Uncertainty Analysis Of AnnAGNPS Model Parameters In Chakou Watershed

With the effective control of point source pollution,non-point source pollution has become the primary factor causing environmental pollution and is the key to the current environmental pollution control.The AnnAGNPS model is one of the most widely used and successful models in many non-point source pollution models.It is an important tool for quantifying and managing non-point source pollution.The model needs many parameters and the structure is complex.The inaccuracy of parameters and input information will directly lead to the uncertainty of the model for non-point source pollution simulation.The uncertainty analysis of AnnAGNPS is a process to identify the source of the model uncertainty and measure of uncertainty.The uncertainty of the model mainly contains three aspects:the uncertainty of the model's own structure,the uncertainty of the model parameters,and the uncertainty of the model's input information.It is difficult to accurately quantify the uncertainty of the model's own structure.Generally,the uncertainty is indirectly reduced by improving the structure in the development of the model or comparing model selection.Therefore,how to accurately quantify the contribution of parameters to the uncertainty of each output variable of the model becomes one of the urgent problems to be solved.Chakou watershed was located in the typical loess hilly-gully region,and was selected as study area.In this study,ten parameters of AnnAGNPS model was selected and Morris classification method was used to analyze the sensitivity of the parameters to the simulation results of the model.The uncertainty analysis method of FOEA(first-order error analysis)and Bootstrap were adopted to identify important uncertain parameters and evaluate the contribution of parameters to the uncertainty of the model output variables.The following results were obtained.(1)The number of runoff curves have great positive influence on model output;Channel Manning coefficient and field water-holding capacity have negative influence on model output.The most sensitive parameters which affecting the surface runoff output are the number of runoff curves and the field capacity.The most sensitive parameters which affecting the sediment and nutrient output are the number of runoff curves,soil erodibility factor,tillage management factor,topographic factor,soil and water conservation factor.(2)The model can better reflects the surface runoff process(R2= 0.99,Ens= 0.99).The simulation errors of sediment,nitrogen and phosphorus are larger,but to some extent,it can reflects the trend of soil and water loss and nitrogen and phosphorus pollution,and has certain reference value to control and manage soil erosion and non-point source pollution(3)The number of runoff curve and field water holding capacity had great influence on the uncertainty of runoff simulation.The number of runoff curve,terrain factor,soil erodibility factor,tillage management factor,soil and water conservation factor and chemical fertilizer application amount had great influence on the uncertainty of sediment,nitrogen and phosphorus load calculation(4)The number of runoff curve had the most significant effect on the uncertainty of the model output,which indicated that the runoff generation process was the driving force of erosion and transportation,affecting sediment and the runoff of various nutrients.The uncertain parameters of sediment and nitrogen and phosphorus were consistent,which showed that the sediment was the carrier of nitrogen and phosphorus output.In short rainstorm events,the forms of nitrogen and phosphorus output were drastically dominated by sediment bound state,and the uncertainty of sediment directly affected the accuracy of nitrogen and phosphorus output.(5)The uncertainty of the sediment in the model output was the largest and the runoff uncertainty was the smallest.Based on the results,it showed that the simulation of hydrological and soil erosion processes were the key to model calibration and the uncertainty reduction.In order to control and manage soil erosion and non-point source pollution,the number of runoff curve,fertilizer measures,farmland management and soil and water conservation should be considered.