Green River Basin Transit Loss Evaluation – Wyoming State Engineer’s Office

Follum Hydrologic Solutions (FHS) in collaboration with Precision Water Resources Engineering (PWRE) completed a comprehensive transit loss evaluation for the Green River Basin on behalf of the Wyoming State Engineer’s Office. The project addressed Wyoming’s need for improved, defensible water accounting in support of Colorado River Compact administration and long-term water management. The study initially focused on the Upper Green River Basin upstream of Fontenelle Reservoir and was later expanded to include the Green River downstream to the Green River gage at Green River, Wyoming. FHS delivered a multi-decade (1991–2023) water balance analysis that integrated observed gaging records, reconstructed streamflow, and satellite-derived evapotranspiration data to quantify reach-scale losses associated with bank storage, riparian evapotranspiration, surface evaporation, and groundwater interactions.

In addition to the water balance analysis, the FHS Team developed fully documented RiverWare® models representing the Green and New Fork River systems, including a novel incremental transit loss methodology to evaluate losses associated with conserved water releases. The project also included a metering and gaging gap analysis that identified priority locations where additional monitoring would significantly improve water accounting accuracy. Together, these tools and analyses provided the Wyoming State Engineer’s Office with a transparent, science-based framework to estimate transit losses, support operational decision-making, prioritize infrastructure investments, and strengthen Wyoming’s ability to understand its water deliveries under interstate agreements.