COMPARISON OF DIFFUSE DISCHARGE FROM SHALLOW-WATER TABLES IN SOILS AND SALT FLATS

Diffuse (evaporative) discharge of ground water is of interest in the management of local or regional ground waters, and soil salinity. However, past studies show that discharge may vary greatly between soils in agricultural areas and salt flats for similar water table depths. Low discharge from salt flats has been previously attributed to the effect of salt crusts, yet possible soil hydrological reasons for these differences have not been examined. Steady-state hydraulic theory describing the relationship between discharge and water table depth is reviewed. The minimum water table depths required for the theory to be applied are defined in terms of soil parameters. Relationships between discharge and water table depth are then used to analyse the results of previous diffuse discharge studies. It is shown that discharge from both bare agricultural soils and salt flats is consistent with this theory. Unsaturated hydraulic conductivity of three salt flat soils, determined from measurements of discharge and soil matric suction, showed that low discharge fluxes recorded from the sites were due to low soil permeability. The relationship between discharge flux and water table depth calculated for these sites also described discharge from other salt flats, implying that low hydraulic conductivity caused low discharge from these areas as well. The reasons for the low hydraulic conductivity of salt flat soils are not clear, and need to be investigated to determine if it is a general property of soils in these areas, or results from the high salinity levels.