Annual precipitation on the Caribbean island of Puerto Rico has decreased steadily during the twentieth century, on average by 16%. The reduced rainfall manifested itself in the form of regular water rationings for millions of inhabitants during the 1990s. This chapter examines the link between the reduction in precipitation and the land-cover change using a combination of energy balance measurements and meso-scale atmospheric modeling. The explanation of the reduction in precipitation proved to be different than expected. Based on measurements made earlier over rain forest and pasture in Amazonia, a forest-covered island was expected to be cooler because of the higher transpiration of forest compared to grassland. The opposite proved to be the case: transpiration by a coastal wetland forest was less than that of an adjacent, well-watered grassland. In addition, the forest's albedo was 8% lower than that for the grassland. Together, these two factors caused the sensible heat flux over the forest to be twice that over the grassland. The surface energy balance observations over forest and grassland were used in a meso-scale atmospheric circulation model (RAMS) to simulate the meteorological effects of island-wide deforestation. The simulations indicated that the development of a sea breeze during the day dominates the climate on the island. In model runs in which the island was assumed to be completely covered with forest, the sea breeze was considerably stronger than when the vegetation had been transformed to grassland. Along the sea breeze front, convergence caused upward air motions. […]
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