Reclamation Information Sharing Environment (RISE)

This Technical Memorandum focuses on the development of daily open water evaporation rates and volumes from 13 lakes and reservoirs in the Klamath River Basin that have changed between predevelopment and current conditions. Daily evaporation rates and volumes were estimated for all large waterbodies in the upper Klamath River Basin that have significantly changed between pre-development and water years (WY) 1981–WY 2020. A large effort was made to quantify the uncertainties related to the estimated evaporation rates and volumes which arose from biases in forcing data and unknown waterbody mean depths. The largest source of uncertainty was the unknown fluctuations in pre-development water surface elevations as the uncertainty due to forcing data biases was greatly reduced through bias correction of the gridMET climate data. Results suggest that while evaporation rates were similar during WY 1981–2020 and pre-development conditions, annual mean pre-development evaporation volumes were 36 to 87 percent higher than current conditions volumes. Approximately 161 to 339 TAF more evaporation occurred annually under pre-development conditions than occurred from WY 1981–2020. This is primarily due to the drainage of pre-development large shallow lakes with high evaporation volumes. The modeling data associated with this report is available at https://data.usbr.gov/catalog/8500/item/133811. This work is part of a larger effort, the Klamath River Basin Revised Natural Flow Study. This study combines seven different hydrologic modeling tools to develop natural streamflow estimates for the Klamath River Basin from WY 1981 through WY 2020. The overarching purpose of the study is to apply current science, methods, and tools to develop natural streamflow estimates, while addressing known limitations of previous estimate and incorporating recommendations provided by the National Research Council.