Quantitative evaluation of alternative formulations of the watertable fluctuation method of recharge estimation

Accurate estimation of groundwater recharge is critical for ensuring the sustainable management of water resources. The watertable fluctuation method (WTFM) is widely used to estimate distributed groundwater recharge in unconfined aquifers, yet its implementation varies significantly, particularly in the extrapolation of recession curves to estimate groundwater discharge during recharge events. Despite the method’s popularity, the accuracy of its most commonly applied variations has not been systematically compared. This study evaluates six WTFM variants by applying them to 1,000 model-generated hydrographs to identify the most accurate approach. Recharge estimation error is characterised according to model input parameters transmissivity, specific yield, recharge, aquifer length, and distance between the observation well and the groundwater discharge boundary.

Results show that the RISE method, which neglects ongoing discharge during recharge events, performed the poorest, underestimating gross recharge by an average of 22%. The commonly used exponential local recession curve method also underestimated recharge, with an average error of 14%. In contrast, the fixed-timestep master recession curve method emerged as the most accurate, underestimating recharge by only 4% on average. This method’s assumption of higher discharge rates during a rising watertable aligns more closely with Darcy’s Law if water bodies receiving groundwater discharge have approximately constant water levels. Notably, for all WTFM variants tested, the greatest range of error was found to occur near groundwater discharge boundaries, where aquifers with high transmissivity produced the greatest underestimation of recharge.

These findings provide valuable insights for improving the accuracy and reliability of WTFM applications in groundwater recharge investigations. This study offers actionable insights for hydrologists and water managers seeking robust recharge assessment methods.