Estimating Extractable Soil Moisture Content for Australian Soils

The amount of water that can be stored in soil and actively used by plants, is a key parameter in hydrologic models and is important for crop and pasture production. Often the active soil moisture store is estimated from laboratory measurements of soil properties. An alternative approach, described in this paper, is to estimate the extractable soil moisture capacity from direct measurements of soil moisture content in the field. A time series of soil moisture values, over the depth of the soil, shows the actual changes in water content. The difference between the wettest and driest profiles is an estimate of the dynamic soil moisture store. We have gathered data on extractable soil water capacity for 180 locations over Australia and have compared our values with published results from the Atlas of Australian Soils, derived from profile descriptions and pedotransfer functions. Preliminary results show that data from the Atlas of Australian soils provide a useful lower bound for measured dynamic soil moisture storage, but of the sites examined, 42% had extractable stores greater than two-times the AAS values. This was due to estimates of total soil depth that were underestimated in the AAS results compared to the active depths apparent in the data. These depths were strongly related to vegetation type.