Land Management Impacts on Evaporation from Shallow, Saline Water Tables

Abstract

Rising water tables and associated soil salinisation are an increasing agricultural and environmental threat in Australia. Water tables will continue to rise over the coming decades (and in some cases centuries) because of the practical limits to reducing recharge in affected catchments, and the time lag between recharge and discharge in the landscape. Thus, land management techniques must be identified to minimise the detrimental impacts of this process. It has generally been assumed that plants overlying shallow water tables take up and transpire more groundwater than would have been evaporated if soils were bare. However, a collation of results from 20 field studies shows that trees, shrubs and crops rarely transpire more than 500 mm yr -1 of saline (electrical conductivity > 5 dS m-1) groundwater. These rates were generally similar to potential groundwater evaporation rates from bare soils with water tables between 1 and 2 m depth. To experimentally verify this conclusion, evaporation from shallow groundwaters under three different land uses (trees, pastures and bare soil) were compared over 18 months at three sites in southeast Queensland. Evaporation of groundwater from trees equalled that from bare soil at one site, but was three times greater than evaporation from bare soil at the other two sites. Groundwater evaporation rates were lowest from pastures at all sites, and were negative (indicating net recharge of the water table) at one of the sites. Transpiration of groundwater continually increases root zone salinity which, in turn, limits water uptake. This process can be modelled to show the interacting effects of water table depth and salinity, plant salt tolerance and soil hydraulic properties on groundwater evaporation. Results of model simulations and the field studies indicate that evaporation of groundwater is most likely to be higher under vegetation than bare soils where water tables are relatively deep and of low salinity, roots can penetrate to the water table, and soils are of low permeability. In other situations, salinisation of the root zone soil limits groundwater evaporation rates from plants, so that rates are no greater than those from bare soils.