Allocations of river and groundwater have been traditionally treated separately in Australia and many other parts of the world even though in many regions groundwater and river systems are hydraulically connected. Groundwater extractions in areas where river systems are hydraulically connected can cause substantial impacts to river flows particularly base flows or low flows, which are considered to be ecologically important.

Traditional groundwater modelling approaches tend to be undertaken on time scales of weeks or months which are not sufficient to demonstrate the impacts of groundwater extractions in many river systems, particularly where flows are ephemeral. This research considers the impacts of groundwater extraction on surface water flows using a simple conceptual modelling approach called IHACRES_GW. The Cox’s creek catchment in the Namoi river basin, Australia is

used as a case study. The research effectively demonstrates that groundwater extractions are having significant impacts on baseflows in this area and that current policies will not be effective in reducing these impacts. The research also demonstrates the potential of such an approach to be used in conjunction with traditional groundwater models when setting allocation limits and considering sustainable groundwater yields.

The application of the IHACRES_GW model in the Cox’s Creek subcatchment demonstrated that groundwater extraction affects the frequency, timing and magnitude of baseflow events, and that the impacts vary not only as a consequence of the extraction rates and other losses to groundwater storage, but also according to the groundwater recharge rates. The legacy that historical rates of extraction have on overall groundwater storage volumes and associated baseflow discharges is a function of the net recharge to the exploited aquifer system versus loss as a consequence of extraction and other groundwater losses. It can take decades or

longer to recharge aquifers to pre-drought storage levels if groundwater resources have been heavily and/or overly exploited. Conversely, during wetter climatic periods, particularly when associated with flooding and increased groundwater recharge, groundwater storages may be replenished within a relatively short time. Although groundwater recharge rates are not required for estimating sustainable pumping rates, they are critical for an accurate assessment of groundwater-river interactions and sustainability assessments.

Application of the IHACRES_GW model to the Cox’s creek catchment has been able to show that the estimated sustainable yields of 7,200 ML for zone 2 and 18,600 ML/yr for the combined zone 2 and zone 9, covering the entire Cox’s creek catchment are likely to have significant impacts on surface water resources in the area. The lower limit in zone 2 is likely to reduce baseflows by 6,000 ML/yr and lead to slower recoveries of the river system following drought periods. The model shows that for extraction rates greater than 9,000 ML/yr the groundwater and surface water systems would be permanently disconnected. It suggests that a limit of between 7,000 and 8,000 ML/yr across the whole subcatchment (including both zone 2 and zone 9) would be most appropriate.

If results of this research were to be adopted by government the outcomes for irrigation industries are likely to be variable. In some areas this could mean reduced groundwater allocations, particularly during dry periods, although this is likely to be offset in other parts of the industry by increased surface water availability and improved capacity of the river system to recover from drought. It is possible that some impacts could be offset by allowing greater surface water access, although the nature of impacts from such a scheme would need to be tested.