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).
|Number of pages||11|
|Journal||Journal of Irrigation and Drainage Engineering|
|Publication status||Published - 2011|