Impacts Of Climate Change And Human Activities On The Hydrological Processes In The Tao River Basin

Climate change and human activities are the two major driving factors that deeply influence the terrestrial processes. Separation of their individual effects is of great importance for water resources management and social sustainable development. In this study, we selected the Tao River Basin (TRB) as the study area. Along with the understanding of the geomorphological heterogeneity in the Loess Plateau and the Qinghai-Tibet Plateau (Gannan Plateau) in the TRB, we used methods of trend estimation and anomaly analysis to clarify the characteristics of hydro-meteorological elements in the area. And, methods of correlation analysis and PCA were adopted to determine the land use/land cover change (LULCC) with its driving facters in the diverse geo-ecological regions of the TRB. According to researches above, and based on the framework of the Soil and Water Assessment Tool (SWAT) we established the hydrologic model in the TRB. The Nash-Sutcliffe efficiency (NSE) and Relative coefficients (R2) were adopted to estimate the precision of the SWAT model’s outputs during the periods of calibration (1981-1995) and validation (1996-2010), by means of comparison between simulated and observed series on the three hydrological monitoring stations in the upper, middle and lower reaches of the TRB. Assisted with the calibrated model SWAT, we designed different simulation scenarioes to seperate influential contributions of climate change and human activities to basin hydrologic variations including runoff, evapotranspiration and soil moisture. Conclusions are summaried as the following.1) The air temperature significantly increased during 1951-2010 and the increasing rate in the loess area was higher than that in the Gannan Plateau in the TRB. The increasing of air temperature mainly occurred in cold seasons in the loess area, while in warm seasons in the Gannan Plateau. The precipitation variabilities were quite different in the two regions. There was an increasing trend in the Gannan Plateau and a decreasing trend in the loess area. The annual increasing and decreasing of precipitation in the two regions were mainly due to the increasing and decreasing in summer and autumn, respectively. Annual runoff was decreased in the both regions of the TRB. The decreasing rate in the Gannan Plateau was larger than that in the loess area. The runoff reduced in the most of monthes in a year, especially in warmer time in the two regions although there was a slight increase in colder time in the loess area.2) LULCC analysis in the TRB during 1981-2010 identified an overall decrease of forest land and grassland and an increase of the cultivated land, which indicated a general expansion of land utilization and the guradual intensity of human activities over the past 30 years, despite the obvious heterogeneities in time and space. Correlation and PCA analysis suggested that the TRB exhibits an interactive characteristic of a combined set of "regional economy + farming factors + urbanization and climate" driving mechanism. The vegetation activities were strengthened across the whole TRB, while the magnitude of increase in the Gannan Plateau was higher than that in the loess area, with significant heterogeneity in both time and space. Over the 30-year period, climate change led to the variations of vegetation growing season. There was a very high probability that germination was shifted earlier in both regions, while withering or dormant season was delayed in loess area. On the whole, regional vegetation dynamics in the TRB were due to the combined effects of climate change and human activities. Temperature increase contributed more to vegetation change than other factors. Human activities exerted trivial effects on vegetation dynamics, and more than 90% was by climate change.3) SWAT model was performed with satisfactary accuracies for relatively high value of NSE and R2 in calibration and validation periods and proved to be a powerful tool for assessing the hydrology variation due to the impacts of climate change and human activities. The outputs suggested that precipitation is concentrated in the upper reaches of Gannan plateau area and relatively rare in the loess region downstream. Affected by the regional water-heat condition and underlying surface coverage, the evapotranspiration consumption was largest in the mountain forest of middle reaches, second in the upper reaches, lowest in the area of downstream. Soil thickness in the middle and upper reaches are relatively small, soil water storage capacity (the unsaturated zone interception) is smaller than that the lower loess area where features relatively thick loess deposition. There was a decrease of runoff in the Gannan Plateau to some extent and a slight increasing of runoff in the loess area during 1981-2010.4) Influenced by the interaction of climate change and human activities, the hydrological sections (runoff, soil water contents and evapotranspiration) responded diffenently in the TRB. Climate change controlled and influenced the surface runoff more significantly than human activities, especially in the Gannan Plateau, although human activities’ impact presented a gradual intense in the whole TRB over the passed 30 years. There was a decrease of evapotranspiration in both the two regions, mainly due to the impact of climate change in 1990s and LULCC by human activities in 2000s. The soil water content variabilities were quite different in the two regions. Human activities resulted in a decrease in 1990s and climate change led to an increase in 2000s in the loess area. The decrease of soil water content in 1990s in the Gannan Plateau mainly caused by climate change although in 2000s, dominated by LULCC by human activities.