Abstract
Summary Accurate quantification of evaporative losses to the atmosphere from surface water bodies is essential for calibration and validation of hydrological models, particularly in remote arid and semi-arid regions, where intermittent rivers are generally minimally gauged. Analyses of the stable hydrogen and oxygen isotope composition of water can be used to estimate evaporative losses from individual pools in such regions in the absence of instrumental data but calculations can be complex, especially in highly variable systems. In this study, we reviewed and combined the most recent equations required for estimation of evaporative losses based on the revised Craig–Gordon model. The updated procedure is presented step-by-step, increasing ease of replication of all calculations. The main constraints and sources of uncertainties in the model were also evaluated. Based on this procedure we have designed a new software, Hydrocalculator, that allows quick and robust estimation of evaporative losses based on isotopic composition of water. The software was validated against measures of field pan evaporation under arid conditions in northwest Australia as well as published data from other regions. We found that the major factor contributing to the overall uncertainty in evaporative loss calculations using this method is uncertainty in estimation of the isotope composition of ambient air moisture.
Highlights
While transpiration is generally considered the largest contributor to continental water flux (Lawrence et al, 2007; Jasechko et al, 2013), evaporative losses from surface water bodies and subsequent water level fluctuations in rivers, lakes and wetlands are important elements of the terrestrial hydrological cycle (Gammons et al, 2006; Hamilton et al, 2005)
We present a user-friendly software (Hydrocalculator) that allows quick calculation of both the estimated evaporative loss (f) in a non-steady-state condition when the water body volume changes over time, and the estimated evaporation over inflow ratio (E/I) in the steady state condition, based on the stable isotopic composition of inflowing and outflowing water (Supplementary data 1, 2, 3)
Under steady-state condition, a water body is constantly replenished by inflowing water and the water level remains constant as the evaporative losses from the pool are compensated by inflow that equals or exceeds evaporation
Summary
While transpiration is generally considered the largest contributor to continental water flux (Lawrence et al, 2007; Jasechko et al, 2013), evaporative losses from surface water bodies and subsequent water level fluctuations in rivers, lakes and wetlands are important elements of the terrestrial hydrological cycle (Gammons et al, 2006; Hamilton et al, 2005). The evaporative loss from a pool or a lake over a fixed period of time can be calculated relatively if the transient stable hydrogen and oxygen isotope compositions of water are known or if the stable isotope composition of inflowing and outflowing water is known (Craig and Gordon, 1965) This method is inexpensive as the calculation requires the stable isotope analysis of just two water samples (assuming there are no additional new water sources during the study period), basic weather data, and estimation of the stable isotope composition of ambient air moisture. We assessed the potential influence of uncertainty in air temperature and humidity measurements and of ambient air stable isotope composition estimations and variations on the computed f and E/I values, which demonstrates how this approach might be applied to other climatic conditions
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