Impacts of Climate Change and Human Activities on the Three Gorges Reservoir Inflow
Identifying changes in runoff and quantifying the impacts of climate change and human activities are of great significance for water resources planning and management in a river basin. In this study, an inflow series of the Three Gorges Reservoir observed from 1951 to 2016 is used to identify the trend and abrupt change point by using statistical methods. Based on the meteorological data, soil type data, and land use data during the same period, the Soil and Water Assessment Tool (SWAT) model
... Tool (SWAT) model is established to quantitatively attribute changes in the Three Gorges Reservoir inflow to climate change and human activities separately and discuss the differences between the two-stage method, which divides the whole study period into two stages to analyze the reasons for runoff evolution, and multi-stage method, which divides the whole study period into more stages to consider the temporal and spatial variation of land use/cover (LULC). The results show: (1) During the study period, a significant decrease is detected in the Three Gorges Reservoir inflow and the decrease rate is 7.7 km 3 per ten years, annual total precipitation decreases by −13.5 mm per ten years, and annual average temperature increases by 0.1 • C per ten years. (2) Contribution of climate change and human activities is around 7:3. Climate change is the main reason for the decrease in the Three Gorges Reservoir inflow. (3) Results of stages in multi-stage method are different from the result of two-stage method. Accumulative results of multi-stage method and result of two-stage method are consistent. There are some changes in results of every stage, which are different from the accumulative results. Water 2017, 9, 957 2 of 14 activities, while other watersheds are the opposite. Reviewing extensive literature, quantitative methods used to assess impacts from climate change and human activities on runoff generally fall into two categories: statistical methods and simulated methods. Statistical methods are based on observed data, while simulated methods are based on various hydrological models, and have gone through a transformation from methods based on lumped hydrological models to methods based on distributed hydrological models. Statistical methods generally use regression to assess impacts on runoff based on observed meteorological data and hydrological data. The regression is established between runoff and one or more meteorological factors. For example, Revelle and Waggoner  used a multiple regression to assess impact of climate change on runoff in the upper reaches of the Colorado River. The climatic elasticity method, which was first proposed by Schaake  and continually extended, is another classic method. These statistical methods have some simple principles and are easy to use. However, these methods are based on, and limited to, the data series, because longer series bring better regression. Data uncertainty and number of driving factors used surely could influence the results. Apart from the impacts from climate change, the rest changes in runoff are attributed to human activities. Such methods have not explained in what way, how much, and how human activities factors influence the runoff, which greatly reduces the reliability of the analysis results. The hydrological processes are very complex and highly non-linear. Models based on the linear system theory generally fail to represent the non-linear processes  . During the history of hydrological models, many famous and excellent lumped hydrological models sprang up, for instance, Tank model , Stanford watershed model  , and Xinanjiang model  . These models consider the processes from runoff generation to runoff concentration and have a certain summary of specific ones including precipitation, evapotranspiration, infiltration, surface runoff, groundwater discharge, and so on. The lumped hydrological models often regard a watershed as a whole, and spatial variations are rarely taken into account. There are many empirical formulas in the models which lack explicit physical meanings. Lumped hydrological models are used to assess impacts from climate change and human activities on runoff in various watersheds across the world because of simplicity [13, 14] . Compared to other hydrological models, distributed hydrological models can describe mechanisms of hydrological processes as well as spatial heterogeneity. With the help of reams of information obtained from Geographic Information System and Remote Sensing, distributed hydrological models are generally effective and accurate to simulate the hydrological processes. As a result, distributed hydrological models are more effective to assess impacts from climate change, LUCC and other human activities  . The Soil and Water Assessment Tool (SWAT) is a representative one. SWAT is a river basin or watershed model that has been widely used to predict flow, sediment, and agricultural chemical yields from watersheds of various sizes, particularly in large, complex watersheds. It is the most vigorous method for simulating the long-term impacts of watershed management (e.g., dam building, timber harvesting, and irrigation) on hydrologic process  . In China, economy is booming. The great developments in agriculture and industry, together with the climate change, have caused varying degrees of changes in hydrological processes  . The Three Gorges Reservoir is currently the largest water conservancy project in the world and attracts flocks of scholars and researchers. Runoff in the upstream Yangtze River flows into the Three Gorges Reservoir in the form of reservoir inflow finally. Great changes have been observed in the Three Gorges Reservoir inflow  . However, limited studies have been conducted to seek the specific reasons and assess the impacts. This study selects the changing Three Gorges Reservoir inflow as the study case and assesses the impacts from climate change and human activities on it by using SWAT. In previous studies, when assessing impacts on runoff via the SWAT, land use/cover (LULC) is considered stable during a certain period, and one piece of LULC map is enough to represent the period [16, 19] . However, it is always being blurred and muddled how long the period should be. The two-stage method and the multi-stage method are two parties holding different opinions. Both methods are based on the premise that LULC remains stable during a certain period. Divergence between the two is how long the certain Water 2017, 9, 957 3 of 14 period continues. LULC is considered stable during a long period, while the stable condition lasts shorter in the multi-stage method. Both methods are discussed in this study. Study Area and Data The Three Gorges Reservoir is located at the mainstream of the Yangtze River, in Yichang City, Hubei Province. It has a total storage of 3.93 × 10 10 m 3 and a total installed capacity for the hydraulic turbines of 2.25 × 10 7 kW. It is a multi-purpose water conservancy project, including flood protection, electric power generation, shipping, water supply, recreation, and ecological protection. In this study, the upstream Yangtze River is selected as the study area. As shown in Figure 1 , the study area is located in the region within 97.37 • E-110.18 • E and 21.13 • N-34.33 • N, with a total area of 1 × 10 6 km 2 covering nine provinces. The elevation of study area ranges from −22 to 7143 m.