Use of Stable Isotopes Deuterium and Oxygen-18 to Derive Evaporation from Flood Irrigation on the Basis of Pan Evaporation Techniques

The loss of water to the atmosphere during flood irrigation occurs through evaporation and transpiration. Whereas transpiration can be estimated through the FAO56 methodology, actual evaporation is difficult to quantify in water balance studies. In this study, two analytical models, previously developed to quantify evaporation from lakes on the basis of stable isotopes, were applied to determine evaporation losses from four flood irrigation sites of varied characteristics. Evaporation losses were determined by empirical relationships derived between heavy isotope enrichment and percentage of water loss in evaporation pan experiments. Validation of the two isotopic models in this setting was achieved by comparison with conventional nonisotopic methods, carried out in parallel. Results showed that heavy isotope enrichment of applied irrigation waters varied among each of the study sites. Isotope enrichment was notably different among irrigation bays that drained rapidly [+0:05 to +0:18 per mil (‰) for Δ ^18OO and +1:7 to +2‰ for Δ ²H] from those in which ponding occurred for up to 18 h after application (+1 to +2‰for Δ ^18OO and +2 to 7.5‰for Δ ²H). When compared with local pan enrichment, these isotope enrichments corresponded to evaporation losses of 0.2-2.7% (0.5-4 mm) and 2-5% (4.5-7 mm), respectively. This study demonstrated that the use of stable isotope data for irrigation waters provided valuable new insights into evaporation losses across different flood irrigation systems. The use of these techniques may be useful in suggesting which management strategies are most effective in improving water use efficiency and water quality.