| In recent years,the imbalance between water and heat caused by the climate change and the transformation of natural underlying surfaces is increasingly intensified by human activity.Not only the distribution of water resources is changing drastically,but also causing the meteorological disaster events occurring frequently.Under such a background,a series of water resources problems such as the drastic reduction in river runoff,the excessive exploitation of groundwater,etc.have become more frequently.In the arid and semi-arid regions of China,where has suffered from the water shortage for a long time,the water issues faced are often even more severe.Inner Mongolian,as the animal husbandry production base,with the change of regional hydrological cycle,is also emerging the hydrological and environmental problems such as river disconnection,flood raging,grassland degradation,etc.These problems have aggravated the contradiction between people and the natural world in a harmonious coexistence and severely restricted the development of regional socioeconomic development.In order to solve the problem of water resources caused by environmental changes,we need to start with the water distribution pattern and changes at the scale of basins,then further discussion on the driving mechanism of hydrological process changes.In this study,we take the Xilin River Basin,a typical steppe-type basin in arid and semi-arid region,as the study area,and apply the commonly used methods of hydrological in the world to analyze extreme precipitation,the spatio-temporal changes of the precipitation at different scales of precipitation,and the causes of runoff changes at the base of the long-term hydrological and meteorological observation data,then we also established the SWAT model to simulate the runoff of the study area.On this basis,a combined model of surface water and groundwater in the Xilin River Basin was established in combination with the Visual MODFLOW model,and the surface water and groundwater of the basin were discussed as a whole,which can provide a theoretical support for the efficient use of water resources in our study area.The results of the study are as follows:1.we use the gridded precipitation dataset?with a resolution of 0.5°×0.5°?of the eastern part of inland river basin of Inner Mongolian Plateau from 1961-2015 as the basis and adopt the methods of climatic diagnosis?e.g.,the Modified Mann-Kendall method,principal component analysis,and correlation analysis?to analyze the spatial and temporal variations of 6 extreme precipitation indices.Furthermore,we analyzed the relationship between El Ni?o–Southern Oscillation?ENSO?events and the observed extreme precipitation.The results indicated that the gridded dataset can be used to describe the precipitation distribution in our study area.In recent 55 years,the inter-annual variation trends of extreme precipitation indices are generally dominated by declination,in particular,the decreasing regions of consecutive dry days?CDD?accounts for 91%of the entire basin.Contrary to CDD,the spatial distribution of the other 5indices are gradually decreasing from northeast to southwest,and the precipitation intensity?SDII?ranges from 3.8-5.3 mm/d,with relatively small spatial differences.To some extent,CDD and R95P can used to characterize the extreme precipitation events.Moreover,the number of days with heavy precipitation?RR10?,SDII and R95P are more susceptible to the ENSO events.In addition,the moderate El Ni?o event may increase the probability of drought,while the La Ni?a events may increase the risk of flood in the study area.2.The annual precipitation of Xilin river presented a nonsignificant decreasing trend and four abrupt changes took place in 1974,1980,1989 and 1998 during recent 56 years.There were a major cycle of 28a which occurred in the period from middle 1970s to the late 1990s and after the late 1990s,respectively.At the seasonal scale,the characteristic of summer precipitation was basically consistent with that of the annual precipitation,the rainfall of autumn had a large fluctuation,and the precipitation of the other two seasons both increased,especially in winter,the precipitation presented a significantly increasing trend.The precipitation in August of the flood season presented a significant decreasing trend and changed in 1998,and the first major cycle was 12a.The precipitation in November and December of the non-flood season demonstrated a significantly increasing trend,of which the precipitation in December in 2000 showed a significant abrupt change.The first major precipitation cycles during the two months were 30 a and 9 a,respectively.The Hurst indices of the precipitation series at different time scales were all greater than0.5,indicating that the precipitation had a persistent trend,the change of future precipitation will be consistent with that of the past.3.The large fluctuation of runoff in the Xilin River Basin directly reflects that the grassland ecology has been seriously damaged.Analyzing the main driving factors of runoff changes is of great significance to the analysis the process of water cycle changes in the river basin and the restoration of grassland ecosystems.Based on the runoff and meteorological data with series from 1970 to 2014 of a typical steppe watershed Xilin River,the Elastic Coefficient Method was used to quantify the contribution of climate change and human activities on runoff changes in this study.The results indicated that the annual runoff present a significant decrease trend during recent 45 years,and occurred mutations in the years of 1979,1986 and 1998 respectively.A quantitative analysis of the runoff changes in each study period with the point of mutation as the cut-off point.For the period 1980 to 1986,the contribution rate of human activities accounted for 46.57%,which reached 91.46%for the period 1987 to 1998 and climate change just had a percent of 8.54%.Compared with the reference period,the runoff decreased 45.56%for the period of 1999 to 2014,66.16%of the runoff variation was caused by human activities,although lower than the previous but still taking the leading role.In brief,human activity is the main driven factors which induced runoff changes.Overall,human activity is the main driven factors which induced runoff changes.4.Based on the date of DEM,soil type,landuse and hydro-meteorology,we established the SWAT model in the upper reaches of the Xilin River Basin,which take the Xilinhot Hydrological Station as the boundary to simulate the annual runoff.The model divides the basin into 7 sub-basins and 214 HRU,and SWAT-CUP software was used to determine the parameters of the model during the model identification period,and the determination coefficients and Nash efficiency coefficients were selected to evaluate the suitability of the model in the basin.In the identification period,the R2 and ENS values of the model are 0.87 and 0.79,respectively.During the validation period,the R2 and ENS values of the model are 0.78 and 0.68,respectively.Although the simulation accuracy is not so high,they all meet the accuracy requirements of the SWAT model.5.Based on the calibrated SWAT model,assigned the HRU to a spatial attribute that corresponds to the CELL in MODFLOW with the aid of Arcgis.The groundwater recharge and evaporation output from the SWAT model are put into the recharge package and the evaporation package of the Visual MODFLOW,and then established the conceptual model of groundwater for heterogeneous-isotropic-two dimensional unsteady flow in the upper reaches of the Xilin River Basin,which was applied to the calculation of the groundwater level in the study area.By fitting the simulation flow field and the measured flow field during the identification period,the parameters that meet the actual hydrogeological conditions in the study area were finally determined.At the same time,the coupled model was proved can be reflected the movement characteristics of groundwater well by fitting the water level of the observation wells during the validation period. |
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