Optimal management of water movement in irrigation bays

Abstract

Salinisation of Australia's agricultural lands and inland waterways is a widespread problem and its management involves significant financial resources. A key component in understanding catchment salt export is interpreting the on-land processes which determine runoff and groundwater recharge/discharge. This paper presents a decision support system for irrigation management based around a predictive model for water and salt movement within irrigation bays. The “bay model” employs a physically-based formulation, where: groundwater movement is explained by Darcy's equation; capillary rise by a quasi–equilibrium approach and overland flow by a Mannings type equation. The bay model is tested through application to a field site where water and salt transport processes were intensively monitored over a two-year period. Model predictions of overland flow and transport, groundwater and unsaturated water movement are shown to be in close agreement with field observations. An objective function is defined which incorporates irrigation efficiency (drain flow), the soil moisture deficit after irrigation, and the duration of surface ponding during irrigation events. The response of the constraints which make up this objective function are investigated with respect to irrigation practice and, as a result, the objective function is refined. This objective function is then incorporated into an optimisation algorithm (Powell's method) which is used to identify the management arrangements which lead to efficient water use.