Contribution Of Water Vapor Recirculation To Atmospheric Precipitation In Inland River Basin,Alpine Region Of China

Precipitation is an important part of the water cycle and water balance,which fundamentally determines the abundance of water resources in a region.Water vapor recirculation is the process of local evaporative water vapor returning to the surface in the form of precipitation.In the context of climate warming and dramatic shrinkage of the cryosphere in the alpine region,the structure,velocity,spatial and temporal processes,elemental composition of the water cycle and its hydrological and ecological effects have changed,and its impact on the ecological evolution process in the cold region has first emerged,making the study of the hydrological and ecological processes in the cold region the focus of global change and regional response research in recent years.In view of this,the systematic study of water vapor sources,transport paths and water vapor recirculation processes on precipitation is an important basis for a deeper understanding of the spatial and temporal changes in regional water resources,and will provide a theoretical basis for the scientific formulation of water resources utilization countermeasures.To this end,the scientific issues of regional water vapor recirculation change processes and possible mechanisms in the alpine endorheic zone are focused on.Based on the theoretical framework of atmospheric water circulation,the water vapor recirculation model is used to calculate the regional water vapor recirculation rate in the alpine endorheic zone,analyze the environmental effects and evolutionary mechanisms of water vapor recirculation,and quantify the contribution of water vapor recirculation to precipitation,and the main conclusions are as follows.（1）Climate warming and humidification have become the most dominant features of the alpine endorheic zone.The annual mean temperature has increased at a rate of 0.4°C/10a,and this increase is likely to come mainly from the contribution of the increase in the annual mean minimum temperature.During the same period,the rate of increase in mean annual minimum temperature was 0.52°C/10a,which is 25%higher than the mean annual temperature.Annual mean precipitation in the alpine endorheic zone increased at a trend of 5.4 mm/10a from 1969 to 2017,with significant interdecadal and interannual differences in precipitation.Evaporation in the alpine endorheic zone showed a decreasing trend from 1969 to 2017（-9.19mm/10a）,with an annual mean The average evapotranspiration is 989.70 mm.there is an obvious geographical variation in the average evapotranspiration,and the high values are mainly distributed in the Qinghai Lake basin and parts of the Qaidam basin,while the northern part of the Qiangtang basin has lower evapotranspiration.The warming trend of the extreme temperature index is obvious,and the warm index and growing season length have been on an increasing trend since the 1970s,while the slope of some cold indices（cold day days and cold night days）,freezing days,frost days and daily temperature difference have been on a decreasing trend for many years.Growing season length,extreme temperature warming index and some cold indices（extreme low of daily maximum temperature）have warmed more in the northern part of the alpine endorheic zone,and the number of freezing days and frost days have warmed significantly in the southwestern and southern part of the alpine endorheic zone.The characteristics of extreme precipitation changes are more complex,but the most obvious one is the enhanced precipitation activity,and precipitation is developing in the direction of high precipitation amount,high number of days,strong intensity and high extreme values.（2）The increase of water vapor content and water vapor transport flux provides favorable conditions for the increase of precipitation in the alpine endorheic region.1979-2017 water vapor content is increasing,and the increasing trend reaches 0.15mm/10 a.The multi-year average atmospheric water vapor content varies between3.7-5.3 mm.Spatially,the whole layer of atmospheric water vapor content in the alpine endorheic region is about 0-10 mm,which is significantly lower than other regions at the same latitude,and the overall spatial pattern shows an increase from the center to the surrounding area.The external water vapor sources controlling the meteorological and climatic patterns of the alpine inland flow region in the past 40years are:1)the mid-latitude westerly transport flux;2)the southwest warm and humid water vapor transport flux from the Indian Ocean-Arabian Sea and the Bay of Bengal;and 3)the northwest dry and cold water vapor transport flux from the Eurasian continent.The main sources of water vapor transport fluxes in the four seasons are generally consistent with the annual average,but there are differences in intensity,showing that the maximum is in summer,followed by autumn and spring,while the minimum is in winter.The net water vapor flux is in surplus in the alpine endorheic zone,i.e.,water vapor sinks into the alpine endorheic zone.The west,north and south boundaries are the water vapor input boundaries of the alpine endorheic zone,and their water vapor income and expenditure are 539.4×10⁶kg·s^-1,276.3×10⁶kg·s^-1 and 718.1×10⁶kg·s^-1,respectively,while the east boundary is the output boundary of water vapor flux,and its water vapor income and expenditure is-986.3×10⁶kg·s^-1,and the net income and expenditure is 547.5×10⁶kg-s^-1.（3）In the context of climate warming,water vapor in circulation precipitation has become an important component of regional water resources.The water vapor recirculation has fluctuated significantly in the past 40 years,and the average value of multi-year water vapor recirculation rate is 24.5%,which is equivalent to about 66mm of precipitation,and varies in the range of 12%-43%overall.In terms of spatial distribution,the water vapor recirculation rate as a whole tends to decrease from the middle to the surrounding area,with the high value center mainly occurring in the central part of the Qiangtang Basin.The contribution of water vapor recirculation rate gradually increases with elevation below 3500-4000m,but starts to decrease above4000m;the water vapor recirculation rate gradually decreases with increasing slope,and the highest slope interval of water vapor recirculation rate occurs at The highest water vapor recirculation rate occurs at 0-5°,reaching 27.2%;the multi-year water vapor recirculation in the study area has a tendency to increase against the background of increasing vegetation area,indicating that good ecological protection is an important support to ensure the stable supply of regional water resources.The enhanced anticyclonic circulation in Eurasia,increased potential height,weakened water vapor transport and increased regional sea surface temperature create favorable background conditions for the study of water vapor recirculation increase.The total water vapor recirculation precipitation production volume is about 68.2 billion m³,which is equivalent to the surface water resources of the whole Qinghai Province（67.541±2.661 billion m³）,reflecting that water vapor recirculation is a very important water resource.