River-aquifer interactions in a semi-arid environment stressed by groundwater abstraction

The connections between rivers and aquifers need to be understood and quantified for successful water management. However, the interactions are complex and remain poorly understood in many catchments worldwide. To advance our understanding of river-aquifer interactions in semi-arid environments stressed by groundwater abstraction, the Maules Creek Catchment, Murray Darling Basin (Australia) is investigated.First, an analysis of existing records (precipitation, evapotranspiration, river flow, groundwater levels, and groundwater abstraction) shows that the groundwater levels have predominantly declined due to groundwater abstraction rather than changes in the climate. Furthermore, the river has altered from being gaining to losing during low-flow periods, with a significant delay in this transition since the onset of abstraction. Next, two novel methods are presented: (1) using pairs of temperature time series at the river-aquifer interface to quantify the interactions and thermal diffusivity; (2) using hydrographs recorded at the upstream and downstream ends of a river reach to estimate the total recharge and rate of recharge. Both methods offer several advantages over previously developed approaches. Abstract Finally, the spatial and temporal variability in the interactions are determined. It is found that high and low river flows correspond to high and low riverbed interactions, which were strongly losing during high-flows, and slightly losing, neutral, or gaining under low-flows. It is demonstrated that episodic high-flow events account for a majority of the total groundwater recharge. It is suggested that differences in permeability between river bed/banks and the floodplain control the efficiency of recharge from the river. The results highlight that up-scaling of the interactions, especially during high-flows, is problematic. Implications for the modelling and management of water resources are discussed throughout. The process understanding gained, and methods developed, can be applied to other semi-arid environment stressed by groundwater abstraction worldwide