Testing water banking recovery policies to secure drought water supply in semi-arid areas with a changing climate

Groundwater storage in many semi-arid areas is diminishing due to declining and more variable recharge. Managed aquifer recharge including water banking offers a potential response to sustain groundwater supplies. The effectiveness of this approach depends critically on how recovery credits are defined. A review of current recovery credit arrangements reveals that storage retention is currently not included in credit specifications for any jurisdiction. Storage retention depends on local hydrogeology and is often poorly defined. This paper applies a simple water balance equation to climate-driven sequences of water recharged to and recovered from a water bank. Two fundamental parameters, maximum recovery entitlement (anthropogenic) and storage retention (hydrogeological), are used to test drought-response performance of one conceptual and four historical managed aquifer recharge and water banking schemes in Australia and USA. These case studies have records of up to 67 years and all exhibit climatic change. They are used to illustrate the impact of the two parameters that could potentially be used to manage recovery credit, in securing future groundwater supplies. Results demonstrate the significant, but not always failsafe, ability of water banking to secure drought supplies under a non-stationary climate. These examples show that aquifer water retention characteristics, that are site-specific and currently rarely quantified, are very important for effective siting and operation of water banks for long-term groundwater security. Many of the reviewed jurisdictions prescribe a time limit of very few years in which a storage credit can be recovered, and thereby remove the incentive for banking water to buffer inevitable future drought. Where losses from an aquifer become desirable gains for streams, publicly funded water banking has distinct advantages over private water banking that focus on only the retained storage credits. Case studies demonstrate water banking potential under climate change at locations where aquifers have high storage retention.