Stable isotope variations in a tropical ‘waterscape’

Abstract:

Isotopic variations and hydrological fluxes remain poorly understood across tropical ‘waterscapes’, particularly in Central America. Here stable isotopes (δ¹⁸O and δ²H) in groundwater, surface water, and rainfall are used to produce isoscapes for Costa Rica, from which we estimate precipitation-groundwater (P-GW) ratios to elucidate recharge mechanisms and produce a grid-based and isotope-informed runoff estimation evaluated against gauging station data. Surface water and groundwater isoscapes reflect the strong orographic separation into the Caribbean and Pacific slopes as induced by different air masses sources transporting moisture with contrasting isotopic ratios as depicted in the rainfall isoscape. P-GW ratios reveal that groundwater recharge is biased towards intensive and more depleted monthly rainfall. Spatially, groundwater isotopic ratios indicate a weak influence of secondary evaporation across the Caribbean slope suggesting rapid recharge via preferential flow paths, whereas in the Pacific slope, there is a clear evidence of secondary evaporation implying a more parsimonious recharge/discharge process.