Accurate assessment of water budgets is critical for effective management of water resources, especially on small, densely-populated islands with extremely limited storage capacity such as Puerto Rico. A water budget defines a balance between inputs, outputs, and storage. The water budgets described herein provide a generalized summary of the inputs, extractions, and outputs from four watersheds in and near the Luquillo mountains using rainfall, runoff, and public-supply extraction data as well as estimates of groundwater losses and inputs such as cloud drip and infiltration from septic tanks. Mean annual rainfall accumulation during a 7-year study (1991 to 1997) ranged from 1,722 mm in the Canóvanas watershed, to 4,235 mm in the Icacos and Mameyes watersheds; the Cayaguás watershed had 2,172 mm. Combined runoff, groundwater flow and withdrawals ranged from 47 to 73 percent of inputs (combined rainfall, cloud drip and septic tank infiltration). Evapotranspiration, calculated as the water budget residual, amounted to 27, 40, 44, and 53 percent of total moisture inputs in the Icacos, Cayaguás, Mameyes, and Canóvanas watersheds, respectively.