A Scalable Water Balance Model Within the Russian River, California, Basin

The availability of long term hydrologic data sets provides opportunities for quantifying hydrologic variability and examining evidence for changing conditions following land surface alterations and climate change. The Russian River basin in California is typical of many watersheds on the fringe of expanding metropolitan regions with competing demands for water from municipal, agricultural, recreational, and environmental constituencies. The Russian River basin is convenient for analysis because there is limited water imported and exported. One of the major challenges faced by water resources engineers within this basin is to understand the conditions necessary for the restoration of salmon and steelhead trout that have life cycles dependent upon migrations between the upper reaches of the watershed and the Pacific Ocean. The amount, timing and duration of surface water flows are frequently cited as some of the key factors controlling fishery health and recovery. The Russian River basin has numerous long-term data sets on flow, precipitation and water quality measures that have been gathered into a data cube for analysis and synthesis. Results to date have focused on comparing annual water balances for sub-watersheds that span over an order of magnitude in area. The data reveal a simple scalable relationship that annual runoff depth equals annual precipitation minus approximately 450 mm of water. This 450 mm of annual water demand represents a basin-wide integration of soil and groundwater dynamics and transpiration by vegetation. There is little evidence of human alteration of this relationship over a 60 year record. This scalable relationship is used to investigate how runoff from the basin would respond to changes in annual precipitation.