The impacts of “far away” groundwater pumping on stream flow in Australia
Abstract
Analysis methods to define the impact of groundwater pumping on stream flow have been well known for many decades. These analytical analysis methods generally deal with the case of bores located relatively close to streams, ie. typically up to a few km from a stream. In the simple unconfined aquifer case the impact is generally 100%, albeit often with a significant time lag of from months to decades. The case where groundwater pumping is located far from a stream, ie. anywhere in the catchment, is rarely considered. In this idealised case the steady state impact of groundwater pumping is identical regardless of the distance of the bore from the river. In reality hydrogeological complexity and a myriad of other geological features results in a more complex environment where many different time and impact effects are experienced. These factors (i.e. discharge to evapotranspiration (ET), to oceans, to another catchment, to deep groundwater systems, semi-confined aquifers and disconnected streams) are discussed for Australia. The typical situations throughout Australia where groundwater pumping may occur anywhere in a catchment are examined and the factors which operate to reduce the impacts below 100% are discussed. The impacts can range from 0% to 100% depending upon these hydrogeological factors. It is postulated that groundwater ET across typical Australian catchments may operate as a significant mitigating process to reduce the impact of groundwater pumping on streams. The critical significance of the location of bores relative to where groundwater ET is occurring and the streams is illustrated by a simple numerical model. Even though ET may be important, there will often still be an impact on the streams due to groundwater pumping virtually anywhere in a typical catchment, albeit usually with a large time lag. The need for catchment scale water balances which include groundwater ET is emphasised.
Author
Dr Richard Evans