Study On Water Supply And Base Flow Division Of Typical River Basins In The Southern Slope Of Qilian Mountains

With the impact of global climate change and human activities,the global water cycle and water resource pattern continue to change,water resource allocation has become a serious problem of regional social economic development.The south slope of Qilian Mountains is the water conservation area,the habitat of wild animals and plants and the birthplace of many water systems in the arid region of western China.Its water resources protection is related to the ecological security of western China.However,the southern slope of the Qilian Mountains is located in the Qinghai-Tibet alpine region,with large elevation drop,poor hydrology conditions and complex natural environment,which makes the long-term observation stations of precipitation,river water,groundwater and other different water bodies in this region less,and the spatial distribution is very uneven.It seriously affects the in-depth study of regional water resources problems such as water cycle change mechanism and water migration process.Located in the south slope of Qilian Mountains,Amidongsuo small watershed has complete cold temperate coniferous forest and steppe communities.Its natural environment and river hydrology（melting period from March to April,rainy period in May to October,freezing period in November-February）have the representative characteristics of many valleys developed rivers.Based on this,this study monitored the flow rate and discharge of the Amidongsuo small watershed at a specific time,and collected samples of precipitation,river water,soil water,groundwater（well water and spring water）.The stable isotope technique of hydrogen and oxygen was used to determine the samples of different water bodies in the study area.Based on the characteristics of δ¹⁸O and d-excess,the recharge sources of river water in different periods are analyzed,and the contribution rates of different water sources to river water in different periods are quantified.Finally,the base discharge of a hydrological year in a small watershed is calculated by the hydrological model,and the following conclusions are obtained:（1）Summer half year（January-June）>winter half year,the average value of dexcess of deuterium in summer half year（January-June）is less than that in winter half year.The variation of δ D and δ¹⁸O is positively correlated with the air temperature and amount of precipitation in Qilian Mountains,but the d-excess of deuterium is affected by evaporation and air humidity and fluctuates repeatedly.Compared with the previous studies on the evaporation line of precipitation in the Qilian Mountains,it is found that the slope of the evaporation line of precipitation in the Amidongsuo small watershed is higher,which indicates that the precipitation in the Amidongsuo small watershed in the Qinghai-Tibet Plateau is more strongly evaporated than that in the whole Qilian Mountains.（2）The delta δD in river water increased（January-March）→decreased（MarchApril）→increased（April-July）→ decreased（July-October）→increased（OctoberNovember）→decreased（11-12）;The change trend of δ¹⁸O in January to July is the same as that of δ D,except that δ¹⁸O decreases suddenly in July to August,increases in August to October and decreases in October to December.This difference reflects the complexity of water supply sources in small watersheds,and the variation of isotopic signal δD and δ¹⁸O in the same period is also affected by special factors.（3）Soil water was limited by sampling period and cost,and only soil samples were collected from July 2021 to September 2021.When collecting soil samples,Alpine grassland,Alpine shrub and Picea crassifolia were selected from the bottom to the top of the slope,and each sampling site was stratified on 0-50cm,with a layer of 10cm.It was found that there were no significant differences in the isotopic compositions of oxygen and hydrogen in different vegetation types.This indicates that the source of soil water supply during the study period is relatively simple.In addition,there is no significant difference in soil water isotope signal values at 0-50 cm depth.This indicates that the soil was not replenished by groundwater during the study period,and the main recharge source was precipitation.（4）The stable isotopic composition of groundwater（well water and spring water）is positively correlated with temperature and the variation of precipitation.With the increase of temperature and precipitation,δD and δ¹⁸O are enriched.With the decrease of temperature and precipitation,δD and δ¹⁸O are poor.It shows that the isotopic fractionation effect becomes stronger with the increase of groundwater temperature and water infiltration.This makes the water rich in δD and δ¹⁸O.（5）By comparing and analyzing the characteristics of stable isotopes of hydrogen and oxygen in different water bodies at different periods,it is found that the water in the ablation stage is supplied by groundwater（well water,spring water）and precipitation,the proportion of which is 89.8%and 10.2%respectively.In the rainy period,the river water has three recharge sources:precipitation,soil water and groundwater（well water and spring water）,in which the recharge ratio of precipitation to river water is 76.8%,groundwater（well water and spring water）to river water is 1.8%,and soil water to river water is 21.4%.The replenishment ratio of precipitation to river water in freezing period was 8.8%,and that of groundwater（well water and spring water）was 91.2%.It shows that the river flow depends on precipitation during rainy period,while the river flow during ablation and freezing period depends on groundwater（well water and spring water）.（6）The total recharge of underground water in Qilian small watershed is 92.1×103 m3/a by using the underground runoff modulus method.It can be seen that the Amidongsuo small watershed in the Qilian Mountains has relatively rich groundwater resources,and in the dry season of the river（freezing period and melting period）,the groundwater contributes as much as Ninety percent to the river water supply.It provides sufficient water supply for the rivers during ablation and freezing periods,which shows that the river base discharge is rich and the water resource potential is good in the Amidongsuo small watershed of Qilian Mountains.