Simple method for estimating groundwater recharge on tropical islands

Simple equations relating recharge estimates from previous soil water-budget studies in Hawaii, USA, to mean annual rainfall, soil infiltration, and infiltration minus potential evapotranspiration (PE) were developed. Tests indicate that the equations can be used to estimate groundwater recharge on other tropical islands, but each equation has a different degree of accuracy. In general, the equations will have smaller relative errors in regions having wet climates. The rainfall equation is the least accurate; its use is limited to applications that do not require the highest accuracy. The infiltration and infiltration-minus-PE equations are more accurate and may be useful for regional recharge estimates in areas having mostly high rainfall or a mix of high to moderate rainfall. Transferability of the equations is limited to areas where the predominant climate and soil characteristics are generally similar to those in Hawaii. The equations also may not be accurate enough for estimates where high spatial or temporal detail or accuracy is required. Despite these limitations, the equations can provide a simple and quick generalized estimate of regional recharge for many tropical islands. Because of their simplicity, the equations may be particularly useful for providing a quick and independent estimate of recharge to compare with estimates determined by other methods.