UNDERSTANDING RECHARGE AND STREAMFLOW GENERATION IN SMALL WATERSHEDS USING STABLE ISOTOPES OF WATER: STUDIES IN THE LUQUILLO MOUNTAINS, PUERTO RICO

Watersheds in the Luquillo Mountains of Puerto Rico have been studied for decades within the USGS Water, Energy and Biogeochemical Budgets program, the USFS Experimental Forest program, and the NSF Long Term Ecological Research and Critical Zone Observatories programs. In 2005, regular sampling of cloud water, rainfall, and streamflow for stable isotope analysis began in the Rio Mameyes and the Rio Icacos/ Rio Blanco watersheds; groundwater and throughfall have also been measured. Objectives of the study include establishing the isotopic signatures of precipitation from different climate patterns, determining which climate patterns are important in maintaining streamflow and groundwater recharge, and using water stable isotopes to investigate watershed response to precipitation. Precipitation inputs have a wide range of stable isotope values, from fog/cloud water with δ²H, δ¹⁸O averaging +3.2 ‰, ‑1.74 ‰ to tropical storms with rain values as low as ‑66.4 ‰, ‑9.45 ‰. Temporal records of isotopic composition in rain show a ‘reverse seasonality’ compared to higher latitudes, with higher isotopic values in the winter and lower values in the summer. Rain isotopic values correlate strongly with local and mesoscale weather patterns and with cloud altitude (and therefore atmospheric temperature). This correlation allows us to assign isotopic signatures to different sources of recharge, and to investigate which climate patterns contribute to streamflow and groundwater recharge. Rainfall in the Luquillo mountains is frequent and streams are flashy, indicating little short-term storage capacity in the watersheds. Contributing factors may be thin or low-permeability soils and consistently high saturation levels. Long-term average streamflow isotopic composition indicates a disproportionately large contribution of trade-wind orographic precipitation to streamflow, highlighting the importance of this climate pattern to the hydrology of the watersheds. Hydrograph separation experiments will yield information on streamflow dynamics, with quantification of contributing sources determined from water isotopes and solute chemistry. Research in these watersheds that encompass ridgetop to shoreline processes within ~13 km has led to insights that may not have been possible with studies on a larger scale.