Reclamation Information Sharing Environment (RISE)

Reservoir evaporation is a critical but poorly quantified component of the western U.S. water budget, and operational estimates have long relied on static monthly rates, pan coefficients, or simplified equations that miss heat storage, wind effects, and seasonal variability. This report presents a physics-based framework for estimating daily evaporation at 247 Bureau of Reclamation reservoirs across 17 western states using the Daily Lake Evaporation Model (DLEM), a modified Penman model that accounts for heat storage and fetch effects. Forced by RTMA and gridMET meteorological data, DLEM produces daily evaporation time series from 1980 to present, delivered through a web application and API.