Impacts Of Small Watershed Environmental Changes On Hydrological Processes In The Yangtze River Delta Region

For recent years, the global climate is changing with main characters of warming and extreme weather events. Natural ecological system and social economy development is suffering significant influence. Meanwhile, industrialization has accelerating urbanization and made impervious area increasing. Furthermore, forest, farmland and other land use/land cover changes caused by human activities, and the construction of water conservancy projects, also has far-reaching influence on nature and society. "Water cycle research in the changing environment" is a core scientific problem in the global water system plan (GWSP). Under the background of environmental changes, temperature increased significantly, but pan evaporation reduced obviously. Water problems such as frequent drought and flood disasters, water quality deterioration, make the conflict between supply and demand of water acute.The Yangtze River Delta is one of the most developed regions in China. With the expansion of the urban areas, the land use changed a lot. At the same time, climate is changing. Flood/drought disasters, water resources and water environment problems are growing and affecting people's lives and property, so it is imperative and significant to study the impacts of the regional environmental changes on hydrological processes. Most rivers in this region are located in plain river network area, and flow is controlled artificially. So the upstream of Hengtangcun hydrological station in Xitiaoxi watershed is chosen as the study area, where the long-term hydrological data and meteorological data provide an opportunity to assess the impact of environmental changes on hydrological processes.The study is supported by related national natural science foundation, and commonweal and specialized programs for scientific research, Ministry of Water Resources of China, hydrological processes were observed, simulated and analyzed. This main technique route "drive-process-impact" is through the full article. Mathematical statistical procedures, geographical information system, watershed hydrological models, historical data and experimental data, were used to qualitatively and quantitatively investigate impacts of environmental changes including natural changes and human activities on hydrology processes. The case study can enrich water cycle research in the changing environment and provide a useful tool for regional flood control and drought disaster reduction, as well as water and land resources management and sustainable development in this region.Based on the analysis of environmental changes, hydrology experiment and hydrological models were applied to reveal the impacts of environmental changes on the hydrological processes. Firstly, Environmental changes are analyzed. Then experiment study on a small urbanized watershed and comparative watersheds with different underlying surfaces is carried out. Impacts of human activities, especially land use change on small watershed storm-runoff processes are simulated and analyzed. The hydrological model was set up and used to simulate runoff under different scenarios of environmental elements combination. Using the new proposed variables separation analysis method, the influence of each main environmental element on runoff is calculated. Impacts of environmental changes on water quality are also analyzed. The main results are as follows:(1) The mean annual temperature in the study area is increasing from 1965 to 2007, and the increasing rate is far higher than that of the world. Since 1975 the mean annual temperature shows obvious increasing. Since 1996 the trend has increased at a significant level. The precipitation has no significant changing trend during the whole period 1967-2007. But divided the series into two parts, the first half is increasing and the second decreasing. In 2002 decreasing mutation occurred in the precipitation time series. The pan evaporation is decreasing from 1969 to 2007, and the trend tends to be obvious from 1984. Decreasing mutation occurred in 1988. From 1985 to 1996 and from 1996 to 2002, the main land use transfer types were both farmland to forest and forest to farmland. However, the total area of farmland and forest changed little as a result of approximatively conversion area. Overall, the hydrologic environment changed a lot in the study area.(2) Hydrology experiment study on a small urbanized watershed and comparative watersheds with different underlying surfaces is carried out in the study area. Storm Water Management Model is used to simulate and analyze several selected typical storm-runoff events in the small urbanized watershed. The result shows a decrease in lag times, increase in storm flow volumes and peak flows, which has greatly increased the pressure on flood prevention. The study of comparative watersheds with different underlying land use shows the same results. Through the analysis of the sample results of water quality, the distribution law of water quality indexes including ammonia nitrogen, total nitrogen, nitrate, potassium permanganate index and total phosphorus in different land use types and rainfall conditions is understood. With experiment continued, more data and analysis will further the research.(3) The study area is divided into 20 subwatersheds based on SWAT2009 according to the 20 water quality monitoring sections, and then each subwatershed is divided into hydrological response units. Meteorological database and soil database are set up. According to the land use maps of 1985,1996 and 2002, the study period is divided into 3 parts:1978-1987,1988-1997 and 1998-2007. Using historical data, the data of the experiments and the CUP program, selected parameters of the model are calculated or calibrated. Then monthly runoff is simulated, and the applicability of the model evaluated. The values of Deterministic Coefficient and Nash-Suttcliffe coefficient show its well applicability to investigate the impacts of environmental changes on long-term hydrological processes in the study area.(4) Analysis of water quality index indicates that water quality condition is good in the study area. The water quality of sections in the densely populated areas is poor. Several sections near farmland had higher nitrate as a result of nitrogen fertilizer utilization. Potassium permanganate index fluctuated before 2002 and had dropped stably after 2002. Dissolved oxygen in the study area was increasing from 2002. Ammonia nitrogen began to decline slightly since 1998. The above reflects the water quality had become better. Nitrate was increasing before 2002 because of overuse of nitrogen fertilizer year by year. As for monthly distribution, total nitrogen was decreasing, while index values of potassium permanganate index, biochemical oxygen demand after 5 days, ammonia nitrogen and total phosphorus were the highest in May, which was caused by intensive use of fertilizer. Dissolved oxygen in July and September was lower, as a result of continuing high-temperature, typhoon or non-point source washed out during heavy rains.(5) For annual runoff series of 1972-2009, the value is decreasing from 1994, and the trend tends to be obvious since 2006. The interannual variability is obvious. Based on the simulated runoff by the established model, the new proposed variables separation analysis method, was used to study the impact of each main environmental element on runoff. From the scenario COM03 to OBS02, the impact of land use change, climate change and operation of reservoirs on the runoff is-0.02% to-0.07%;-1.45% to-1.46% and -0.61% to -0.66%, respectively. The total impact is -2.13%, with the contribution of the three factors 0.94% to 3.29%,68.08% to 68.54%,28.64% to 30.99%, respectively. From the scenario COM06 to OBS03, the impact of land use change, climate change and operation of reservoirs on the runoff is -0.18% to -0.37%; -10.94% to -11.14% and-7.70% to -7.76%, respectively. The total impact is -19.07%, with the contribution of the three factors 0.94% to 1.94%,57.37% to 58.42%,40.38% to 40.69%, respectively. In addition, the two reservoirs played a significant regulatory role in monthly runoff distribution.