Quantitative Evaluation Of Soil Erosion And Nutrients Loss For Small Watershed Of Loess Plateau Based On AnnAGNPS Model

Soil and water loss are a global environment problem. They lead not only to soil erosion and land productivity reduction but also severely destroy the ecological environment, and erosive sands fill up in reservoir, impoundment and river bed and heavily effect reservoir volume, ship transportation and so on. On the other hand, through soil and water loss, water environment has deteriorated because a variety of nutrient and pollutants are deposited into rivers, lakes, and coastal waters or introduced into ground water.Based on 3S (GIS, Remote Sensing, GPS) technology, the Annualized Agricultural Non-Point Source Pollution model (AnnAGNPS) was applied to evaluate runoff, soil loss and nutrient loss from Zhuanyaogou watershed (29.16km²) on the Loess Plateau, China. The study provide basis for the control of soil and watershed loss and non-point source pollution.The database of AnnAGNPS model was set up in Zhuanyaogou watershed by analyses of rainfall characteristics, segmentation of simulating cell, translating of soil relative parameters, translating aerial photograph, characteristic of land use management, and so on.The hydrology and water quality of runoff events were measured on gauging station and the runoff plots in the watershed and the rules of runoff, sediment, nitrogen and phosphorus load were analyzed.The sensitivity analysis investigated the runoff and sediment yield to changes in runoff curve number for different land use type, and the order of sensitivity was loess hilly land>bare rock>potato field>panic field>uncultivated land> Robinia pseudoacacia and Populus L. forestland>crude grass land>shrub land.Simulations were compared with measured values in order to test AnnAGNPS. The test results showed that AnnAGNPS adequately simulates long-term runoff and sediment yield. AnnAGNPS simulated runoff over a three-year period (1988-1989, 2004) was 110% of the measured total runoff, and simulated sediment yield over a three-year period (1988-1989, 2004) was 90% of the measured total sediment yield. The model is adequately used to quantitatively evaluate runoff loss and soil erosion for in hilly and gully regions on the loess plateau. AnnAGNPS simulated total nitrogen and total phosphorus load in 2004 was 58% and 108% of the measured total nitrogen and total phosphorus load respectively. The enor of simulated total nitrogen was larger, but it was still within error range. The model may simulate nitrogen and phosphorus loss for in hilly and gully regions on the loess plateau.After AnnAGNPS test, the spatial distribution of soil erosion and nutrient loss was simulated for the subwatershed cell of different land use scenarios in Zhuanyaogou watershed. The degree of spatio-temporal variability of runoff, soil erosion, total nitrogen and total phosphorus and the capability of environmental attributes to predict that spatial variability were studied. The results show that average runoff was 23.4mm·yr^-1 and 22.4mm·yr^-1, respectively, average soil erosion intensity was 3590Mg· km^-2·yr^-1 and 3512 Mg·km^-2·yr^-1, respectively, average total nitrogen erosion intensity was 1295 kg· km^-2·yr^-1 and 1319 kg·km·^-2·yr^-1, respectively, and average total phosphorus erosion intensity was 1748kg'km^'yr"1 and 1767 kg*km"2|yr"\ respectively for land use scenarios in 1987 and 2004. The runoff, soil erosion, nitrogen erosion and phosphorus erosion present obvious difference among different land use types. Loess hilly land is distinctly higher than other land use types in average soil erosion intensity, total nitrogen erosion intensity, and total phosphorus erosion intensity of loess hilly land, and bring into soil, total nitrogen, and total phosphorus erosion yield are about 90% of erosion yield. The runoff of loess gully land, bare rocks and resident was bigger than that of slope farmland, dam land and terrace. Because land use is affected by topography index of slopes, aspects elevations and so on, soil loss and nutrient loss have distinct topography difference.The risk region of soil erosion was identified. The risk region covers 36.65% (1987) and 35.63%(2004) of the watershed, and average soil erosion intensity are 9663 Mg'km^'yr"1 (1987) and 9670 Mg*km^<sup>*yr (2004), and mainly distributes on loess gully land and uncultivated land. Because of the intense bed and bank erosion, the reach presents the soil erosion risk. Based on the soil erosion region, the land use scenarios are planned. The result showed that average soil erosion intensity of the watershed decrease to 64.51 % to 2004, and warping dam benefited from blocking water and sand, with the result that soil erosion risk obviously descend.AnnAGNPS model is successfully applied to research and control soil and water loss in hilly and gully regions on the loess plateau, and is a useful tool for optimal watershed management, which is used for reference in other comparable areas.