Flowmeter records of groundwater extraction for urban use, from the Cities of Orland, Williams, and Willows 

Estimated based on the typical field accuracy of meters, and the typical combined accuracy of volumes in urban distribution systems (accounting for metered groundwater, metered deliveries, unmetered deliveries, and apparent losses)

Groundwater extraction for agriculture and managed wetlands was estimated through a land use-driven water budget approach based on applied water demand within corresponding land use areas (Land IQ 2018 informed by jurisdictional areas and GSP land use analyses), after accounting for available surface water supplies. Total water demand was quantified based on OpenET ET data. The portion of ET met by precipitation (ETPR) was quantified from total ET, PRISM gridded precipitation data, and representative soil/crop parameters. The portion of ET met by applied water (ETAW) was quantified as the difference between total ET and ETPR. Total applied water and the portion met by groundwater were estimated based on efficiencies calculated from the GSP groundwater model in recent years, considering ponding operations in managed wetlands.

Typical uncertainty when calculated from a land surface water budget approach, considering the combined uncertainty of other water budget flow paths following the procedure described by Clemmens and Burt (1997). The uncertainties of other flow paths are quantified based on measurement device accuracy, technical literature, or professional judgment.

Estimates of groundwater extraction in urban areas where flowmeter records were unavailable were estimated based on annual population data and monthly per capita water use requirements. Annual population data were obtained from the California Department of Finance. Average monthly per capita water use rates in 2016-2021 were quantified from population data and available pumping data in the Cities of Orland, Williams, and Willows, and then adjusted according to the relationships between cities/unincorporated areas identified during GSP development.

Typical uncertainty when calculated from population and per capita water use data, considering the combined uncertainty of those data sources.