basis for soil erosion control and ecological construction of the loess region.Flume experiment,flume experiment with soil feeding hopper and soil physical and chemical properties test were adopted in this study.Five soils（Yangling clay loessial soil,Changwu loessial soil,Ansai loessial soil,Dingbian sanddy loessial soil and Shenmu sandy loessial soil）,five slopes（15.84%,21.26%,26.79%,32.49%,38.39%）,five unit flow discharges(0.00156 m² s^-1,0.00200 m² s^-1,0.00244 m² s^-1,0.00289 m² s^-1,0.00333 m² s^-1),and five sediment loads（10%T_c,25%T_c,50%T_c,75%T_c,90%T_c）were combined to study the process of soil detachment and sediment transport by rill flow on the loess region.Soil detachment capacity by rill flow,sediment transport by rill flow,rill erodibility parameter and rill detachment threshold parameter,contribution of sediment load to soil detachment by rill flow,and equation of the process of soil detachment by rill flow were investigated and analyzed.The main results are showed below:1.Soil detachment capacity by rill flow on the loess region were studied,and equations of soil detachment capacity by rill flow on the loess region were established:（1）The relationships between soil detachment capacity by rill flow and slope and flow discharge are binary power function for each soil（R²:0.9685⁰.9886,P<0.01）.The power exponents of slope are in the range of 0.9509 to 1.1322,and the power exponent of unit flow discharge are in the range of 1.1585 to 1.3247.The dynamic equation of the five soils are linear equation between soil detachment capacity by rill flow and stream power（R²:0.9263⁰.9764,P<0.01）.（2）Soil detachment capacity by rill flow of Shenmu sandy loessial soil is biggest(2.2145 kg m^-2 s^-1),followed by Yangling clay loessial soil(1.9320 kg m^-2 s^-1),Dingbian sanddy loessial soil(1.6093 kg m^-2 s^-1),Ansai loessial soil(1.1189 kg m^-2 s^-1),and Changwu loessial soil(0.9435 kg m^-2 s^-1).There are significant differences among Yangling clay loessial soil,Changwu loessial soil,Ansai loessial soil,and there aren’t significant differences among Yangling clay loessial soil,Dingbian sanddy loessial soil and Shenmu sandy loessial soil.There isn’t significant difference between Changwu loessial soil and Ansai loessial soil,but the two soil have significant difference with Dingbian sanddy loessial soil and Shenmu sandy loessial soil.（3）Soil detachment capacity by rill flow is negative correlated with soil cohesion and effective silt content（P<0.05）,and is positively correlated with effective median size and effective sand content（P<0.01 and P<0.05）.（4）The soil detachment capacity equation of the loess region is a quaternary power function,which indicate the variation of soil detachment capacity with slope,unit flow discharges,soil cohesion and effective median size（R²=0.9748,NSE=0.9595,P<0.01）,or is a ternary power function which indicate the variation of soil detachment capacity with stream power,soil cohesion and effective median size（R²=0.9712,NSE=0.9560,P<0.01）.2.Sediment transport capacity by rill flow on the loess region were studied,and equations of sediment transport capacity by rill flow on the loess region were established:（1）The relationships between sediment transport capacity by rill flow and slope and flow discharge are binary power function for each soil（R²:0.9486⁰.9884,P<0.01）.The power exponents of slope are in the range of 0.3704 to 0.9209,and the power exponent of unit flow discharge are in the range of 0.9101 to1.3469.The dynamic equation of the five soils are linear equation between sediment transport capacity by rill flow and stream power（R²:0.8071⁰.9484,P<0.01）.（2）Sediment transport capacity by rill flow of Shenmu sandy loessial soil is biggest(2.5733 kg m^-1 s^-1),followed by Dingbian sanddy loessial soil(2.4066kg m^-1 s^-1),Ansai loessial soil(1.8332 kg m^-1 s^-1),Changwu loessial soil(1.8665 kg m^-1 s^-1),and Yangling clay loessial soil(1.6039 kg m^-1 s^-1).There aren’t significant differences among Yangling clay loessial soil,Changwu loessial soil,Ansai loessial soil,and there aren’t significant differences among Dingbian sanddy loessial soil and Shenmu sandy loessial soil,but there are significant differences between the first three and the last two.（3）Sediment transport capacity by rill flow is negative correlated with silt content（P<0.05）,and is positively correlated with median size and sand content（P<0.05）.（4）The sediment transport capacity equation of the loess region is a ternary power function,which indicate the variation of sediment transport capacity with slope,unit flow discharges,and median size（R²=0.8544;NSE=0.8922;P<0.01）,or is a binary power function which indicate the variation of sediment transport capacity with stream power and median size（R²=0.6882,NSE=0.7217,P<0.01）.3.The rill erodibility parameter and rill detachment threshold parameter were revealed:（1）Rill erodibility parameter of Shenmu sandy loessial soil is the biggest(0.4005 s²m^-2),followed by Yangling clay loessial soil(0.3508 s² m^-2),Dingbian sanddy loessial soil(0.3014 s² m^-2),Ansai loessial soil(0.2281 s² m^-2),Changwu loessial soil(0.1963 s² m^-2).The rill erodibility parameters of the soils are gradually increased according to soil particle size from fine to coarse,and geographical locations from south to north except Yangling clay loessial soil.（2）Rill erodibility parameters of the five soils are negatively correlated with soil cohesion and effective silt content（P<0.01 and P<0.05）,and are positively correlated with effective median size and effective sand content（P<0.05）.（3）Rill detachment critical stream power of Changwu loessial soil is the biggest(1.0352 W m^-2),followed by Ansai loessial soil(0.9522 W m^-2),Dingbian sanddy loessial soil(0.5255 W m^-2),Yangling clay loessial soil(0.3549 W m^-2),Shenmu sandy loessial soil(0.3326 W m^-2).Rill detachment critical stream power of Changwu loessial soil and Ansai loessial soil are very close.Rill detachment critical stream power of the five soils are gradually decreased according to soil particle size from fine to coarse,and geographical locations from south to north except Yangling clay loessial soil.（4）Rill detachment critical stream power of the five soils are positively correlated with soil cohesion and effective silt content（P<0.01 and P<0.05）,and are negatively correlated with effective median size and effective sand content（P<0.05）.4.Contribution of sediment load to soil detachment rate by rill flow were quantified:（1）Soil detachment rate by rill flow decreased with sediment load increased under different soils,unit flow discharges and slopes.Soil detachment rate by rill flow decreased linearly with the increase in sediment load（R ²:0.8797⁰.9971,P<0.05 and 0.01）.The mean decrease rates of detachment rates under the five sediment load 10%T_c,25%T_c,50%T_c,75%T_c,90%T_c（T_c:Sediment transport capacity）were 17.51%,32.56%,51.27%,66.15%and 81.64%,respectively.（2）For the condition of typical loess soil Ansai loessial soil,the comprehensive response correlation of soil detachment rate to sediment load and stream power is a binary linear equation.Sediment load is a resistance factor to the process of soil detachment and stream power is a driving factor.The contribution rate of sediment load to soil detachment rate by sediment-laden rill flow for all combinations is 37.07%and stream power is58.24%.The contribution rate of stream power is 1.6 times of that of sediment load.（3）For the conjoint analysis of the five soils,the comprehensive response correlation of soil detachment rate to sediment load and stream power is also a binary linear equation.The contribution rate of sediment load to soil detachment rate by rill flow for all combinations is31.26%and stream power is 55.44%.The contribution rate of stream power is 1.8 times of that of sediment load.5.The model equations of the process of soil detachment by rill flow were established:（1）The model equations of the process of soil detachment by rill flow were established based on the typical loess soil Ansai loessial soil.The model equations of the process of soil detachment by rill flow on loessial hillslope is a binary power equation,in which soil detachment rate response to soil detachment capacity and sediment deficit rate:D_r=0.8099D_c^1.0258（1-G/T_c）^0.6616（R²=0.9730;P<0.01;n=125）.（2）The model equations of the process of soil detachment by rill flow were established based on the conjoint analysis of the five soils.The model equations of the process of soil detachment by rill flow on the loess region is a binary power equation,in which soil detachment rate response to soil detachment capacity and sediment deficit rate:D_r=0.8427D_c^0.9867（1-G/T_c）^0.6426（R²=0.9488;P<0.01;n=145）.This equation is suitable for simulating the soil detachment rate by rill flow on the loess regions under the experiment condition.