Phosphorus Losses Risk Assessment Of The Chakou Small Watershed Based On Ann-AGNPS Model

The Chakou small watershed is a typical loess hilly-gully region, the widespread attention about nutrient loss is caused by soil erosion has been received, phosphorus is one of the main elements in soil nutrient loss, it’s of great importance that research the phosphorus losses in loess hilly-gully region. This paper was the simulation forecast the phosphorus loss loading based on the non-point source pollution Ann-AGNPS model for Chakou small watershed, this paper analyzed the phosphorus loss risk rule at the time scales and the space scales in the watershed.Firstly, the main conclusion on phosphorus loss risk assessment at the time scale is as follow:(1) The yearly average phosphorus loss loading has significant correlation with the yearly average runoff in the watershed;(2) Every year, the main period of phosphorus loss was from June to September, accounted for99.63%in the total phosphorus loss, and the most serious period of phosphorus loss was in July, accounted for76.53%in the total phosphorus loss;(3) In the single rainfall event, phosphorus loss was given priority to with the particulate phosphorus, compared with the runoff, the presence of the phosphorus loss peak lagged. In addition, the average contents of the dissolved phosphorus in the runoff was0.12μg/mL,more than the critical concentration of phosphorus of lake eutrophication(0.02μg/mL), therefore, the phosphorus loss had caused the water eutrophication in the watershed.Secondly, the main conclusion on phosphorus loss risk assessment at the space scale is as follow:On the spatial distribution, the phosphorus loss load of the southeast in the watershed was higher than that of the northwest, additionally, the phosphorus loss was more serious in the middle-south of the small watershed. The total phosphorus loss load was similar to the particulate phosphorus loss load in certain extent.(1) In the watershed, the grassland was the main land use types caused phosphorus loss and its phosphorus loss contribution rate reached78.46%. The phosphorus loss load per unit area was in turn:the residential area> water area> cultivated land> grassland> orchard> forest land;(2) The influence of different soil species on the phosphorus loss was in the sand and silt content of the soil, according to the results of the analysis, it was concluded that the medium thickness loess gray brown soil texture was the most important species caused phosphorus loss;(3) The increase in the vegetation coverage, significantly reduced the total phosphorus loss load, with the reduction of the particulate phosphorus was the most obvious. In forest,the vegetation coverage was increased respectively by6.17%,16.41%,16.41%,the particulate phosphorus loss load in turn reduced96.20%,97.95%,95.61%; the dissolved phosphorus loss load in turn reduced0.00%10.71%10.71%; Total phosphorus loss load in turn reduced88.92%,91.35%,91.35%. It showed that the vegetation coverage effectively reduced the loss of the particulate phosphorus;(4) In the same Land use, the more increased the slope, the more increased the phosphorus loss load, phosphorus loss load began to decline after reaching the critical slope. Through the research, the critical slope caused the phosphorus loss was between the8and15degree in the Chakou small watershed, including the phosphorus loss largest load per unit area in the slope was35.24kg/(hm2a)-1.