Isotopic Characteristics Of Precipitation And Moisture Recycling Ratio In The Altay Mountains Based On IsoGSM2

Precipitation is the main input of surface water and the important component of water cycle.The spatiotemporal variation of precipitation affects the total amount of regional water resources and water cycle process.As a contributing source of precipitating water vapor,locally recycled water vapor from surface evaporation and transpiration plays an important role in terrestrial water cycle,especially in central Asia.As a component of water molecules,stable isotopes of hydrogen and oxygen are often used as tracers to explore the hydrological cycle process.Therefore,the precipitation and water vapor isotopes can be examined to estimate the contribution rate of recycled water vapor to precipitation,i.e.,moisture recycling rate,which is helpful to understand the hydrological and environmental changes in central Asia under a warming climate background.Taking the Altay Mountains in central Asia as the research area,this paper analyzed the spatiotemporal distribution characteristics of precipitation and water vapor isotopes using IsoGSM2 simulations during 1979-2020.The contribution rates of water vapor from various sources to precipitation in the Altay Mountains were calculated in the three-component isotope mixing model.The temporal and spatial variations and influencing factors of moisture recycling rate in the Altay Mountains were then analyzed to clarify the hydrological cycle process in the Altay Mountains.This work provides a reference for the study of modern atmospheric processes,and is also useful for the application of isotope-enabled atmospheric circulation models in water resources and climate reconstruction.The research of this paper mainly draws the following conclusions:（1）The isoGSM2 simulation overestimates the precipitation isotope values in the Altay Mountains,but generally reflects the basic characteristics of precipitation isotopes.The measured meanδ¹⁸O of precipitation in the Altay Mountains was-13.1‰,ranging from-28.8‰to-3.2‰on an annual basis;the corresponding meanδ¹⁸O simulated by IsoGSM2 was-12.7‰,ranging from-19.6‰to-6.6‰.The measured and simulated precipitation isotopes showed seasonal variation with enrichments in summer and depletions in winter.The most enriched month appeared in July and August,and the most depleted appeared in January and February.The slopes of the meteoric water lines based on the measured and IsoGSM2-simulated data were 7.8 and 7.2,and the intercepts were 3.8‰and 2.4‰,respectively,which were lower than the global meteoric water line.According to the relationship between precipitation isotopes and meteorological elements,precipitation isotopes had temperature effect on a seasonal scale,but had no amount effect.（2）The δ¹⁸O values in the total,daytime and nighttime precipitation in the Altay Mountains simulated by IsoGSM2 were-13.4‰,-13.2‰and-13.6‰,respectively,with relatively larger values in daytime and lower values in nighttime.The interannual variation of precipitation isotopes for the three series all showed a depleting trend,especially the daytime series.The precipitation isotope values were high in the south and low in the north.For the interannual tendency,low values occurred in the western part.The temperature effect of precipitation isotope was obvious from a spatial perspective,but not for an interannual scale.The isotopes in water vapor were more depleted than those in precipitation.The meanδ¹⁸O values in the total,daytime and nighttime water vapor were-25.6‰,-25.5‰and-25.7‰,respectively.Spatially,depleting trends can be seen in most areas,and the depleting tendency was strong in the south and weak in the north.There was a significant temperature effect on the spatial distribution of water vapor isotope,but this was not the case for interannual variation.There was a positive correlation between precipitation isotopes and water vapor isotopes.The interannual decreasing trend of precipitation isotope was more obvious than that of water vapor isotope.（3）Based on the IsoGSM2 simulation,the isotopes of water vapor in advection,surface evaporation and plant transpiration in the Altay Mountains were determined.The meanδ¹⁸O values for advection,surface evaporation and plant transpiration were estimated as-25.5‰,-32.9‰and-23.3‰,respectively,and the meanδ²H values were-187.7‰,-196.7‰and-166.1‰,respectively.The isotope values in plant transpiration were the highest,and those in surface evaporation were the lowest.The water vapor isotopes of these three fluxes showed a decreasing trend in the interannual variation,especially plant transpiration and surface evaporation.The mean contribution rate of water vapor was 95%for advection,1.3%for surface evaporation and 3.7%for transpiration.The contribution of advection showed an increasing trend and played a dominant role,while the mean values were high in the west and low in the east.The contributions of surface evaporation and plant transpiration have decreased with a spatial distribution of increment from west to east.The interannual tendency rates of surface evaporation and transpiration were mainly negative.The annual mean moisture recycling rate in the Altay Mountains was 5%and fluctuated between 3%and 7%,showing a decreasing trend on an interannual scale.Regarding the spatial distribution,the moisture recycling rate was high in the west and low in the east.Most areas showed a negative tendency except for some southern parts.The water vapor recirculation rate was negatively correlated with meteorological factors including temperature,precipitation and relative humidity,which are not statistically significant.