Data driven model development for estimating salt export from irrigation areas

Land and Water Management Plans (LWMPs) were developed for the Murray region irrigation areas in 1995 to improve economic and environmental sustainability. These LWMPs needed to be assessed under the Salinity and Drainage Strategy (SDS) to determine their effect on salinity in the Murray River. The assessment required estimating flow and salt exports for a number of scenarios. The LWMP areas are dominated by shallow watertables with an underlying rising trend coupled with climatically driven fluctuations. Salt is exported by two pathways: seepage of groundwater directly into drainage lines; and mobilisation of salt to the surface and subsequently in surface runoff. The assessment required the development of a robust modelling system to estimate changes in water levels over time, and estimate salt export by the major pathways. The assessment also had to be flexible enough to account for complex management changes described in the LWMPs. After initially adopting a complex integrated modelling system with limited success, a revised modular modelling approach was adopted where model components represent catchment hydrologic response in a simple manner, and encapsulate available data. Historical monthly groundwater levels were interpolated spatially resulting in a verifiable estimate of watertable behaviour, and simple assumptions were used to predict future watertable changes. These groundwater levels were used as inputs to surface runoff models coupled with simple estimates of salt washoff, and to estimate seepage of saline water into drains. The effect of assumptions in the model components were tested by sensitivity analysis.