River-groundwater interaction: a practical management approach

摘要

In much of Australia, groundwater and surface water are interconnected and interchangeable resources. This physical reality is however not reflected in our water management systems whereby groundwater and surface water are managed as separate resources. This paper proposes a methodology for managing groundwater — surface water interaction. It is proposed that all groundwater bores be classified into four zones to reflect the relative significance of their impact on surface waters. The proposed zones would be dependent upon the level of groundwater and stream interaction and may range from an area covering bores which should be managed under surface water extraction rules, through to areas where groundwater extraction has no discernible impact on surface water. It is proposed that the zonal classification be adopted when considering new licences/trades/restrictions/cancellations, as follows: Zone 1- this applies close to streams where groundwater extraction has an almost immediate impact upon stream flow. All existing groundwater licences in Zone 1 would be managed under surface water extraction rules. Zone 2- this zone would deal with all groundwater usage that would impact on stream flow over the critical low flow period of the stream. It would be expected to cover groundwater extraction where impacts would be typically felt within 6 months. Zone 3- this zone would deal with long term (100 year) impacts of groundwater use on stream flow. This would often cover all other groundwater users in a surface water catchment with licences generally managed to annual allocations. Zone 4- this zone is where there is no discernible impact of groundwater use on the stream. The zone would not necessarily be a certain distance from a stream, but would apply to certain hydrogeological situations, for example, deep confined aquifers or disconnected streams. Various analytical methods which are available to calculate the impacts of groundwater extraction on stream flows are discussed. In many catchments, any groundwater use, regardless of the distance of the bore from the river, will result in a reduction in stream flow. The time lag between commencement of pumping and the impact on streamflow is considered particularly with respect to Zone 2 where the impacts of groundwater use on streams is recognised as occurring within the short term planning period, but no tools have been available for conjunctive management. As the distance between a pumping bore and stream increases so does the lag or delay between the commencement of pumping and the impact on streamflow (ie reduced discharge and/or induced recharge). This lag between pumping groundwater and the impact on streamflow means decisions on how to manage streamflow impacts due to groundwater pumping need to be taken well in advance of the commencement of groundwater pumping. Criteria that could be used to develop operational rules (ie restrictions) that could be applied to groundwater users in order to manage streamflow impacts are proposed.