Long-term annual groundwater storage trends in Australian catchments
Introduction
Groundwater represents a large proportion of readily available freshwater resources on a global scale and is the only available water resource in some areas [1]. During prolonged dry periods, groundwater sustains low flows in streams and changes in climate and land use can significantly affect groundwater storage and low flow regimes [2], [3]. Characteristics such as the magnitudes and durations of low flows have been widely used to determine water allocation and ecological water requirements [2]. It is predicted that increasing human consumption and climate change will have profound effects on future water resources, although the predictions are associated with large uncertainties [4], [5], [6], [7]. Most studies of these impacts on hydrology have focused on surface water and much less is known about the impact on groundwater [8], [9]. In any event, the identification of trends in groundwater requires long time series of observations (viz. at least half a century to a century). Unfortunately, long-term direct measurements of groundwater levels are rare and often too short to make reliable inferences of groundwater trends at regional scales. This is certainly the case for many regions in Australia, where groundwater monitoring started in the 1970s and 1980s with discontinuous measurements. There is also the issue of scale; measurements of groundwater level in single isolated wells represent local scale, whereas water resources management requires information at the catchment scale, so that a wide network of many wells would be needed but is rarely available.
As noted, during dry periods low flows of a river derive primarily from water released by the upstream groundwater aquifers. Consequently, on the basis of hydraulic groundwater theory, this principle has been applied [10] to develop a method that estimates groundwater storage changes using daily streamflow data from a catchment. Because streamflow measurements typically commenced much earlier than groundwater records, this method can provide regional estimates of groundwater storage for much longer time periods than for those obtainable from groundwater wells. Applications have already been made to catchments in Mongolia [11], Japan [12] and USA [3], [13] to study trends in groundwater storage. Over the past 50 years, many catchments in Southern Australia have experienced declines in annual streamflow, mostly due to reductions in annual rainfall [14], [15], [16] but also due to reductions in groundwater storage [17]. Significant changes in low flows have been observed in these catchments with some perennial streams even becoming ephemeral [15]. In Australia many regions are dependent on groundwater resources for irrigation and domestic water use, and a number of the regions comprise significant groundwater-dependent ecosystems. A better insight into how groundwater systems in Australia have been changing will be critical in developing sustainable water resources management plans. The objectives of this study are (1) to test the groundwater storage trends derived from baseflow observations [10] against observed groundwater level data and (2) to examine long-term groundwater storage trends during the past century in selected catchments in Australia.
Section snippets
Catchment description and data
This study selected 17 catchments that have at least 45 years of unregulated daily streamflow records and the area of the catchments ranges from 196 to 8358 km2 (Fig. 1). Unregulated streamflow is defined as streamflow that is not affected by human control or diversion. The selection of the catchments was also based on consideration of availability of continuous daily streamflow records, if possible though not absolutely necessary, groundwater level data, and a distribution of hydroclimatic
Groundwater storage trends estimated from base flow
The low flow hydrograph can be expressed as a function of time, as:where Q is the rate of flow and t is the time . For convenience of comparison with other fluxes in the water cycle such as rainfall and evaporation, in what follows, is transformed to flow per unit of drainage area , and denoted by , in which is the area of the catchment.
Probably the most commonly used functional form of in hydrology is of the exponential type, namelywhere K is
Characteristic drainage time scale
The values of the catchment drainage time scale estimated by means of the method described in the previous section are listed in Table 2. Fig. 2, Fig. 3 show examples for the Barron River and Hunter River. For the 17 selected catchments, the drainage time scale K varied between 25 and 101 days with an average value of 51 days. These values are consistent with those obtained in other studies (e.g. [3], [10], [11], [13, Fig. 1]), where it was found that was on average 48 days, with an
Conclusions
This study examined groundwater storage trends in 17 selected Australian catchments over the last 45 to 97 years from measured daily streamflow. The method is based on the concept that base flow in a natural river system is directly controlled by groundwater storage and hence measured streamflow can provide quantitative estimates of catchment-scale groundwater storage evolution. The selected catchments have at least 45 years of continuous daily streamflow data and represent different
Acknowledgments
This study was supported by the CSIRO Water for a Healthy Country Flagship. We would like to thank Anthony O’Grady for discussion and suggestions on catchment scale evapotranspiration from groundwater table. We would like to acknowledge the Department of Primary Industries, Parks, Water and Environment, Tasmanian, Department of Environment and Primary Industries, Victoria, Department of Primary Industries, New South Wales, Department of Natural Resources and Mines, Queensland, Department of
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