Hydrological Responses To Land Use Change And Climate Variability In The Chaohe Watershed, Beijing, China

Intensive climate changes and human activities have increasing effects on water environment and water resource in Northern China. Going with climate changes, it is important to carry on reasonable land use for utilization of water and soil resource, ecological restoration and adaptive watershed management. In this study, we analyzed hydrological responses of land use change and climate variability in Chaohe watershed located in up Miyun Reservoir. Results of the study can be theoretical basis for improving water quantity and quality by using adaptive watershed management in Northern China. The trend of the time series (1961-2009) and change points of climate in terms of precipitation and potential evapotranspiration (PET) and annual, wet season, and dry season streamflow of Chaohe watershed were examined by using the non-parametric Mann-Kendall test. In addition, we analyzed the changes of land use over the period. A method based on water balance was used to quantify the respective contribution of land use and climate changes to mean annual streamflows. Linear regression model was used to examine their effects on sediment yield reduction. Under different scenarios of climate variabilities and landuse changes, SWAT model was used to simulate and predict their effects on water yield and sediment yield. The main results are as follows:(1) From 1979 to 2009, the typical characters of land use changes were that woodland, shrubland and urban increased rapidly while grassland and farmland decreased in Chaohe watershed. Grasslands transferred to scrublands and woodlands; scrublands transferred to woodlands; farmlands were transferred to grasslands. Different policies and intensive human activities such as Grain for Green and ecological restoration lead to land use changes.(2) By using Mann-Kendll test and change point test for climate, streamflow and sediment yield in Chaohe watershed, results showed that:there were no significant changes in annual and wet season precipitation and PET from 1961 to 2009. In contrast, the annual and wet season streamflow decreased significantly from 1961 to 2009 with the change points occurred in 1999. In dry season, streamflow declined significantly going with precipitation and PET increasing significantly, and change points happened in 1999,1979 and 1987 respectively. Annual and wet season sediment yield both declined significantly and changed point occurred in 2004. It was estimated the land use/cover change contributed for 56.1% reduction in mean annual streamflow, while the rest was explained by climate variability. Land use changes overtook 95.8% of reduction for the declining of annual sediment yield. We concluded that the effect of land use change on reduction of water resources and sediment yield outweighed that from climate variability. (3) SWAT model can be suitable for use in Chaohe watershed. The simulated results of SWAT model showed that runoff depth gradually declined from southeast to northwest, meantime higher elevation, less runoff. Spatial distribution of runoff depth was mainly decided by spatial distribution of precipitation. In addition, land use played an important role on spatial distribution of runoff depth. Spatial distribution of sediment yield mainly has been affected by precipitation, terrain and land use.(4) Under extreme scenarios of land use, the value of runoff of different land use followed the order of shrub land>grassland>woodland>farmland. Compared with land use in 1999, shrub land and grassland could increase 158.2% and 4.1% runoff, respectively, when farmland and woodland declined runoff 41.7% and 23.7%, respectively. Under extreme scenarios of land use, annual sediment yield followed the order of farmland>grassland>shrub land> woodland. Compared with land use of 1999, farmland increased sediment yield 85.9%, while grassland, shrubland and woodland decreased sediment yield 32.1%,45.4% and 57.0%, respectively.(5) The simulated results of scenarios of climate variability on water and sediment yield showed that runoff was more sensitive to precipitation than to temperature. Precipitation increased 10% while runoff increased 23.9%. But 1℃increase of temperature lead to 6% reduction of runoff. Sediment yield was more sensitive to precipitation than runoff to precipitation. Precipitation increased 10% while sediment yield increased 44%。(6)The aim of watershed management is to supply more water and control soil erosion in Chaohe watershed as a water resource field for Miyun Reservoir, so we designed spatial optimized land use scenario based on extreme scenarios and characters of land use with different terrain. Compared with landuse of 1999, both farmland area and woodland area decreased in optimized scenario, when shrubland and grassland increased. Results of runoff depth and sediment concentration in optimized scenario showed that runoff depth increased 34.54% while sediment concentration declined 14.16%. Besides precipitation, both land use and terrain affected the distribution of runoff depth. Runoff played a domain effect on the distribution of sediment concentration in research watershed.