The Impacts of Climate Variability and Land Use Change on Streamflow in the Hailiutu River Basin

The Hailiutu River basin is a typical semi-arid wind sandy grass shoal watershed in northwest China. Climate and land use have changed significantly during the period 1970-2014. These changes are expected to impact hydrological processes in the basin. The Mann-Kendall (MK) test and sequential t-test analysis of the regime shift method were used to detect the trend and shifts of the hydrometeorological time series. Based on the analyzed results, seven scenarios were developed by combining different land use and/or climate situations. The Soil Water Assessment Tool (SWAT) model was applied to analyze the impacts of climate variability and land use change on the values of the hydrological components. The China Meteorological Assimilation Driving Datasets for the SWAT model (CMADS) was applied to enhance the spatial expressiveness of precipitation data in the study area during the period 2008-2014. Rather than solely using observed precipitation or CMADS precipitation, the precipitation values of CMADS and the observed precipitation values were combined to drive the SWAT model for better simulation results. From the trend analysis, the annual streamflow and wind speed showed a significant downward trend. No significant trend was found for the annual precipitation series; however, the temperature series showed upward trends. With the change point analysis, the whole study period was divided into three sub-periods (1970-1985, 1986-2000, and 2001-2014). The annual precipitation, mean wind speed, and average temperature values were 316 mm, 2.62 m/s, and 7.9 degrees C, respectively, for the sub-period 1970-1985, 272 mm, 2.58 m/s, and 8.4 degrees C, respectively, for the sub-period 1986-2000, and 391 mm, 2.2 m/s, and 9.35 degrees C, respectively, for the sub-period 2001-2014. The simulated mean annual streamflow was 35.09 mm/year during the period 1970-1985. Considering the impact of the climate variability, the simulated mean annual streamflow values were 32.94 mm/year (1986-2000) and 36.78 mm/year (2001-2014). Compared to the period 1970-1985, the simulated mean annual streamflow reduced by 2.15 mm/year for the period 1986-2000 and increased by 1.69 mm/year for the period 2001-2014. The main variations of land use from 1970 to 2014 were the increased area of shrub and grass land and decreased area of sandy land. In the simulation it was shown that these changes caused the mean annual streamflow to decrease by 0.23 mm/year and 0.68 mm/year during the periods 1986-2000 and 2001-2014, respectively. Thus, the impact of climate variability on the streamflow was more profound than that of land use change. Under the impact of coupled climate variability and land use change, the mean annual streamflow decreased by 2.45 mm/year during the period 1986-2000, and the contribution of this variation to the decrease in observed streamflow was 27.8%. For the period 2001-2014, the combined climate variability and land use change resulted in an increase of 0.84 mm/year in annual streamflow. The results obtained in this study could provide guidance for water resource management and planning in the Erdos plateau.