The watertable fluctuation method of recharge estimation: A review

•The watertable fluctuation method for recharge estimation has many different forms.•Despite widespread application, systematic testing has been limited.•The method has been modified in several ways to apply to non-ideal situations.•The method often produces estimates that differ significantly from other approaches.•The optimal approach for extrapolating the recession curve is presently unclear. Accurate groundwater recharge estimates are vital for the management of groundwater resources. The watertable fluctuation method (WTFM) is one of the most widely used techniques for estimating distributed recharge. This is likely due to the simplicity of its formulation and the limited input data requirements, which include groundwater level measurements and a specific yield estimate. The method is presented in alternative forms within published accounts of its use, including adaptations for non-ideal situations (e.g., aquifers subjected to extensive pumping). This review summarises the development of the WTFM and its variants, applied to the estimation of distributed recharge, and compares resulting recharge estimates from alternative forms of the method with other recharge estimation techniques. The review finds that the WTFM has been modified to account for shallow watertable conditions, deep unsaturated zones, groundwater pumping, seasonal variations in evapotranspiration, and aquifers with hydraulic properties that vary with depth. A key component of the WTFM is the projection of the antecedent recession curve to account for groundwater discharge during periods of watertable rise, for which there are multiple approaches. The best choice for projecting the recession curve remains unclear. Opportunities to develop the WTFM and improve its reliability include modifications to account for common boundary effects (e.g., proximity to rivers, etc.), evaluating the accuracy of recession curve projection techniques, assessing the spatial scale for which WTFM estimates of recharge apply, and incorporating more complex storage terms into the WTFM. Despite the extensive use of the WTFM, systematic testing has been limited and is therefore needed, including for the many alternative forms of the WTFM.