Transport of salts in catchments and soils

Abstract

Rainfall-runoff, and stream discharge-salinity relationships of forest catchments are shown to be consistent with mean sub-soil hydraulic conductivity and rain intensity-duration data. Similar analyses should be applicable in the study of dryland salinity problems. Stream water salinity did not vary with discharge rate in a catchment with highly weathered acid soils, that had long solute travel times and where depth dependent exchange reactions occurred during transport of anions. Such anion exchange behaviour is rare in irrigation soils, which usually differ in clay mineralogy, pH and base saturation. It is shown for Yandera loam that, under flow rates and solute concentration gradients which occur in irrigated conditions, the convective solute flux exceeds the diffusive flux by two orders of magnitude. An order of magnitude calculation shows that in most cases this is likely for the dryland situation too. Consideration of the concept of critical depth of water table for surface soil salinisation indicates that such depths may not differ as much from the irrigation case as is expected from isothermal flow theory.