Responses Of Runoff And Sediment To Land Use Cover Changes In The Upper Reaches Of Beiluo River Based On Swat Model

The runoff and sediment is a comprehensive index of watershed hydrological processes. Under the combined effect of climate change and human activities, the runoff and sediment discharge in the Yellow River has decreased in recent decades. With the implementation of the Grain for Green Project, the land use and land cover in the Beiluo River have changed dramatically, and the runoff and sediment discharge have decreased dramatically as well. In this paper, the dynamics of climatic and hydrological regime of the upper resches of Beiluo River were investigated by mathematical statistics methods, based on the meteorological and hydrological data from 1971 to 2012 and three periods of land utilization data(1990, 2005, 2010). In order to improve the applicable method to estimate runoff in the upstream of Beiluo River, the Soil Conservation Service Curve Number(SCS-CN) was optimized and validated by rain simulation experiment. Then, based on the Soil and Water Assessment Tool(SWAT) to quantitativelyevaluate the impacts of climate and land use cover change on runoff and sediment-change. The results were shown as follows:(1)The land use and land cover conditions of the upper reaches of Beiluo River have changed significantly. Based on 1990, 2005 and 2010 land use data analysis results show that, with the implementation of the Grain for Green Project, cultivated land was continuously decreasedfrom 1483.95km2 to 929.43 km2, while the change rate is 37.37% from 1990 to 2005. Forest land was increased from 79.07km2 to 1025.88 km2, while the change rate is 1197.41% from 1990 to 2005(2)Theclimateshowed a trend ofwarming and dryingduring 1971-2012, which mainly showedno significant decrease in precipitation and significant increase in temperature. Precipitation has declined by-5.52mm/10 a, average temperature, maximum temperature and minimum temperature increased by 0.237℃/10a、0.202℃/10 a and 0.213℃/10 a. Break point analyses reveal that, more rapid increases for average temperature, maximum temperature and minimum temperature began in 1996, 1996, and 1988.(3)During 1971-2012 period, runoff and sedimentwere decreased significantly by-0.014 billion m3/10 a and-9.449 million t/10a(P<0.01). Break point analyses reveal that, there was anabrupt change occurred around 2002 for annual runoff and sediment.Comparingto the period of 1971-2001, runoff and sediment were decreased by41.5% and 79.6%, respectively, during 2002-2012, while the base flow increased from 34% to 51%.(4) Human activities were the primary factor of runoff and sediment reduction.Effects of rainfall and human activities on runoff and sediment were evaluated by double-mass curve analysis.It was shown that human activities were the primary factor of runoff and sediment reduction with a contribution rate of 86.27% and 97.07%,respectively.(5)Parameteroptimization and improvement of SCS-CN model in the upper reaches of Beiluo River. We modified standard SCS-CN model by rain-simulated experiment, and the resultswereλ=0.15 and 75 and 90 of CN of grassland and bare land.The optimizated SCS-CN model were more accurate for estimating runoff in the upstream of Beiluo River.The relative errorRe has dcreased from 34.38% and 20.91% to 0.50%和 2.06%.(6)Using observed value of runoff and sediment to simulate the annul and monthly runoff and sediment during 1992-1999 in the upper reaches of Beiluo River, relative error(|Re|) of the model was lower than 25%, determination coefficient(R2) and efficiency coefficient(Ens) were both above 0.65. It indicated that SWAT model can be applied to simulating runoff and sediment in the upstream of Beiluo River. Then, running model under the same climate circumstances, changes of land use pattern had a strong impact on watershed hydrological processes. Compared with observed data, the contribution rate of Land UseCover Change(LUCC) to runoff reduction in 2005 and 2010 were 47.32% and 48.61%, respectively. For the same scenarios, the contribution rates of LUCC in sediment reduction were 57.21% and 59.19%, respectively.