Analytical model of salinity risk from groundwater discharge in semi-arid, lowland floodplains

River regulation and irrigated agricultural developments have increased saline groundwater discharge to semi-arid floodplain environments, resulting in soil salinization and vegetation dieback. There is a need for rapid assessment tools to identify areas of floodplain vegetation at risk from salinization. This article describes the development and testing of a simple, one-dimensional analytical model that distributes floodplain groundwater discharge as seepage at the break of slope, evapotranspiration across the floodplain and as groundwater flow into and out of the river. The analytical model provided comparable estimates of groundwater discharge to MODFLOW (R(2) = 0.998) over a broad range of floodplain scenarios. Using regional scale geographic information system (GIS) data along an 85-km long reach of the lower River Murray in South Australia, the model predicted the location of most (66%) of the unhealthy trees affected by seepage and soil salinization near the edge of the river valley and more than 75% of the unhealthy trees across the broader floodplain. The model predicted 98% of the observed variance in measured kilometre-by-kilometre river salt loads, identifying 'hotspots' of salt input to the river. It can therefore be used as a rapid assessment tool to assess the potential salinity risk to the floodplain vegetation and river from irrigation developments and river management. This approach can be applied more broadly, although the impact of wetlands and oxbows, which may act to intercept groundwater within the floodplain, is not taken into account. Copyright (C) 2009 John Wiley & Sons, Ltd.