Spatial distribution of transfer pollution absorbed phosphorus load in slope farmland of purple soil in Three Gorges Reservoir Region

Adsorbed nonpoint source (NPS) pollution in the Three Gorges Reservoir Region, China has been increasingly recognized as a threat to aquatic environment in recent years due to the eutrophication problem of its tributary. The origin of the adsorbed NPS pollution is soil erosion. The Three Gorges Reservoir receives a significant sediment yield from soil erosion, and slope farmland is its main source. Many water and soil conservation measures have been suggested for controlling soil erosion and NPS pollution in slope farmland. Computer-based modeling is considered as a cost-effective tool for predicting the effect of measures prior to their implementation. Therefore, in this study, based on the topographic index, the spatially distributed equation of pollutant export coefficient was proposed to study the spatial distribution characteristics of absorbed phosphorus load of slope farmland in the Three Gorges Reservoir Region and predict abatement effects of new cultivation modes on pollution load in the entire Three Gorges Reservoir Region. Then the model composed of absorbed pollution load model with improved pollutant export coefficient and RUSLE was constructed and the simulation results of the model were verified. Based on this, the spatial distributions of absorbed phosphorus load from 2001 t0 2010 in the Three Gorges Reservoir Region were simulated by applying the model and GIS technology. The results showed that spatial distributions of absorbed phosphorus load were very obvious in the Three Gorges Reservoir Region and the regions of Kaixian, Yunyang, Fengjie and Zigui produced more absorbed phosphorus loads than the other regions. Furthermore, the model was also applied to predict the effect of new cultivation mode, that is, the mode of large cross slope combined with small along slope and mode of ridge combined with hedgerow, on reducing the absorbed phosphorus load of purple soil in the Three Gorges Reservoir Region. Under climatic and land use condition from 2001 to 2010 in the Three Gorges Reservoir Region, for slope farmland with purple, average absorbed phosphorus load for traditional farming technology was 15 309.70 t/a, unit grids (1 km2) absorbed phosphorus load was 1.62 t/km2. Compared with the traditional mode of farming, the absorbed phosphorus pollution load for the mode of large cross slope combined with small along slope and ridge combined with hedgerow, has reduced to 9 032.72 t/a and 5 511.49 t/a, unit grids (1 km2) , absorbed phosphorus pollution load was 0.96 and 0.58 t/ km2. The absorbed phosphorus load would be decreased by 41% for the mode of large cross slope combined with small along slope or 64% for the mode of ridge combined with hedgerow if the mode was used.