Publication Date: 
December 2013
Grantham, TE, JH Viers. Evaluation of Water Rights to Reveal Hidden Patterns and Trends in Water Resources Management. EOS TRANS. AGU, 93(53): H31C-1181

California's current water management challenges are strongly influenced by the legacy of 19th century legal doctrines, which aim to maximize the appropriation of available water for human uses and benefits. As a consequence and over time, most of the water flowing through the state's rivers and streams has been dedicated through water rights to human uses, limiting potential for sustainable water management and climate change adaptation. This history is recorded in a database of water rights, which contains approximately 50,000 records of diversion and storage projects dating from the early 19th century to the present day. Although water rights records are rarely considered in regional water budgeting, information on the location, age, purpose, and face values of water rights offer insight into the spatial and temporal dimensions of water-use pressures and can be used to identify opportunities for management interventions. Using the state water rights database, we calculated cumulative face-value rights at the HUC12-watershed scale and compared water appropriation volumes with modeled predictions of surface water availability. Total annual allocations of current water rights exceed 340 million-acre feet (4 x 1011 m3), approximately five times the state's mean annual runoff. The total volume of face-value water rights is 25 to 1,000% of annual water availability in major river basins (greater than 400 km2), with the greatest degree of appropriation observed in tributaries to the Sacramento and San Joaquin Rivers and coastal streams in southern California. Trends in water rights appropriation since 1900 indicate periods of rapid demand growth between 1925-1940, 1950-1960, and 1975-1985, which mirror major dam building eras. We illustrate how information on the spatial distribution of water rights, appropriation volumes, and priority of use, can guide strategic planning for re-allocating water for environmental benefits and other management objectives. We also describe how this effort can improve reporting and processing of, and access to, state water rights data for improved decision-making and management of water resources.