Study On The Best Management Practices Of Land Use In Small Watershed Based On AnnAGNPS Model

The best management practices (BMPs) of land use in small watershed in Zhuanyaogou watershed, which is located in the middle reaches of the Yellow River, were studied by using AnnAGNPS model. This study will not only help to transform an innovative thinking in the comprehensive management of small watershed in the middle reaches of the Yellow River but it will also have an important theoretical and practical significance in coordinating the contradiction between land use and ecological protection in loess plateau to achieve sustainable land development.From the theoretical and practical point of view, we designed seven kinds of land-use scenarios of returning arable land and unused land to forests to adjust land use types based on land slope. Four kinds of results came from the simulation:the present status of 2012, situations of runoff, sediment, nitrogen and phosphorus erosion in each scenario. Evaluating the outcome from the aspects of total quantity and grade distribution for each scenario, we made a comprehensive comparison and assessment under these seven scenarios and present status, and came to the main conclusions:(1)Through simulation and analysis of the land use status in 2012, it can be seen that the average annual runoff in the watershed is 26.20mm, the annual average sediment erosion intensity 3695.13t/km2·a, the average annual nitrogen erosion intensity 1465.04kg/km2·a, and the average annual phosphorus erosion intensity 1795.33kg/km2·a. Moderate and above erosion areas of runoff, sediment, nitrogen and phosphorus are small, but are of a large proportion.Average runoff in resident, loessgul and scabland is above the mean value; average erosion modulus of sediment, nitrogen and phosphorus in loessgul and abandoned land is higher than mean value, all at least severe level. Therefore, optimizing land use should be based on the erosion space, selecting loessgul, abandoned land and sloping land as the key areas.(2)In terms of the total amount of runoff and erosion, the output for each scenario was lower than that of the land use status in 2012. And what’s more, as the forest scenarios were designed based on the previous one, the output of each scenario was relatively lower than the previous one. Magnitude changes as scenario 3 and 4 had, while scenario 5-7 had fewer changes and closed to the results of the fourth one, which was mainly for the reason that the land use types modified in the last three scenarios were designed to take up a small proportion of the total area.(3)In the sight of runoff and erosion grades, the area proportion of medium and above runoff was decreased in sequence, resulting in corresponding decrease in the proportion of runoff.Particularly the largest one changed into the smallest one. The area proportion of medium and above erosion intensity of sediment decreased in sequence, and corresponding decrease was also in proportion of erosion--the largest one reduced to a small one.Areas of the strongest ones were decreasing successively. Apparently the quantity of sediment decreased as well. Nitrogen and phosphorus erosion areas in severe and above places became smaller and smaller one by one and the largest and larger ones were turning into small ones.Besides, severe and above erosion reduced in sequence as well. Four kinds of output were extremely obvious in scenario 3and 4.(4)By comparing the output of the seven scenarios, allowing for soil loss in loess plateau and Demanding for food proved in previous studies, we chose scenario 4 as the best land-use management practices at last after referring to the current demand for arable land. That is to return slope and arable land(more than 15°)and unused land(more than 25°)to forests. The arable land area is 308.79 hectares at last, decreasing by 22.59% compared with the present situation. Forest land increases by 61.19% and unused land decreases by 38.60%. The output of runoff, sediment, nitrogen and phosphorus reduces by 61.03%,92.68%,89.15% and 81.27% respectively in comparison with present situations.In respect of grade distributions, we’d better decrease mean grades without exception, make runoff come to a very small level and sediment erosion mainly below the strong degree and give nitrogen and phosphorus erosion priority to a slight degree.