SALT ACCUMULATION IN SEMIARID FLOODPLAIN SOILS WITH IMPLICATIONS FOR FOREST HEALTH

Dieback of native Eucalyptus largiflorens forests is an increasing pro blem on the floodplains of the lower River Murray, southern Australia. Salinisation of floodplain soils, as a result of the changed hydrolog ical 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 gr oundwater levels beneath floodplains to rise. The higher water tables have resulted in increased discharge of the naturally saline groundwat er in the floodplains by evapotranspiration, and the decreased-inciden ce of large floods has reduced floodwater recharge and hence leaching of salt from floodplain soils. Use of soil physical properties for a r ange of floodplain soils, combined with measurements of groundwater di scharge from bare and vegetated sites, suggests that the time-scale fo r complete soil salinisation can, at worst, be less than 20 years. Mor eover, salt accumulation at most sites will continue to occur as the p resent flooding regime (of which there is limited scope for improvemen t) 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 accumul ation. Failure to implement schemes which lower the water tables benea th the floodplain may, in the long term, cause serious damage to these important riparian forests.