Predicting Irrigation Return Flows to River Systems: Conceptualisation and Model Development of an Irrigation Area Return Flow Model

Irrigated agricultural uses 72% of total water diversions in Australia and its potential to generate returns flows to river systems in terms of both quantity and quality is significant. Increased hydraulic loading under irrigation and changes in land use has lead to high water tables and land salinisation and sodification. Drainage schemes have been implemented to reduce the consequences of land salinisation. These drainage schemes contribute large amounts of salt, nutrients and sediments into natural water courses and have lead to a decline in water quality in rivers and reduced health in riverine ecosystems.The implications of management and interventions to drainage systems are complex and often have the potential to cause significant impacts on stakeholders in the system unless careful consideration is given to all aspects of the system. There is a need to clearly understand the tradeoffs between management options and interventions and impacts on drainage return flows, and to conduct water accounting (quantity and quality) at these scales.Hence tools or frameworks which allow all aspects of the drainage intervention to be considered and trade-offs between stakeholders investigated allow improved decision making to safeguard against solutions which only address the symptoms of a particular problem, leading to a further different set of problems, often transferring the problem downstream.This paper presents the conceptualisation and model development of an irrigation return flow model, called "Tiddalik", for the prediction of drainage return flow volumes and salt loads to streams and river systems. Conceptualisation of the major drivers of return flows are presented along with the interaction of land use management variables that determine generated return flow volumes and salt loads.The Tiddalik model can be used to look at a range of management and operational options for meeting license conditions that are applied to return flows from irrigation areas. These may include flow conditions and/or quality conditions such as salinity limits/EC credits. Various scenarios are presented from increasing irrigation efficiency to large scale land use changes (i.e. changed cropping systems, drainage implementation) for their effect on drainage volumes and salt loads.Core building blocks of the model which include evapotranspiration, soil water balance, upflux, watertable, subsurface drainage, irrigation system and on farm storage/recycling system modules and their limitations are described and discussed.The Tiddalik model seeks to provide a transparent framework whereby users have the ability to investigate management options and trade-offs for meeting environmental targets in relation to drainage return flow quantity and quality.