Stable Isotopes (δ18O and δ2H) Help to Delineate Flow Paths and the Importance of Different Climate Patterns in Watersheds of the Luquillo Mountains, Puerto Rico

Precipitation isotopic signatures can help determine the relative importance of different rainfall regimes in the interactions between water, soils, and ecosystems in watersheds. The tropical forest in the Luquillo Mountains of Puerto Rico can receive over 5000 mm of precipitation per year. Recent modeling studies indicate that global climate change or local land use changes may lead to a decline in precipitation amounts. Weather analysis showed that 29% of rain input to the Luquillo Mountains was trade-wind orographic rainfall, and 30% of rainfall could be attributed to easterly waves and low pressure systems, with the remainder from fronts, troughs, and isolated thunderstorms. Trade-wind orographic precipitation usually occurs as frequent, low-intensity and low-volume rain events, whereas easterly waves and low-pressure systems have higher volume and more intense rainfall. To help determine the importance of different precipitation types in the forest water cycle, monthly precipitation samples from a network of rain and cloud water collectors and stream samples from two watersheds were collected and analyzed for δ18O and δ2H. Weekly throughfall and rain samples were also collected at one site during five periods of different rainfall intensity to determine whether isotopic fractionation occurs when rain falls through the forest canopy. Seasonal rainfall sources have distinct isotopic signatures, partly due to differences in cloud height associated with the seasonal climate patterns. Monthly precipitation samples during the dry season had average isotopic values of -1.5‰ δ18O and +2.3‰ δ2H, associated with the weather pattern of trade-wind showers and fronts. Rainy season precipitation, from easterly waves and low pressure systems, had average monthly values of -3.7‰ δ18O and -16‰ δ2H. Precipitation during months with significant low pressure systems had average values of -5.9‰ δ18O and -36‰ δ2H. Isotopic composition of stream water at higher altitudes in the Icacos and Mameyes watersheds suggests that the low-intensity trade-wind showers contribute a greater proportion of stream baseflow than do high-intensity rain events. High-intensity rain events run off quickly and may not effectively infiltrate the saturated tropical soils. The seasonal cycle of rain isotopic composition is also reflected in the streams and can be utilized to estimate contributions from shallow and deep flow paths for both watersheds.