Water Balance

Modelling the dynamics of the water cycle at a national scale, with the capacity to model land cover and land use effects on the water balance, was recognised as being critical to investigating the impacts of climate and land use change on landscape water balances. A steady-state catchment water balance modelling approach (PDF - 901kb) has been applied to estimate transpiration, soil evaporation, runoff, deep drainage and irrigation. Under this approach, precipitation is equal to total evaporation plus runoff and drainage to below the root zone (deep drainage). Total evaporation (evaporation plus transpiration) is determined by water supply (rainfall and soil water) in dry environments and energy supply (radiation) in wet environments.

Total evaporation (evapotranspiration) rates are determined by a land-use-dependent parameter, or water coefficient. For example, all other things being equal, forests will transpire more water than grasslands. The value of this parameter describes the influence of catchment land characteristics, or land use, on evapotranspiration.

Potential irrigation demand in any particular region is calculated by ‘irrigating’ relevant pasture, crop and horticulture cells in a land use raster dataset according to the amount of water needed to raise actual evapotranspiration to potential evapotranspiration.

An estimate of return flow from irrigation (PDF - 177kb) is calculated as the sum of runoff and deep drainage amounts generated from modelled irrigation water demand in irrigated areas.

Demand for water from rural communities and urban areas has been estimated using Australian Bureau of Statistics and National Land Water Resources Australia data on water use and demographics, further spatially analysed where necessary by the Bureau of Rural Sciences (BRS).

Data on the location, type and volume of water storages has been sourced from Geosciences Australia and the Australian National Committee on Large Dams, while information on irrigation licence allocation and use has been obtained from the Australian National Committee of the International Commission on Irrigation and Drainage.

Groundwater information, including recharge volumes based on analyses of chloride sampling using mass-balance equations, is being added as it becomes available from BRS analyses of data held under the Australian Water Data Infrastructure Project.