Near surface water balance in the Northern Murray-Darling Basin.

The water balance allows the calculation of deep drainage from other components of the hydrological cycle. Deep drainage has been linked to outbreaks of dryland and irrigated salinity. Until recently, deep drainage was not considered to be an issue on the alluvial plains of the Northern Murray-Darling Basin. Recent simulation studies and calculations using the water balance suggest that substantial deep drainage occurs under irrigated agriculture. However, these estimates have large uncertainties due to possible errors in measurement, calculation and due to spatial variability. On a catchment scale the relative area under a certain land use as well as the connection to local groundwater and the influence of anomalies such as prior streams needs to be considered. This paper discusses the current state of knowledge on the water balance in the Northern Murray-Darling Basin and highlights the need for a concentrated effort to measure all the components of the water balance in this area, as well as the effect on shallow groundwater quality and levels.