Variations of Precipitation Stable Isotope and Vapor Origins Revealed by Deuterium Excess in Changsha

Variations of both stable isotope and deuterium excess(denoted as d) in precipitation as well as their relationship with precipitation, temperature and humidity were analyzed by using daily precipitation stable isotope data from January 1, 2010 to May 31,2012. The results show that both stable isotopes and d in precipitation indicate obvious seasonal variation in the monsoon system, high value in the summer half year and low value in the winter half year. There are precipitation amout effect and humidity effect, in addition,anti- temperature effects in the summer half year and temperature effect in the winter half year. Considering both d and δD in precipitation with atmospheric humidity, we deduced that the main causation of stable isotopic variations in precipitation is related to the property of rainfall air mass. In the summer half year, the water vapor is transported by the southwest and southeast monsoon from low latitude oceans, with high humidity, low stable isotopic ratios and d owing to the rainout of vapor on the transport way. Then, in the winter half year, the vapor is primarily from the westerly transportation and the replenishment of reevaporated vapor in inland, with low humidity, high stable isotopic ratios and d values in precipitation.