Study On Water Cycle And Water Resources In Songhua River Basin Under The Changing Environment

The Songhua River Basin(SRB) is one of China’s most important commodity grain production bases. It is also the largest inland wetland region. In recent years, the characteristics of hydrologic cycle and water resources in the SRB have changed due to the climate variation and human activities, which made a serious threat to agricultural, industry, and ecosystem. This dissertation analyzed the changing characteristics of the hydrologic process in the SRB based on the long term hydrometeorological data, then assessed the individual contributions of climate and human factors to the runoff change through a hydrologic sensitivity analysis, and finally simulated the runoff under different landuse and climate change scenarios with the SWAT model. We aimed to clarify the response of runoff to the changing environment in the SRB and suggest some adaptive management strategies to avoid the risk of water resources shortage in this region. Through the research, we mainly obtained the following conclusions:(1) Spatiotemporal variability of climate and streamflow in the Songhua River Basin over the past 50 years The modified Mann-Kendall tests, flow duration curves and correlation statistics were performed to identify the long-term trend and interrelation of the hydrometeorological variables, based on the long term(1960-2009) data from 37 meteorological stations and 33 major river gauge stations across the SRB. The results showed that the average temperature in the SRB was 3.48°C, and has steadily increased in the past five decades with a increasing rate of 0.41°C/10 a, while precipitation fluctuated greatly among the years and the decades with a declining trend of 5.8 mm/10 a. Overall, the average, highest and lowest discharge at most gauge stations across the SRB showed a downward trend in the past five decades, with a significantly decreasing trend in the Lower Songhua River. Seasonally, the declining trend in discharge was prevalent in spring. Compared with that before 1990, the intra-annual nonuniformity of runoff increased, and the flow duration analysis showed a decrease in high flow, but an increase in low flow after 1990 at most mainstream stations of the SRB. Because precipitation in this river basin is concentrated during the summer and fall months, annual discharge was closely and positively correlated with precipitation amount occurred during these two seasons.(2) Separating the impacts of climate variation and human activities on runoff in the Songhua River Basin Combined the analysis of moving t-test and precipitation-runoff double cumulative curve, we assumed 1974 was the changing point of runoff in the SRB. Consequently, the 50-year study period was divided into two time series: 1960–1974 where minimal human activities took place and 1975–2009 where extensive land use change occurred and river engineering projects were undertaken. Based on a hydrologic sensitivity analysis, we found the contribution of climate variation and human activities to the runoff change varied temporally and spatially. Climate variation played a bigger role in runoff reduction in the upper Second Sonhua River Basin and Lower Songhua River Basin(63%–65%) between 1975 and 1989, as well as in runoff increase in the Nenjiang River Basin(85%–86%) between 1990 and 1999. While, human activities contributed more during the other periods.Spatially, the effect of human activities on runoff decline was relatively stronger in the lower basin areas in the 1960 s and 1970 s while showing an increasing role in the upper basin areas in the past two decades.(3) Response of runoff to the landuse change in the Nenjiang River BasinThe analysis of the landuse in 1975, 1986, 1996 and 2000 in the Songhua River Basin showed that landused had changed a lot, especially in the Nenjiang River Basin(NRB). Therefore, we took NRB as the research area to study the impact of landuse change on runoff. The results showed that dryland, forest, grassland, and wetland were the main landuse type in the NRB. Generally, the drastic changes happened during 1975-1986 and 1996-2000. The evapotranspiration varied successively from the scenarios of wetland, forest, grassland and cultivated land. The surface runoff was largest when the whole basin was dryland, and least when it was forest or wetland.The groundwater changes in the orders of forest, grassland, dryland, wetland and paddy land.When the landuse was changed to the conditions of 1975, 1986, and 1996, the streamflow increased in every month of the year, while it differed among the extrem scenarios. The runoff under the wetland scenario was lowest corresponed to all the probobility. For the other four scenarios, the runoff during wet season declined in the order of cultivated land, grassland and forest, while the order was forest, drland, grassland, and paddy land for the runoff during the dry season.(4) Streamflow prediction under different climate scenarios in the Songhua River Basin According to the future climate change scenarios(RCP4.5 and RCP8.5), the average temperature during 2020-2049 in the SRB will increase compared with the current condition, and it will increase more under the RCP8.5 scenario. Spatially, the increment will be larger in the NRB and the Lower Songhua River Basin. In addition, the temperature will have an increasing trend in 2020-2049, and the increasing rate is larger under the RCP4.5 scenario, with the largest rate occurs in the NRB. The change of future precipitation will be more complicated. There will be a decreasing trend in the average precipitation of SRB, while the trends will vary in different subbasins. Streamflow of Dalai station, Fuyu station, and Jiamusi station will all show a decreasing trend in the future 30 years. Compared with that during 1980-2009, the streamflow of Dalai and Jiamusi station under RCP4.5 and RCP8.5 scenarios will respectively decrease by 33%~38% and 16%~24%, while the streamflow of Fuyu station will increase by 17.7% and 9.7%.Through the above results and analysis of the problems and uncertainties in the water resources of SRB, we proposed the management strategies resources adaptive to the changing environment. These strategies are based on the principles of strengthening the measurement systems, optimizing the development pattern of land, water and economy, and establishing a water-conserving society.