Journal of Hydrology, 150 (1993) 589 614 0022-1694/93/$06.00 © 1993 Elsevier Science Publishers B.V. All rights reserved 589 [31 Salt accumulation in semi-arid floodplain soils with implications for forest health Ian D. Jolly*, Glen R. Walker, Peter J. Thorburn CSIRO Division of Water Resources and Centre for Groundwater Studies, PMB No 2. Glen Osmond, S.A. 5064. Australia (Received 15 December 1992; revision accepted 20 May 1993) Abstract Dieback of native Eucalyptus largiflorens forests is an increasing problem on the floodplains of the lower River Murray, southern Australia. Salinisation of floodplain soils, as a result of the changed hydrological management of the River Murray, appears to be a primary cause of the dieback. Regulation of the River Murray has reduced the frequency of large flood events by a factor of approximately three and caused groundwater levels beneath floodplains to rise. The higher water tables have resulted in increased discharge of the naturally saline groundwater in the floodplains by evapotranspiration, and the decreased incidence of large floods has reduced floodwater recharge and hence leaching of salt from floodplain soils. Use of soil physical properties for a range of floodplain soils, combined with measurements of groundwater discharge from bare and vegetated sites, suggests that the time-scale for complete soil salinisation can, at worst, be less than 20 years. Moreover, salt accumulation at most sites will continue to occur as the present flooding regime (of which there is limited scope for improvement) appears incapable of providing the leaching required to counteract accumulation. The analyses carried out here suggest that the 'critical' water table depth (below which groundwater discharge is balanced or exceeded by floodwater recharge) needs to be increased by 14 55% (the more clayey the soil, the larger the increase) to prevent salt accumulation. Failure to implement schemes which lower the water tables beneath the floodplain may, in the long term, cause serious damage to these important riparian forests. Introduction Dieback of native riparian forests on the floodplains of the River Murray (Australia's longest river) is an issue of considerable concern. Margules and Partners et al. (1990) estimated that approximately 18000ha of floodplain vegetation is severely degraded by factors such as saline groundwater, drown- ing or waterlogging, water stress, logging or clearing, and grazing. They concluded that the issue of greatest concern for the long-term health of * Corresponding author. Telephone: (08) 303 8700. Fax: (08) 303 8570.