STABLE ISOTOPE MEASUREMENTS OF RAIN, CLOUD WATER, AND STREAMS IN THE LUQUILLO MOUNTAINS, PUERTO RICO

Quantifying water budget components in forested mountain catchments is difficult in areas with cloud water input, as there are no standard methods for measuring cloud water deposition to forests. Isotope hydrology methods may be useful, but few studies on the isotopic composition of cloud water have been done. This investigation uses δ¹⁸O and δ²H of water to estimate contribution of orographic (including cloud water) and synoptic-scale precipitation to stream flow in a mountain range in the trade wind latitudes. 11 precipitation collectors (9 rain, 2 cloud water) placed along windward-leeward elevation transects in the Luquillo Mountains of eastern Puerto Rico have been sampled monthly for δ¹⁸O and δ²H since April 2005. Isotopic signatures correlate with summer season, dry season, and low-pressure system origins for rainfall. Dry-season precipitation was enriched by as much as 5‰ in δ¹⁸O compared to precipitation from a low-pressure system. The 12-month volume-weighted average δ¹⁸O gradient is 0.13‰ per 100 m over the 1050 m altitude range of the study area. Cloud water isotopic signature was enriched by an average of 0.4‰ in δ¹⁸O compared to rain for the Pico del Este site (1050 m), but not for the Bisley site (480 m), suggesting that the efficacy of passive cloud water collectors for isotope sampling varies with wind exposure. Samples were taken from the Mameyes and upper Icacos catchments to determine precipitation sources contributing to flow in the rivers in February 2006 (dry season). The R. Mameyes system was sampled at 8 sites from 4 to 770 m. The upper R. Icacos system was sampled at 5 sites from 450 to 990 m. In the Mameyes catchment, stream chemistry was consistent with an increasing proportion of groundwater with decreasing altitude. Isotopic composition of stream samples from both the Mameyes and Icacos catchments was similar at all altitudes and fell between the volume-weighted average rain composition and the previous month's rain composition. The R. Mameyes stream samples did not follow the slope of rain isotopic composition with altitude, suggesting that it has a larger proportion of groundwater at lower altitudes and a larger proportion of orographic precipitation at higher altitudes. Additional stream sampling is planned to investigate sources of streamflow during different rainfall regimes.