Use of soil survey information to assess regional salinization risk using geographical information systems

Previous experience in southern and western Australia has shown that tree clearing alters the water balance in a landscape, leading to increased deep drainage and consequent rises in phreatic water tables. Assessment of the risk of regional salinization involves integration of hydrology, hydrogeology, soil and land management issues. An example of the use of soil survey data, integrated with water resources and digital elevation data in a GIS, to estimate the risk of salinization after tree clearing is illustrated for the upper Burdekin river basin in the wet/dry tropics of North Queensland. Soil map unit descriptions include information on soil-landform relations, parent material, soil thickness, depth to bedrock and dominant and subdominant soil types, Such information can be used to qualitatively estimate permeability and drainage classes (high, medium, or low) and to determine likely recharge and discharge areas in the landscape. Analysis of the spatial association between surveyed electrical conductivity measurements and estimated recharge and discharge areas was used to cross-check this qualitative classification, The distributions of saline soils and shallow water tables are used to assess the salinity hazard of the region. Where % total soluble salts (TSS) are >0.25% and water table <6 m deep, a salinity hazard is present, This hazard can translate into a salinity problem if the hydrologic balance is altered so that the salt is remobilized and redeposited in the landscape, An estimate of the likelihood and amount of increased recharge to groundwater after tree clearing was obtained using water balance models with soil physical parameters measured at key sites, Where physical data were not available, surrogate physical properties were estimated from the soil survey data, Results suggest that tree clearing may increase deep drainage by up to 10 times and that in watersheds where shallow unconfined aquifers are present and where %TSS > 0.25%, recharge areas should not be cleared.