Salinity Effects from Evaporation and Transpiration under Flood Irrigation

Transpiration and evaporation rates from irrigated pastures can be adequately assessed by conventional methods and more recently, by the use of stable isotopes Δ ²H and Δ ¹⁸O. However, the salinity effects that transpiration and evaporation have on infiltrating irrigation waters and residual soil-waters have not been independently assessed in a flood irrigation setting. In this study, oxygen-18, deuterium and chloride concentrations of irrigation water, soil-water, and groundwater were monitored with soil-water content over time to independently assess the salinization effects of evaporation and transpiration. This study was carried out across four flood irrigation sites that overlie a heterogeneous loam-sand and limestone vadose zone. Results showed that minor evaporation losses were detected across most flood irrigation sites through the use of stable isotopes Δ ²H and Δ ¹⁸O. The associated increase in chloride concentration of irrigation water as a result of evaporation (minor fractionating water loss) was low (0 mg/l to 129 mg/l) compared to the chloride increase as a result of transpiration (150 mg/l to 2;800 mg/l) noted in shallow soil-water. Across all sites, the fractionating water loss detected in soil-water was minor (< 1‰ Δ ¹⁸O from the source) with isotopic signatures reflecting partially evaporated irrigation waters. The high soil-water chloride concentrations, minor fractionating loss, and corresponding decrease in soil-water content suggest that transpiration is the dominant cause of water loss and therefore the largest contributor to salinity effects during flood irrigation. Salinity effects caused by transpiration (0.4 to 2:6 t/ha) were 3 to 50 times greater than the salinity effects caused by evaporation from irrigation and soil-waters (0.01 to 0:3 t/ha).