Nutrient Losses In Runoff And Its Mechanism Of Chemical Regulation On Loess Slope Land

Serious soil and nutrients losses not only cause soil quality degradation,but also lead to water body pollution in the Loess Plateau.Therefore,accurate prediction of the nutrients losses process is significance for river basin management and agricultural cultivation guidance.Based on indoor tests,field trials and theoretical analysises,the effects of rainfall characteristics,topographic conditions,land cover,and polyacrylamide(PAM)applications on the characteristics of nutrient transport with overland flow were investigated.Mathematical models on nutrients transport with overland flow were proposed based on convection diffusion model,and the associated model parameters were estimated by comparing measured data.The main results include:(1)The impacts of raindrop,scour flow rates,and chemical regulation on water,soil and nutrients losses were investigated under raindrop splashing and water flow scouring.Runoff rates and sediment,nutrient losses increased with an increase of raindrop diameters under the effect splash.Sediments and nutrients losses were reduced by covering gauze and applying PAM on the soil surface.For covering gauze,this decrease was caused by the drcrease of raindrop kinetic energy,but for PAM applitions it was caused by the increase of soil erosion resistance.Slope erosion and nutrient losses were increased with an increase of water discharge.The runoff sedimentation rates and ammonia-nitrogen losses were reduced 27.2%-98.6%and 40.0%-63.4%by PAM application,respectively.(2)The effects of rainfall intensities,slope morphology,and chemical control measures on runoff,runoff sediment contentes,and nutrient losses were systematically analyzed and the appropriate chemical regulation method in the test area was proposed.Slope flow velocities and sediment losses were decreased with an increase of PAM application rates,but runoff rates and nutrient losses was decreased first and increased later with it.Suitable PAM application rate on the loess slope is 1-2g/m2,no more than 4g/m2.The order of soil and nutrient losses was roughly convex slope>concave slope>straight slope for different slope types.The efficacy of PAM on soil and runoff nutrient losses was most obvious on the convex slopes.(3)A soil hydraulic conductivity model considering the variation of soil surface bulk density with rainfall time and a slope flow generation model based on the motion wave equation and Green-Ampt infiltration model were established.The soil hydraulic conductivity and runoff rate can be well described through these two models,respectively.There is a linearly increasing relationship between the model parameters of the density decay rates in surface soil and raindrop diameters,and the maximum changes of the soil surface bulk density were decreases exponentially with an increase of PAM application rates.The two proposed models provided basic research methods for calculation of complex slope sediment production processes and solute transport processes.(4)The dynamic models of soil particles and nutrients transport with surface runoff were proposed based on the time-varying erosion rate,which can provide scientific basis for the management of soil nutrient losses.The models were validated and parameters were reversely estimated by fitting test data of raindrop splashing,water erosion,and field rainfall experiments.The results show that the depthof the exchange layer was increased with an increase of the rainfall intensity and the diameter of the raindrop,but decreased with an increase of PAM application rates.Comparing with the complete-mixing depth model,the nutrients transport process was better described by the simulation effect of the proposed nutrient dynamic model.