Quantifying The Moisture Recycling Fractions From Lake Taihu With An Isotope-based Method

Contribution of lake evaporation to precipitation at downwind site,referred to as moisture recycling,is an important part of local hydrological cycle.Accurate quantification of lake moisture recycling can improve our understanding of surface-air interactions,and serve as independent validation on the atmospheric water budget predicted by regional climate models.Isotope-based method is a powerful tool to quantify the moisture recycling fractions of lakes,but still face some challenges:(1)There are still lack of simultaneous measurements on precipitation isotopes at both the upwind and downwind sites of lake.(2)Regarding the estimation of the isotopic composition of lake evaporation,the Keeling plot method remains subject to large uncertainty.(3)How to consider the isotopic kinetic fractionation effect during sub-cloud evaporation and lake evaporation processes in the quantification of moisture recycling is still controversial.(4)What drives the seasonal variability of moisture recycling fractions of the lakes in the subtropical monsoon region is still not clear.The moisture recycling fractions over Lake Taihu,located in the subtropical monsoon region,was estimated using an isotopic method to deal with the issues above.Investigations were carried out as follows:(1)Regarding precipitation isotopes,the temporal and spatial patterns were investigated,and the influence of moisture source and lake evaporation on the variabilities were discussed based on the observation at Nanjing,Liyang,Yixing and Dongshan sites,which located along the prevailing wind direction of Lake Taihu region from August 2015 to December 2017.(2)Based on the high-frequency water vapor isotopes observation data,the best regression approach coupled in the Keeling plot method was chosen,and the Keeling plot methodology system suitable to estimate the isotopic composition of lake evaporation was established.(3)The contribution of evaporation from Lake Taihu to precipitation at downwind site was estimated based on the two-component mixing model,the sensitivity of the estimation to the kinetic effects of surface evaporation and raindrop evaporation was evaluated,and the seasonal variability of the recycling fraction and the driving mechanisms was investigated.Main results are as follows:(1)Regarding the temporal variabilities of precipitation isotopes,the deuterium excess of precipitation was higher in the winter monsoon periods(about 20‰)and lower in the summer monsoon periods(about 10‰),which can be attributed to the difference of moisture sources.Regarding the spatial patterns,the deuterium excess of precipitation at the downwind precipitation site was larger than that of the upwind site,larger than 0.9‰-3.0% during the winter monsoon periods and-0.2‰-2.4‰ during the summer monsoon periods,due to the effect of evaporation from Lake Taihu.The magnitude of spatial variations was larger in the winter monsoon periods,indicating a larger moisture recycling fraction in the winter monsoon periods than that in the summer monsoon periods.(2)Regarding the calculation of the isotopic composition of lake evaporation: The performance of Keeling plot method was improved significantly if ordinary least squares regression or York's Solution regression,with concentration-dependent field characterization of measurement errors,was employed and observation from two heights were used,showing good agreement with the flux-gradient method and Craig-Gordon model.(3)Regarding the quantification of lake moisture recycling in subtropic monsoon region,the moisture recycling fractions were 48.1% ± 13.3%,7.0% ± 2.9% and 37.5% ± 5.3% during the winter monsoon from December 2015 to February 2016,the summer monsoon from June2016 to September 2016,and the winter monsoon from December 2016 to February 2017,respectively.Either employing the kinetic fractionation factors widely used in previous lake studies,or neglecting sub-cloud evaporation will decrease moisture recycling fraction,but it is more sensitive to the former.(4)The moisture recycling fraction was higher in winter monsoon season than that in summer monsoon season,the main reason was that large-scale advection transport less water vapor in winter,and the fraction of water vapor contributed by lake evaporation in the atmospheric water vapor became higher.