Water balance modelling in Bowen, Queensland, and the ten iterative steps in model development and evaluation

This paper describes a Geographic Information System (GIS)-based method of estimating the water balance as applied to a coastal aquifer near Bowen, Queensland, Australia, using the ten interactive steps in development and evaluation of environmental models of Jakeman et al. [2006]. The 220 km Bowen irrigation area is data-rich with 260 observation bores plus stream gauging, metering of irrigation bores and detailed land use mapping. The method uses Darcy’s Law, which defines the volumetric rate of flow through a tube of saturated medium as a relationship between the hydraulic conductivity of the medium, the hydraulic gradient in the tube and the cross-section area of the tube. The groundwater components in the study area are discretised into polygons to which Darcy’s Law is applied to estimate the magnitude of the water balance components. The study calculated that, on average, aquifer recharge is 87% from rainfall and irrigation, 12% from rivers, and less than 1% from lateral inflow from adjacent groundwater storage. Of the groundwater discharges, on average, 66% is due to evapotranspiration, 28% is from groundwater pumping, 4% is submarine discharge, and 2% becomes river baseflow. The method proved cost and time effective and provided important insights to the groundwater dynamics of the area. The modelling approach is generally applicable to data-rich aquifers.