Recent intense and record-breaking droughts have greatly impacted California’s water supply, driving new large-scale water conservation initiatives. Informing and analyzing these initiatives requires data at similarly large scales, which is primarily only possible with remotely sensed data. Despite its increasing availability, remotely sensed data has not been utilized to the extent that it could be in water conservation, resulting in data gaps and uncertainty. The objective of this dissertation is to evaluate and inform water conservation initiatives using remotely sensed data. In the first chapters of this dissertation, I assessed a fallowing program along the Colorado River intended to produce water savings to protect the Colorado River system. I first identified an appropriate evapotranspiration (ET) model for the fallowing program using the traditional valley-wide consumptive use estimate since ET is a proxy for consumptive use, I then explored how fallowing savings estimates differ between the standard water-balance approach employed by the Federal Government and an ET-based approach, and finally I explored factors that correlate with higher fallowed field water use. My results showed that the traditional approach for calculating fallowing savings produces higher estimates than an ET-based approach, primarily due to differences in estimated fallowed field water use. I also found evidence that fallowed field water use decreases with older field age, lower surrounding field water use, and larger field size. Fallowing programs may benefit by accounting for fallowed field consumptive use when estimating savings and incentivizing keeping fields in fallow and having larger fallowed fields to potentially reduce lost water. The final chapter of this dissertation used remotely sensed irrigated area estimates to inform the Outdoor Residential Water Use Standard (ORWUs) described under the Conservation as a California Way of Life legislation. The ORWUs are designed to set water budgets for retailers based on their irrigated area as measured by a 2018 snapshot image. I used additional years of imagery data to inform this 2018 value and explore how irrigated area varies by year. I found that irrigated area as measured by this type of snapshot imagery is highly variable between years and that the use of 2018 values results in “winners” and “losers” between agencies. Remotely sensed data has the potential to fill critical data gaps and improve water conservation initiatives as shown throughout this dissertation. However, utilizing remotely sensed data requires modeling and data may not be as high-resolution or as temporally available as desired, resulting in uncertainties. Clearly identifying, addressing, and communicating uncertainties and benefits related to remotely sensed data will be key to maximizing the benefit that this data source has to offer for policy makers.

Understanding California’s water balance sheet—how much there is, who has claims to it, and what is actually being “spent”—is key to effectively managing the state’s limited water supply in support of a healthy economy and environment. The latest drought has spotlighted serious gaps in California’s water accounting system. California is a large, geographically diverse state, and its water systems are physically interconnected and institutionally fragmented. Water infrastructure connects the state’s northern watersheds to its southernmost communities, Sierra Nevada rivers to coastal cities, and surface water to groundwater. Additional complexity arises from having hundreds of independently governed water systems, each with its own water accounts; from the widespread practice of managing linked surface water and groundwater as separate systems; and from a lack of clarity on how much water is reserved for environmental purposes. The combination of physical interconnectedness, institutional fragmentation, and water scarcity heightens the need for more effective accounting at the statewide and river-basin levels.  We identify gaps in California’s water information systems and recommend that the state modernize its water accounting, and that key state agencies—supported by an oversight committee of key stakeholders and independent experts—develop and adopt a common accounting framework.