Windward/leeward rainfall gradients in the Luquillo Mountains, Puerto Rico, and implications for water resources and biogeochemical fluxes

Abstract:

The Luquillo Mountains receive the highest rainfall in Puerto Rico and serve as “water towers” for densely-populated urban areas surrounding the range. The region has hosted hundreds of scientific investigations of hydrological, ecological, and geochemical processes in tropical forests, many of which have been included in regional and global models that compare tropical forests to other ecosystems. Precipitation is obviously a critical parameter in such studies, and thus many scientists have developed forest- and watershed-wide estimates of rainfall in the region, primarily using elevation-based models of rainfall. These rainfall estimates have been incorporated into assessments of the effects of land and climate change on water resources. However, in a compilation of all known rain gages in the region, we have found that spatial variability of rainfall is in fact very high, with complexity related to topography and wind, and rainfall amounts were often substantially lower or higher than previous estimates. Similar to other mountainous islands, leeward watersheds in Puerto Rico receive much less (50-70%) of the mean annual precipitation (MAP) received by windward watersheds. Rain fell on a majority of days across the mountains, but on fewer days at leeward sites (~60% of days) than at windward sites (~75%). Annual variability of annual precipitation at all sites was substantial; stormy days (defined as >20 mm d^-1) delivered over half of MAP. The amount of rainfall delivered by different types of storms (e.g., hurricanes/tropical storms, easterly waves, cold fronts) varied across the mountains. Human-induced warming of air and ocean by climate change will likely affect the intensity of various types of storms differently. In eastern Puerto Rico, export of sediment, carbon, and nitrogen are closely related to stream discharge, and discharge is controlled by storm events. Therefore, improved understanding of spatial variability of rainfall is imperative if we are to assess the role that climate change may play in the future of water resources and biogeochemical responses of the Luquillo Mountains.