Groundwater–Surface Water Simulation Case Study : Namoi River

Prediction tools that simulate complex groundwater – surface water interactions are needed to assist in the sustainable allocation of water yet are handled poorly in existing river planning models. Because of this, eWater CRC have been working in partnership with the CSIRO and National Water Commission, to develop a Groundwater – Surface water modelling component for Source. Trials of this Groundwater – surface water (GW-SW) link model have been running in the Namoi River throughout 2010 where real world data was used to simulate the flux of water between the river and groundwater aquifer.

The Trial

The trial focused on the reach between Boggabri and Narrabri, which was ‘predominantly gaining’ (i.e. more water is gained from the aquifer than is lost from the river) during the period of interest. To highlight the differences, Source was calibrated both with and without the Groundwater–Surface Water simulation.

Using data from the Upper Namoi MODFLOW model (NSW Office of Water), exchange fluxes were modelled between the surface water system and the groundwater system. April 1992 to December 1995 was chosen as a suitable period because the majority of flows were confined to within the river channel.

Building the Namoi River model

The conceptual river model of the Namoi was built with data from gauging stations along the reach of the Namoi between Boggabri and Narrabri. Direct losses from evaporation and gains from rainfall were modelled using the climate data at Boggabri. Monthly irrigation diversions were disaggregated to daily flows by using patterns of simulated diversion subject to river flow constraints and the model was calibrated for best agreement with the observed outflows. The Linear Muskingham method with a one-day lag time was used for routing flows.

The impacts of groundwater pumping, irrigation and diffuse recharge were quantified using data from the Upper Namoi MODFLOW model. The cumulative groundwater -surface water exchange flux was estimated by assuming linearity and summing the impact of individual stresses. Daily stage-height time series from the Boggabri upstream node was used to estimate bank storage fluxes across the reach.

The river model was subsequently recalibrated to obtain best agreement with the observed outflows at Narrabri and to include the GW-SW exchange fluxes.

The Results

The Namoi trial of the groundwater module in Source has been very promising. The results clearly indicate that Source can take explicit account of the influence of groundwater processes - a first for any river model - to significantly reduce residual flows to reflect only ungauged quick flows from the reach.

Moreover, this functionality explicitly accounts for the time lags associated with groundwater processes and has the potential to predict future unrealised impacts of processes such as groundwater pumping. This will improve the forecasting of groundwater-surface water fluxes in the Source models.

This module has been developed as part of the GSWIT project funded by the National Water Commission through the Raising National Standards program.

We have released a free public version of Source, which is suitable for IWRM studies and development of customised Decision Support Systems. Source (public version) is a fully-featured hydrological, water balance and water quality tool, an ideal entry point for Transboundary IWRM studies and research based on the leading hydrological and water quality foundations of Source. For more information on this free version of Source, visit us at http://www.ewater.com.au