Changes in the surface water – Groundwater interactions of the Murray-Darling basin (Australia) over the past half a century

Abstract

The Murray-Darling Basin is Australia’s most stressed water resource and most contentious in the sharing of those water resources. Our hydrological models used for water management have no information on the surface water – groundwater interactions within a reach and so implicitly assume that their relationship is stationary. This study seeks to test this assumption by looking at the direction of surface water – groundwater exchanges and how they have changed over the 49-year period from 1970 to 2019 at the whole of basin scale (492 river reaches). This was achieved by comparing the groundwater level at 59,340 bores to the elevation of the surface water at the closest point on the stream network. Each point evaluated on the stream network was classified as either gaining (water from groundwater) or losing (water to groundwater) at an annual scale. These point scale observations were aggregated to the reach scale (as used in river systems models) for analysis of trends though time. The proportion of bores predicting losing conditions was falling through the 1970’s and 1980’s before reaching a minimum of 48% in 1995/96. From this period the proportion of bores predicting losing conditions increased to a maximum at the end of the analysis period of 78% in 2018/19. This finding at the point scale is replicated at the reach scale. The reaches with greater than 80% of bores predicting losing conditions had a minimum in 1993/94 (at 40%) and increased to 66% at the end of the analysis period. These observations are consistent with the management challenges of the time. In the 1980’s and 1990’s gaining conditions leading to increasing salt loads in the streams caused by groundwater discharge were a major problem for downstream water users. Since the turn of the century drought and water scarcity have been the major challenges with falling groundwater levels leading to increases in losing conditions. This study has shown that the surface water – groundwater interactions are not stationary and have been changing through time with many reaches having changed direction from gaining to losing throughout the analysis period. This is highlighting a deficiency in our hydrological models that cannot replicate this change in flux direction and therefore leading to errors in predicting low flows.

Topics & Concepts