Precipitation stable isotope variability and subcloud evaporation processes in a semi-arid region

The stable isotopic (H-2/H-1 and O-18/O-16) composition of precipitation has been used for a variety of hydrological and paleoclimate studies, a starting point for which is the behaviour of stable isotopes in modern precipitation. To this end, daily precipitation samples were collected over a 7-year period (2008-2014) at a semi-arid site located at the Macquarie Marshes, New South Wales (Australia). The samples were analysed for stable isotope composition, and factors affecting the isotopic variability were investigated. The best correlation between delta O-1(8) of precipitation was with local surface relative humidity. The reduced major axis precipitation weighted local meteoric water line was delta H-2 = 7.20 delta O-1(8) + 9.1. The lower slope and intercept (when compared with the Global Meteoric Water Line) are typical for a warm dry climate, where subcloud evaporation of raindrops is experienced. A previously published model to estimate the degree of subcloud evaporation and the subsequent isotopic modification of raindrops was enhanced to include the vertical temperature and humidity profile. The modelled results for raindrops of 1.0 mm radius showed that on average, the measured D-excess (=delta H-2 - 8 delta O-18) was 19.8 parts per thousand lower than that at the base of the cloud, and 18% of the moisture was evaporated before ground level (smaller effects were modelled for larger raindrops). After estimating the isotopic signature at the base of the cloud, a number of data points still plotted below the global meteoric water line, suggesting that some of the moisture was sourced from previously evaporated water. Back trajectory analysis estimated that 38% of the moisture was sourced over land. Precipitation samples for which a larger proportion of the moisture was sourced over land were O-18 and H-2-enriched in comparison to samples for which the majority of the moisture was sourced over the ocean. The most common weather systems resulting in precipitation were inland trough systems; however, only East Coast Lows contributed to a significant difference in the isotopic values. Copyright (C) 2016 Australian Nuclear Science and Technology Organisation. Hydrological Processes. (C) 2016 John Wiley & Sons, Ltd.