Management of salt mobilisation in the irrigated landscape – A review of selected irrigation regions

Abstract

Summary Changed land use and irrigation has increased recharge to the groundwater system in irrigated areas worldwide. This altered hydraulic regime increases salt discharge by either mobilising stored salt or increasing salt loading from irrigation application. The salt is transported to surface drainage and river systems through the following processes; surf ace runoff (salt washoff), groundwater seepage, engineered sub-surface drainage and irrigation channel outfall. This paper has reviewed previous studies of salt mobilisation processes and effectiveness of management strategies to reduce the impact of irrigation areas on downstream water quality. Salt mobilisation is highly variable between irrigation areas. Processes are affected by hydrogeological setting, irrigation and drainage management and climatic trends. The largest salt loads are contributed from areas where groundwater discharges directly to the river as a result of groundwater seepage. This is likely to happen in irrigation developments that are located on the edge of an incised river. In general, the risk from salt mobilisation in surface runoff is less than the risk arising from direct discharge of groundwater. Irrigation and drainage management options are site specific and depend on the dominant salt mobilisation process in the area. Management targeting one salt mobilisation process will impact on the other salt mobilisation processes. This trade off needs to be carefully considered when evaluating management options for particular irrigation areas to control the impact on total salt load. Movement of water to higher value use is occurring as water has become a scarce resource. New irrigation developments need to be tightly controlled to minimise the impact of salt mobilisation on water quality for downstream users and ecosystems.