| Luoyugou watershed, located in the north suburb of Tianshui city, is the common boundary between Longxi Loess Plateau hill-ravine region and Longnan Mountains, which has remarkable representativeness and particularity in climate, terrain and soil erosion. The modelling research about space-time pattern of soil erosion in this watershed can help us not only understand the process and coupling mechanism between eco-hydrology and soil erosion, but also make optimized plans about water soil loss and ecosystem reconstruction. Using 20 years observation data of the watershed and GIS technique of statistics and spatial analysis, this research, which is based on USLE empirical model and WEPP physical model, simulates the space-time pattern of soil erosion, discusses the parameter evaluating method for different factors and the influence for simulating results, analyzes the reason of regional space-time pattern of soil erosion and brings forward the reference project about ecosystem reconstruction and water and soil loss. The main results of this research are:(1) Based on hydromechanics theory, this research propose a new precipitation eroding force index fit for this region - Precipitation Kinetic Energy Index and proves that this index is more effective than other traditional indices by correlated to sediment transferring amounts including three spatial scales of runoff plot, small watershed and middle watershed and three time scales of event, day and year. Precipitation Kinetic Energy Index is the fundamental driving force of soil erosion in this region.(2)Based on the precipitation variation characters with geography location and elevation, the regional precipitation kinetic energy field in flood season is generated monthly by GIS spatial interpolation technique. Calculating monthly correlation coefficient between precipitation kinetic energy at each points and sediment transferring amounts at the watershed outlet, the relative sensitivity of precipitation kinetic energy to sediment transfer at the outlet is expressed in each spatial points. The result shows that the spatial pattern of erosion sensitivity at each point is consistent with the erosion and intensity field observation and simulation of USLE and WEPP.(3)Using optimized algorithm and regarding each spatial unit in the watershed as sediment transfer limited sources, the monthly nonlinear plan equation between precipitation kinetic energy and sediment transferring ratio in the outlet is set up. The distribution map of sediment transfer contribution ratio is generated depending on the precipitation kinetic energy coefficient and accumulation value in the equation and shows that the region having the greater contribution for sediment transfer is located in the watershed middle reaches, spatially in uphill, which has some different conclusion with other methods.(4)When running USLE and its improvement versions, using GIS can quickly calculate terrain LS index and evaluate regional soil erosion. Changing the precipitation index in USLE to the precipitation kinetic energy index proposed by this research can improve the simulation precision.(5)The gully-edge can be recognized using several terrain indices and DEM. Based on the hypothesis that treats gully-edge as fore erosion terrain surface, the gully depth in each point can be calculated and treated as the gully erosion intensity index of geological period.(6)WEPP can generate good forecast result for general erosion events, but for intense rainstorm events the simulation result isn't stable. When accumulating the simulation results of each event to annually, the final result has certain commensurability by comparing with the observation data. So WEPP can be used for the space-time pattern research of multi-year precipitation erosion in Luoyugou watershed(7)The higher resolution of DEM, the better spatial resolution will be obtained in simulation results. But while consider in bigger watershed there are some difficulties for simulation with high resolution and large amount data. The experiment finds that the 22.5m resolution of DEM can be satisfied to the application of WEPP in middle watersheds. The division plan of watershed and the detailed degree of channel getting from DEM will influence the simulation result of sediment transfer in the water outlet.(8)The channel parameters have remarkable influence for simulation results, especially for watershed production of sediment and water. While for different channel ranks the sensitivity of production of sediment and water is different. In branch and tiny channel of first to third channel ranks in Qiaozidonggou watershed, the channel parameters have the most remarkable influence for watershed sediment transfer.(9)WEPP is very sensitive to the soil effective hydrology conductivity. On the base of effective hydrology conductivity calculated from the soil texture, according to the soil structure and land use, the adjustment of effective hydrology conductivity can improve the simulation precision.(10) The annual variation of soil erosion in Luoyugou watershed has four characters: gradualness, periodicity, weak tendency and sudden change. The soil erosion in this watershed has three remarkable developing stages and the gradualness shows that there is 6-8 year-long period. In intensive erosion variation period, the erosion has the three-year period. The main erosion process of this watershed is concentrated in June, July and August and happens in several days extreme rainfall events in flood season.(11)The spatial distribution of soil erosion in Luoyugou watershed has distinct regularity. The intensive erosion region is firstly located in the slope region of middle and lower reaches in the left-shore watershed and then in the mixed soil channel and tiny channel of right-shore watershed. In the common boundary between bedrock and loess/mixed soil region in the left shore there is a remarkable intensive erosion region which is also the high sensitive area of erosion time variation. Land use and slope are the fundamental factors to control spatial pattern of soil erosion in Luoyugou watershed and other factors such as soil has relative small functions.(12)According to the country policy about changing farming to nature plant and the land use situation of Luoyugou watershed, the three scene patterns of land use optimization are designed. The simulation of WEPP shows that, at the bedrock slope higher than 25 degree and mixed soil slope higher than 15-25 degree, the regions changing farming to nature plant have remarkable effect for preventing water soil loss. The design proposal of the land use can be the reference for the plans of changing farming to nature plant and ecology restoration in Luoyugou watershed.
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