Evaluation of ecological flow with considerations of hydrological alterations in the Poyang Lake basin

Abstract

Fluvial hydrological processes within the Poyang Lake basin are significantly altered under the influence of human activities and climate changes. Poyang Lake is the largest freshwater lake and plays an important role in conservation of biological diversity and also in flood mitigation in the Yangtze River basin. In this case, ecological instream streamflow is also altered and the ecological environment is potentially affected under the influences of altered hydrological processes. With consideration of hydrological alterations, re-evaluation of ecological instream streamflow will be of great scientific and practical merit in terms of water resource management and conservation of ecological environment. We use the Hurst coefficient method to preliminary analyze the degree of alterations in five major tributaries of the Poyang lake basin. Then we use eight mutation testing methods to comprehensively investigate change points. We use 15 kinds of probability distribution functions to fit respectively daily flow for each month before occurrence of the hydrological alterations. Finally, the monthly optimal distribution functions and corresponding streamflow with the largest probability are determined and computed, and accepted as the ecological instream streamflow. Results indicate that: (1) main causes behind the hydrological alterations of the Gan, Xin, Rao and Xiu Rivers are the climate changes such as the precipitation changes observed in this study. However, the Hu River is mainly affected by human activities; (2) due to large amounts of water diversion irrigation system, hydrological processes in the Hu River were significantly altered after 1962. The hydrological alterations of the Gan and Xiu Rivers occurred in 1968. The East Asian Summer Monsoon led to anincrease of precipitation during the mid-1960s; The Xin, Rao Rivers were dominated by significant hydrological alterations in 1991. Increase of precipitation in the early 1990s is the main cause behind this hydrological alterations that occurred in the 1960s;(3) The increase of forest coverage and the construction of water conservancy are expected to improve the satisfaction rate of the ecological water requirement in the dry season. The satisfactory rate of ecological water requirement in the dry season is higher than that in the wet season in the Rao, Gan, and Xiu Rivers. However, the Fu and Xin Rivers do not follow this trend. The forestation and increased vegetation coverage can greatly alter the spatiotemporal distribution of water or runoff and can significantly increase the runoff during the dry seasons. Woodland has an important influence on the changes of runoff. Due to the large forest coverage, the ecological instream flow is greatly satisfied during dry seasons in the Rao River when compared to the other four rivers. A difference is found in the Gan River in terms of water requirements of ecological instream flow between wet and dry seasons. The number of reservoirs in the Gan River basin is high when compared to the other four tributaries of the Poyang Lake basin that were considered in this study. Forest coverage in the Xiu River basin accounts for 64.4% of the total area, which is ranked the second largest in the Poyang Lake. However, the difference between wet and dry seasons is small. The quantity and scale of the reservoir in the Xiu River basin falls far behind others. Therefore, the regulation activities of the reservoirs in Xiu River basin is not evident than other river basins. On the contrary, the satisfactory rate of ecological water requirements for the dry season is below that for the wet season in the Fu and Gan River. The rate of forest coverage in those two basins is far less than in other basins. The ability to regulate the forest is limited. These results will provide an important scientific basis for the planning and management of water resources within the Poyang Lake basin under a changing environment.