Assessing the factors controlling rising groundwater levels by combining agricultural modelling with reactive transport modelling

Rising water tables are an important issue in irrigation areas globally, with consequences including waterlogging of the root zone, increased salinity in the root zone, reductions in crop productivity and infrastructure damage. Controlling deep drainage rates from irrigation processes and the subsequent groundwater recharge demands an understanding of interactions between agricultural management, soils and groundwater. It is well established that soil hydraulic properties for sodic soils can vary in response to the application of soil ameliorants. However, impacts of sodic soil amelioration on groundwater recharge rates are not well documented in field studies or modelling studies. In order to determine whether sodic soil amelioration could be leading to rising groundwater levels in an Australian irrigation area, outputs from an agricultural production model, APSIM, were used as inputs for HYDRUS-1D soil water and reactive solute transport modelling. This combined APSIM-HYDRUS modelling was used to assess the relative influence of soil hydraulic properties, sodic soil amelioration, irrigation management practices and crop water uptake on predicted deep drainage rates. The simulated deep drainage rates were compared with monitored groundwater level trends below the irrigated area. The combined APSIM-HYDRUS approach demonstrated the importance of considering soil chemistry impacts on soil hydraulic properties when predicting deep drainage rates However, the simulations indicated that irrigation of sodic soils, even with amelioration through the use of gypsum applications, was not sufficient to explain the rising groundwater levels. Therefore, these modelling results indicated that other factors were also contributing to rising water tables. Identifying the causes of rising groundwater levels is the first step in addressing this important global issue.