Can the dataset of field based recharge estimates in Australia be used to predict recharge in data-poor areas?

Effective management of water resources requires that all elements of the water balance be estimated. Groundwater recharge measurements are difficult, time consuming and expensive. In some cases a field study cannot be justified and simple empirical relationships are used to estimate recharge, and often the value chosen is simply a percentage of rainfall. This paper aims to use a database of 4386 field based estimates of recharge from 172 studies in Australia to produce simple empirical relationships that relate recharge to nationally available datasets and hence can be used to estimate recharge in data-poor areas in a scientifically defensible way. It was found that the vegetation and soil type were critical determinants in forming relationships between average annual rainfall and average annual recharge. Climate zones and surface geology (lithology) were not found to be significant determinants in the relationship between rainfall and recharge. The method used to estimate recharge had an impact upon the magnitude of the recharge estimates due to the spatial and temporal scales over which the different methods estimate recharge. Relationships have been developed here between average annual rainfall and average annual recharge for combinations of soil and vegetation type that can be used with only nationally available datasets to provide a recharge estimate. These relationships can explain 60% of the variance in recharge measurements across Australia. The uncertainty in the recharge estimated using these relationships is generally greater than an order of magnitude. This means that if these relationships are used to help determine water allocations, then the precautionary principle should limit allocations to less than about 5% of the estimated recharge. If allocations are greater than this, a more detailed site specific study is warranted.