The annual amount, pattern, frequency and intensity of precipitation determine ecosystem dynamics and ground water recharge. Global climate change and regional climate variability are influencing precipitation dynamics in our region. To what extent are these factors determining the eco-hydrology of the island needs to be assessed. The location of Puerto Rico in the upper northeastern portion of the Caribbean and the Central Mountain range that divides the island results in the island being affected by both northeasterly trades in the northern part and southeasterly trades in the south. Therefore the origin of precipitation events can vary depending on geographic location, time of the year and strong meteorogical events. In general the natural abundance of ð ¹⁸O and ð D in precipitation fluctuate with latitude and altitude 2, seasons1 and by unusual meteorological events such as monsoons3, hurricanes and tropical storms1,4. Therefore we can utilize the variation of the natural abundance of ð ¹⁸O and ð D in precipitation to trace the origins of precipitation and groundwater recharge1. We analyzed the sources of precipitation on two contrasting sites in Puerto Rico: Northeastern (Rio Piedras) and southwestern region (Guánica Dry Forest, ) as they represent the moist region (1800 mm annual precipitation) and the semiarid region (760 mm annual precipitation) in the island. Rio Piedras and Guánica have a bimodal pattern, usual for the Caribbean region, with precipitation peak in May-June and relative minimum in December-March and July-August. The Guánica Dry Forest, however has unpredictable year to year annual and monthly precipitation. We hypothesized that isotopic signatures of (ð¹⁸O and ðD)of precipitation in the Guánica Dry Forest (GDF) (southwest) and Rio Piedras (RP) (northeast) will reflect the sources and amount of precipitation We also tested the hypothesis that the isotopic signature of groundwater in the Guánica Dry Forest will be similar to the isotopic signature of precipitation because there will be very little evaporation prior to infiltration as Guánica Dry Forest is found on highly permeable limestone bedrock with little soil to retain precipitation.