An analysis of the long-term trend of evaporative water loss in Lake Hefner (Oklahoma) for sustainable water management

Southern Great Plains (SGP), Oklahoma, USA. The region’s semi-arid climate, with marked year-to-year variability in temperature and precipitation, makes its water resources highly climate-sensitive. This study analyzes long-term evaporation at Lake Hefner, a medium-sized municipal reservoir in central Oklahoma, using the Global Lake Evaporation Volume (GLEV) dataset and NASA POWER climate data from 1985 to 2018. The Mann-Kendall trend test and Sen’s slope estimator detected trends, while multiple linear regression identified key climatic drivers. A water balance analysis assessed evaporation relative to inflow and outflow variables. A policy review examined how evaporation is considered in local and state water management. Evaporation increased significantly over the 34-year period (+0.113 mm yr⁻¹, p < 0.05), with the most pronounced trends occurring in autumn and winter. Air temperature was identified as the dominant driver of these changes. On average, evaporation removed approximately 13.0 MCM per year, representing about 19.1 % of total annual outflows. Cumulatively, this results in nearly 440 MCM of water lost over the study period. Despite these findings, evaporation monitoring and forecasting remain absent in current reservoir operations and broader state-level water policy. Given projected warming and rising water demand, evaporation losses are expected to intensify, posing challenges for long-term reservoir sustainability. These findings underscore the urgency of integrating evaporation into future strategies, including the 2025 Oklahoma Comprehensive Water Plan. • Rising evaporation at Lake Hefner signals growing water loss risk. • Autumn and winter show the strongest increases in evaporation rates. • Rising air temperature was identified as the dominant driver of evaporation. • Evaporation removes approximately 19 % of annual outflows. • Current water policies do not address evaporation losses despite rising risks.