Scientific and institutional complexities of managing surface water for beneficial human and ecosystem uses under a seasonally variable flow regime in Mediterranean climate northern California

In mediterranean-climate northern California, agricultural water needs are almost exclusively met locally. Grape growers in this region, including Napa, Sonoma, and Mendocino Counties, have diverted surface water from streams under appropriative and riparian doctrines throughout the 20th Century. Increased diversion requests from the Russian River drainage network (Sonoma and Mendocino Counties), along with protections extended to endangered species and public trust values, prompted the California Water Board to develop new guidelines for diversion from wine country streams. As designed, however, these guidelines do not provide intended ecosystem protections. Reflecting the limited role the Water Board assumes in water management, new guidelines apply only to a subset of water rights, thus ignoring some threats to beneficial ecosystem uses. In 24 of 27 major Russian River tributaries, existing surface water rights exceed expected stream flow during a typical year. Field data illustrate impacts of diversions on flow: in two gauged catchments, diversions reduced flow by up to 100% at times corresponding to peak agricultural water need.; Water Board guidelines also make inaccurate assumptions regarding hydrology and its relationship with ecological processes. Guidelines assume flow is linearly proportional according to a ratio of catchment areas, and that the duration of a particular environmental flow, defined as the flow necessary for upstream salmonid migration, is approximately equal throughout a drainage network. This environmental flow serves as the threshold condition to allow diversion from North Coast streams. Data from eight nested streamflow gauges installed along a spatial gradient in Franz and Maacama Creeks (Sonoma County) indicated that flow is not proportional linearly according to catchment area. Headwater streams had higher peak flows and lower base flows, per area, than downstream channels in 2004 and 2005. Additionally, durations of threshold exceedence at headwater gauges were between 5% and 10% of the downstream exceedence duration in both years. Because of these scaling discrepancies, guidelines may permit diversion from headwater tributaries before ecosystem needs are met. Though the new Water Board management guidelines may not offer intended protections as written, studies such as those conducted here can provide the foundation to change policies to better reflect ecosystem processes.