Impacts Of Climate Change And Land Use/Cover Change On Water Resources In A Karst Graben Basin Based On SWAT Model

The karst graben basin,one of the eight major karst landforms in China,is located in the Yunnan-Guizhou Plateau and the upper reaches of the Yangtze River and the Pearl River,which is an important ecological conservation area in China.Karst water resources are an important basic resources for economic development and ecological restoration in karst graben basin areas.The Nandong Subterranean River Watershed?NSRW?is a example of karst graben basins in China,and it is also a region with severe rocky desertification and serious water shortage.With the increase of population and the development of regional economy and society,the contradiction of supply and demand of regional water resources has become increasingly serious.The rational development and utilization of regional karst groundwater resources is the most important way to resolve the aforementioned contradiction.For the reasonable exploitation and sustainable use of karst ground water resources,we must not only scientifically understand the current status,evolution characteristics and reasons of water resources changes,but also need to make reasonable predictions on the amount of water resources with future changing environments.In this context,according to the characteristics of karst graben basin,the Soil and Water Assessment Tool?SWAT?model was established by investigating the hydrogeological background and natural geo-environmental characteristics and collecting relevant data of NSRW.Based on flow variations,climate change,land use and land cover change?LUCC?analysis,as well as hydrological simulation results of SWAT,this study quantitatively analyzed the causes of water resources evolution and predicted the water cycle process in the NSRW according to future changing environmental scenarios.The main conclusions are showed below:1.Using the determination coefficient?R²?,Nash-Suttcliffe efficiency coefficient?NSE?,and percentage deviation?PBIAS?to evaluate the simulation effect.The evaluation results showed that the R² and NES and PBIAS were 0.77 and 0.76 and 1.9%in the calibration period?1972?1985?,respectively.In the validation period?1990?2002?,the R² and NES and PBIAS respectively were 0.80 and 0.79 and 0.6%.These showed that the model has a good flow simulation effect and applicability in the research area.2.From 1972 to 2002,the proportion of evapotranspiration vs.rainfall in thebasin area was 8.20%higher than that in karst mountainous area from 1972 to 2002,while the flow proportion in the basin area was 11.14%less than that in mountainous area in the NSRW.There is a significant difference in surface and underground flow distribution between the basin and mountainous areas.The surface flow ratio in the basin is 16.34%,while the underground flow ratio is 83.66%.However,in the karst mountainous area,the surface flow ratio is 2%,while the underground flow ratio is 98%.It can be seen that there is a serious lack of surface flow in the NSRW,which is particularly prominent inkarst mountainous areas.3.The annual average discharge in the NSRW was 8.27 m³/s from 1972 to 2014with large interannual change,and alsoshowed a fluctuant decreasing trend.The ordered clustering method and the Mann-Kendall method were used to find the mutation points of the discharge changes.The abrupt years are 1979,2003 and 2009.According to the abrupt year,the status of discharge variations was divided into three periods:base period T₀?1972?2002?,variation period T₁?2003?2008?and T₂?2009?2014?.The mean flow of T₀,T₁ and T₂ are 8.73 m³/s?7.83 m³/s and 6.71 m³/s,respectively.4.The simulation analysises in different periods found that:?1?Compareing with the T₀ period,the precipitation decreased by 1.26%and the temperature increased by0.41?in the T₁ period.The contribution rate of climate change and human activities and LUCC to flow reduction was 23.83%and 76.17%,respectively.However,the LUCC variationsresulted in flow increase,which the contribution rateto flow reduction was-10.36%;?2?During the period of T₂,the precipitation decreased by 16.91%and the temperature increased by 0.89?.The contribution rate of climate change and human activities to flow reduction was 93.98%and 6.02%,respectively.The LUCC variations resulted in flow increase,which the contribution rate to flow reduction was-5.72%.In the word,the flow reduction during the T₁period was dominated by human activities,while the flow reduction of during the T₂ period of was dominated by climate drought.5.Different climate changes and LUCC variations will change the components ratio in water balance processes in the catchment.The rainfall in the watershed is mainly lost in the form of evapotranspiration,and the average proportion is 77.20%.The volume of surface flow generation is very low with an average proportion of 2.14%.The average proportion of underground flow is 17.35%,and the average percentage of infiltration is 4.68%.The soil water storage are in a state of loss,with an average loss of11.08 mm.6.The simulation results on future environmental scenarios showed that hydrological process changes caused by climate change are significantly stronger than LUCC.The hydrological response caused by precipitation changes is dominant in the hydrological response caused by climate change.During the water cycle,infiltration and surface flow processes have the most obvious responses to environmental changes.Under the same climatic conditions,the soil water retention effect in forest land is better than in grassland.Strengthening the exploitation of karst groundwater resources,building and managing water conservancy facilities,developing water-saving irrigation technology in agriculture and strengthening the monitoring and forecasting of drought are important countermeasures to cope with drought in the future.The restoration of forest land in the karst mountainous area is critical to the conservation of water and soil and ecological restoration.