| Land-cover/climatic changes and their impacts on hydrological processes are of widespread concern and a great challenge to researchers and policy makers. The Loess Plateau in China is confronted with severe eco-environmental issues, such as soil erosion and water shortage, resulting in extremely fragile ecosystem. Under the background of climatic variability, large-scaled Grain for Green Project, water and soil conservation measures have been carried out in this region, therefore these ecological restoration programs must bring great impacts on underlying surface of drainage area, so much attention has been paid to reveal the changing processes of runoff and sediment on the Loess Plateau. This paper took Jihe watershed (1019km2), located in the third sub-region of Loess Plateau, as study area, analysed the changing trends of climatic factors and runoff and sediment by the hydrometeorological data in 1962-2008; then discussed the effects of LUCC and climatic changes on hydrological process on the basis of five terms of land uses; afterwards, SWAT model was used to simulate the transport of runoff and sediment under different scenarios of land cover and climate. The objective was to offer some supports on preventing soil and water losses and vegetation establishment. The main conclusions were as follows:1) It was found that the land use structure has taken place huge changes after the five phases of land use maps. The watershed was dominated by sloping farmland and grassland in 1975, accounting for about 80% of the total, while terrace only had a scattered and small distribution; Large-scaled sloping farmland was converted to terrace fore-and-aft the 1990s, and it occupied about 33% of the watershed, the forest land kept steady but slow increase, but it presented clear increase after 2000, increasing by 21.9% compared with 1975, while grassland did not have obvious change. As a whole the forest and grass cover has increased a lot since 2000, showing that vegetation restoration performed very well in the watershed.2) The mean annual precipitation in Jihe watershed was 555.8mm in 1962-2008, presenting slightly decrease nearly the past 50 years, the linear trend coefficient was-3mm/10a, and it was found that it declined evidently in the 1990s, but it was not observed to produce significant trend change through Mann-Kendall test. While the mean, minimum and maximum temperature all increased markedly, and the linear trends of the three items were 0.37℃/10a,0.34℃/10a, 0.39℃/10a respectively, consequently it was estimated that the mean, minimum and maximum temperature moved up 1.74℃,1.83℃,1.60℃respectively during the research period, besides it was observed that the temperature rised distinctly in winter and spring; then MK and t test, combined with hop-parameter, were used to reveal the change point, and it showed the three change points occurred in 1993,1995,1997 respectively, then broke through the significance of a=0.05, indicating significant increasing trend in temperature.3) Runoff and sediment yield in the watershed decreased extremely in the past 47 years, especially had a sharp decline in the 1990s, only accounting for 30.2% and 21.3% of the 1960s respectively, and the variation coefficients reached 75.0% and 87.4% respectively; the two items both produced abrupt change in 1976, and further exceeded the significance of 0.05 in 1993 and 1977 respectively.4) According to linear regression method, precipitation change influencing the runoff reduction contributed 13.5% while human activities contributed 86.5% to runoff; meanwhile water balance method was also adopted to further show the contrast, it showed that the two influencing factor occupied 12.5%,.87.5% respectively, showing that the results of the two methods were consistent, however, it was different in different time, for example, climatic factor contributed 48.2% to runoff reduction in the 1990s. As for sediment yield, still two methods were used to quantify the impacts of rainoff change and human activity on sediment yield, that is, double cumulative curve of rainoff and sediment, and based on USLE, the results were 14.8%,85.2% and 13.5%,86.5%, thereinto, gully entrenchment measure such as dam build effected about 16% sediment reduction in the watershed, and it meant that land use changes effected about 70% sediment reduction in Jihe watershed.5) SWAT model was used to simulate runoff and sidement in the watershed, the results showed that the Nash-Sutcliffe model efficiency coefficient was about 0.7, and relative error was below 15%, coefficient of determination was more than 0.7 both for annual flow and sediment yield in calibration period, so it satisfied the simulating needs, however, it did not perform better in validation period than in calibration time; similarly simulations of monthly flow and sediment did better in calibration period. The simulated and observed values kept fine agreement in trend changes. Uncertainty analysis indicated that DEM resolutions and watershed subdivisions produced little influence on annual flow while obvious influences on annual sediment yield.6) Different land uses have different characteristics of runoff and sediment generation, as regards the main land uses in Jihe watershed, it presented the order of sloping cultivated land> pasture>forest land>terrace, and runoff and sediment yield in upper and middle reach were obviously higher than those in middle and lower reach. The responses of runoff and sediment yields to climatic changes and LUCC were simulated by SWAT with several scenarios, showing runoff was sensitive to rainfall, while the two were less sensitive to temperature rise. The process of runoff and sediment generation had remarkable responses to LUCC, especially for sediment transport, if sloping cultivated land were the only land use type in the watershed, runoff and sediment yield would increase by four and eight times; if the watershed were dominated by terrace or forest land, runoff would decrease by 70-80 percent, and sediment would decrease by about 90 percent, showing the two land uses possesses great effects of water and soil conservation.
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