Estimating cumulative catchment streamflow depletion from abstractions

Abstraction from surface and groundwater bodies alters river flow regimes. The economic and social benefits of abstraction need to be balanced against their consequences for hydrology dependent ecological functions, ecosystem services, cultural values and recreation. However, impacts of an abstraction on flow regimes are often assessed in isolation and so cumulative impacts of many spatially distributed abstractions on the catchment are not understood. While spatially distributed, high-resolution model(s) (e.g. MODFLOW) can be developed to understand the cumulative impacts of abstractions, this is cost prohibitive and the demand for data is high (e.g., system properties, hydroclimatic) to develop such a model at regional scales and, further, such site specific models cannot be transferred to other spatial locations. We have developed a model to estimate cumulative streamflow depletion at given locations of a stream network resulting from both surface and groundwater abstractions. The surface water abstractions directly deplete the nearest river segment with which the abstraction is associated. However, depletion owing to each groundwater take, response times of which can extend to weeks, months or even years following the abstractions, was associated with all river segments which were within a 2 km radius of the groundwater take. The proportion of depletion from each river segment owing to a groundwater take is dependent on distance between well and segment, flow (based on the naturalised 7-day mean annual low flow) and length of the segment within 2 km radius of the well. Two aquifer parameters (transmissivity and storativity) are used for calculating the streamflow depletion. Field tests can be used to measure these parameters but observations are not available for all necessary locations. We used Random Forest statistical techniques to estimate the aquifer parameters at unmeasured locations. Results of the streamflow depletion model are displayed using an interactive application (app). The model can be used to obtain timeseries of cumulative stream depletion at any location in the river network from many spatially distributed abstractions.