Puerto Rico is densely populated, with 3.7 million people living primarily on the mainland island. Forests, which make up about 54.7 percent of the island's land cover, are important for its water supplies. Forest Service researchers at the agency’s Eastern Forest Environmental Threat Assessment Center and their partners used the Water Supply Stress Index (WaSSI) model, which estimates monthly water balance and flow routing, to quantify the amount of water originating at forested watershed outlets. To quantify the population that relies on this water, researchers used water intake location data from the U.S. Geological Survey and data on population served from the Puerto Rico Aqueduct and Sewer Authority. They estimated that all lands in mainland Puerto Rico produce 6.5 billion cubic meters per year of water. Of that total, National Forest System lands (3.3 percent of total land area) provide 219 million cubic meters of water, and commonwealth and private forest lands (40 percent of total land area) provide 2.6 billion cubic metersof water. This water serves a population of 3.6 million people from 113 intakes located downstream of commonwealth and private forest lands. Thirteen of the intakes that serve 780,000 people are located downstream of El Yunque National Forest, which receives relatively high precipitation and produces the highest runoff on the mainland. This analysis provided quantitative information on water yield from forested lands and sources of water supply in Puerto Rico.