Revisiting carbonate island recharge using a combined modelling framework to calculate aquifer storage and recharge

Most communities on carbonate islands face increasing freshwater scarcity because island hydrologic characteristics raise their vulnerability to variations in rainfall, sea level rise, and groundwater extractions. Engineered options for freshwater production can be challenging and expensive, leading many island communities to rely on groundwater for potable water supplies. Consequently, freshwater recharge magnitudes and rates provide critical information for accurate assessments of water resources. Recharge is commonly estimated with water balance models based on low frequency (monthly) precipitation (P) and evapotranspiration (ET) values, but such coarse analyses may miss short-term recharge (e.g., stochastic storm events) in high permeability carbonate island settings. To test the efficacy of water budget techniques and gain new insight on recharge dynamics in carbonate islands, we develop a recharge model framework that requires only high temporal frequency (15 min) groundwater level data to determine aquifer permeability, storage, and daily recharge. We apply the method on San Salvador Island, The Bahamas using 14-months of groundwater level data. We validate our aquifer characteristic assessments with high spatial resolution permeability and porosity measurements from two separate cores. Over the 14-month study period, recharge totaled 428 mm, or 37% of total precipitation, which is ∼2.5–3 times the monthly water budget estimate of ∼164 mm recharge or 12% of precipitation. Most recharge occurs within a day of precipitation, limiting the effectiveness of low-resolution estimates and emphasizing the need for high-resolution recharge for water management. Our recharge analysis also shows a power law relationship between increasing recharge fraction with decreasing precipitation amounts, highlighting limitations of using a single value of the fraction of precipitation that becomes recharge. A sensitivity analysis on the new model framework shows that recharge basin length is the primary source of uncertainty, but this uncertainty may be reduced with data available on small carbonate islands. Our results provide guidance on collecting data to estimate recharge in carbonate island settings and our model framework can improve freshwater management practices on carbonate island settings using easy to deploy and maintain water level loggers. • We have developed a new recharge framework to be used in data poor settings. • We analyzed high frequency groundwater level data to analyze recharge dynamics. • In carbonate islands, water budget techniques may produce erroneous recharge. • Our method can be applied to other remote settings to explore water budgets.