Below-cloud Evaporation Effect On Stable Water Isotopes In Precipitation In The Western Region Of The Monsoon Triangle Based On Stewart Model

Water resources are the cornerstone of the development of human society.The study of water cycle provides a scientific basis for the rational use of water resources.Precipitation is an important part of the water cycle,and precipitation isotopes can reflect the mechanism of regional water cycle effectively.In the arid and semi-arid region,there will be obvious raindrop isotope ratio changes during the falling process due to the influence of the below-cloud evaporation,and quantitative evaluate the change is of great significance for clarifying the regional water cycle,to improve the precipitation prediction and water resources management ability.The ecological environment in the western region of Monsoon Triangle is fragile and sensitive,the water resources situation is severe,and the evaporation is strong.In view of this,from January 2020 to December 2020,we selected Baiyin,Huanxian,Jingning,Kongtong,Maqu and Wudu in the western region of Monsoon Triangle to build a regional precipitation isotope monitoring network,and based on the homogeneity and stratification assumptions of Stewart model.We revealed the characteristics of stable isotopes in precipitation of the Monsoon Triangle,and compared and analyzed the below-cloud evaporation effect under the two assumptions.At the same time,the factors affecting below-cloud evaporation and the key input elements of Stewart model are discussed,and the environmental significance of precipitation and water vapor isotopes are analyzed as well.The research results are as follows:（1）Theδ¹⁸O in precipitation in the study area is between-28.61‰and 12.20‰,with an average value of-8.93‰,TheδD is between-196.20‰and 37.12‰,with an average of-63.18‰.The stable isotopes in precipitation of Baiyin,Huanxian and Kongtong are higher in summer and lower in winter,while those in Jingning,Maqu and Wudu are relatively poorer in summer and richer in winter.The abnormal performance is due to the influence of abundant water vapor brought by the Southeast Monsoon and the Southwest Monsoon.In terms of spatial variation,the stable isotopes in precipitation in Kongtong are the most abundant,while those in Jingning are the most poor;Among the factors of affecting the isotope in precipitation,the relative humidity has a negative correlation with isotope in precipitation,while the temperature,vapor pressure,atmospheric pressure and precipitation have no obvious correlation with it.The deuterium-excess in the study area is between-115.78‰and118.42‰,with an average value of 8.23‰,and is lower in summer and higher in winter.The LMWL based on observations isδD=6.12δ¹⁸O-8.53,the slope and intercept are lower than that of GMWL,and there is a significant difference between LMWL slope and the theoretical slope,indicating that raindrop in the study will suffer from obvious effect of below-cloud evaporation.（2）The temporal and spatial characteristics of below-cloud evaporation based on the homogeneity assumption and stratification assumption of Stewart model are the same in the study area.The isotope enrichment caused by below-cloud evaporation is more obvious from 10:00 to 21:00（UTC+8）,and the intensity of below-cloud evaporation in spring is greater than that in other seasons.The below-cloud evaporation is the most obvious in the Baiyin,while Kongtong is the weakest.Through comparative analysis,the homogeneity assumption often overestimates the isotope change between the cloud base and the ground,while below-cloud evaporation intensity under the stratification assumption corresponds well with the precipitation isotope and with a high correlation coefficient,implying that stratification assumption can more accurately estimate the isotope change of raindrops from the cloud base to the ground.The below-cloud evaporation intensity is positively correlated with temperature（T）,cloud base height(H_cb),but negatively correlated with relative humidity（h）,precipitation intensity（P）and raindrop diameter（φ）.When T>25°C,or h≤50%,or P≤0.1mm/h,orφ≤0.75mm,or H_cb>600m,the isotope change caused by below-cloud evaporation is the most significant.Compared with temperature,relative humidity has similar temporal and spatial characteristics to below-cloud evaporation and there is a high correlation coefficient between them,indicating that relative humidity can be used as an ideal indicator of below-cloud evaporation and is the key input element of Stewart model.（3）Global warming will enhance the below-cloud evaporation effect in the study area.The long time series study of below-cloud evaporation has certain reference value for indicating global warming and regional climate change.At the same time,the below-cloud evaporation effect can be considered to be used to revise short-term and imminent precipitation prediction methods and models to improve the accuracy of precipitation prediction.The negative correlation between water vapor d-excess and CO₂concentration in the study area indicates that urban fossil fuel combustion water vapor has a significant effect on atmospheric water vapor,which will affect the distribution of atmospheric water vapor isotopes.Using water vapor isotopes can refine the study of urban emissions to the atmosphere,and providing a basis for urban environmental protection.