Reference Tuesday: Evaporation Studies

WRCA receives a number of unique research queries, some of which are of interest to the larger population. With that in mind, we present Reference Tuesdays, in which we will post oft-asked or general interest reference questions along with our answers.

WRCA recently received a question about current evaporation data for specific reservoirs in California. There are several sources, but primarily these data are based on pan evaporation studies. There is surprisingly little information about this topic in the literature.

The California Department of Water Resources (DWR) has published several Bulletins (1947, 1959, 1974 and 1979) about reservoir evaporation. These reports summarize all available data on pan evaporation measurements in California. "Evaporation from water surfaces in California" (WRCA call no. R G400 XW7 nos. 54, 54-A, and 73, 73-1, 73-79)

A simple “evaporation” query on the DWR website retrieves several documents that summarize the available data. For example, the “Draft Narrative of Evaporation from Lakes & Reservoirs” for water years 1998, 2000, 2001 are available online via the DWR website.

Also, the CIMIS database calculates evapotransportation (ETo) from sensor readings at over 100 weather stations throughout California. Multiply the ETo number by the coefficient 1.1 to produce statewide average. Actual values may vary depending on the depth and turbidity of the water body in question, and other environmental factors.