Water stable hydrogen and oxygen isotopes in a hydrogeologist toolbox – Hydrocalculator software

Full text: Groundwater recharge in northern Australia is mainly driven by cyclones that are characterised by distinct stable hydrogen and oxygen isotope signatures. These signatures can be used as natural tracers to estimate water budgets but require careful estimation of evaporative losses prior to infiltration. However, determination of evaporation losses and overall estimation of water budgets, particularly in highly dynamic systems, can be challenging due to uncertainties in monitoring of water flow and volume measurements of water bodies. In this study, we developed and tested a new user-friendly software (Isotope Hydrocalculator) based on the revised Craig-Gordon model (CG), which allows quick and robust estimation of evaporative losses based on simple measurements of water isotope composition. We assessed the accuracy of the C-G model and tested the software against results from field pan evaporation experiments in the Hamersley Basin of Western Australia after Cyclones Heidi and Lua (2012). The mean evaporative losses calculated based on changes in the water stable isotope composition were in good agreement with estimates based on water level observations (R2 =0.998 p<0.001). Differences were on average 0.2% and always <2.2%. The main constraint and source of the largest uncertainty is the estimation of ambient moisture isotope composition. Using a mean annual air temperature and relative humidity (24°C, 31%), mean evaporate loss prior to recharge in the Hamersley Basin ranged from 13% to 20%. This relatively small loss compared to the potential pan evaporation (~3000 mm/yr) can only occur due to rapid recharge. (author)