Canal surface evaporation along the China's South-to-North Water Diversion quantified by water isotopes

Abstract

Water stable isotopes have extensive applications in the study of riverine hydrological processes, in particular, isotopic enrichment occurring along flow direction can be used as an indicator to estimate river surface evaporation. However, this application is difficult in natural rivers due largely to complex water exchange along river channel. China's South-to-North Water Diversion Project (SNWDP) is the largest artificial river designed to divert water from south to north through its enclosed, long span open canal, therefore providing a practice to estimate river surface evaporation by using isotope method. In this paper, we carried out hydrometeorological surveys and sampled canal water along the Middle Route of SNWDP (MRP) in two seasons, July 2018 and April 2019, for δ18O and δ2H measurement, then simulated the isotopic enrichment in canal water by using Craig-Gordon (C-G) evaporation model. We found clear increasing trend in heavy isotopes along the long span canal from head water to the end, resulting from evaporation enrichment. We used C-G evaporation model to estimate evaporation ratio E/V of canal water, and results show a ratio of evaporation loss of 2.54%~3.73% in July 2018 and 1.66%~2.39% in April 2019. We also found obvious seasonal differences existed in canal water isotopes, evaporation enrichment and the CWEL (canal water evaporation line), in association with more intensive evaporation in summer. Some large isotope fluctuations along the canal are mostly related to rainfall events, altering the canal water isotope signal. Our result in this study highlights the potential for water isotopes in the application of inter-basin water resources management, in particular, with increasing stress from water shortage and anthropogenic impact.