UPTAKE OF SALINE GROUNDWATER BY PLANTS - AN ANALYTICAL MODEL FOR SEMIARID AND ARID AREAS

An analytical model, based on unsaturated zone water and solute balanc es, was developed to describe the uptake of saline groundwater by plan ts in dry regions. It was assumed that: i. initially, the profile had low water and salt contents to some depth; ii. both water and solutes move upwards from the water table by piston flow due only to plant wat er extraction; iii, the uptake of water concentrates solutes in the so il solution until some threshold salinity is reached, above which plan ts can no longer extract water due to osmotic effects; iv. uptake of t he groundwater does not affect the water table level; and v. uptake of groundwater is only limited by transmission of groundwater through th e soil, Model predictions were compared with measurements of groundwat er uptake made over 15 months at five sites in a Eucalyptus forest in a semi-arid area, using independently measured model parameters. Depth and salinity of groundwater, and soil type varied greatly between sit es. Predicted groundwater uptake rates were close to measured values, generally being within similar to 0.1 mm day(-1). Sensitivity analysis showed that groundwater depth and salinity were the main controls on uptake of groundwater, while soil properties appeared to have a lesser effect. The model showed that uptake of groundwater would result in c omplete salinisation of the soil profile within 4 to 30 yr at the site s studied, unless salts were leached from the soil by rainfall or floo d waters. However, a relatively small amount of annual leaching may be sufficient to allow groundwater uptake to continue. Thus groundwaters , even when saline, may be important sources of water to plants in ari d and semi-arid areas.