Analysis of hydrologic, meteorologic, and isotopic data collected over 3 yr quantifies and explains the enormous variability and isotopic enrichment (δ18O = +16.9, δD = +50.0) of alpine Lake Waiau, a culturally and ecologically significant perched lake near the summit of Mauna Kea, Hawai'i. Further, a simple one-dimensional hydrologic model was developed that couples standard water budget modeling with modeling of δD and δ18O isotopic composition to provide daily predictions of lake volume and chemistry. Data analysis and modeling show that winter storms are the primary source of water for the lake, adding a distinctively light isotopic signature appropriate for high-altitude precipitation. Evaporation at the windy, dry summit is the primary loss mechanism for most of the year, greatly enriching the lake in heavy isotopes.