Lanterna Rally

Author: Tiffany Noel Kocis

Publisher:

Published: 2016

Total Pages:

ISBN-13: 9781369201826

California is the largest agricultural producer in the United States, consuming nearly 40% of California's total annual water supply often during times of the year when surface water supplies are relatively limited (i.e. summer). Across all three sectors of California water use (agricultural, environmental, and urban), groundwater accounts for 38% of the California's water supply during a normal year, reaching upwards of 48% during a dry year. This tremendous use of groundwater has led to groundwater overdraft throughout the Central Valley. Statewide groundwater overdraft is estimated between 500 thousand acre-feet and 2.6 million acre-feet per year and is most severe in the Tulare Lake Basin and the southern Central. To address continued overdraft of groundwater resources, the State of California passed the Sustainable Groundwater Management Act (SGMA) in 2014, which requires identification of surface water available for groundwater replenishment. In light of both groundwater overdraft and the passage of SGMA, this study developed an index to identify and rank watersheds where surface water is potentially available for groundwater recharge projects. The spatial index, the STreamflow Availability Rating for Recharge (STARR), can be used by policy makers and stakeholders to inform water resources management decisions. To develop the STARR and to understand the physical distribution of water available for groundwater recharge projects, the magnitude, frequency, duration, and timing of available high-magnitude flow (above the 90th percentile) were determined for unimpaired and impaired stream gauges throughout the Central Valley. This study found that a single wet year can provide 11.8 MAF of water above the 90th percentile from the Sacramento Valley alone -- 5 times the annual groundwater overdraft in the Central Valley. However, these flow above the 90th percentile in an average year occur only for relatively short times between November and April (25-30 days in an average year with flow above the 90th percentile), and from few storm events (5-7 events in an average year with flow), suggesting a need for coordinated efforts for the local-scale diversion of flood flows onto spreading grounds for groundwater recharge. The STARR index incorporates the magnitude, duration, and frequency of high-magnitude flows to rank watersheds in terms of high-magnitude flow availability for groundwater recharge projects. The STARR indicates that the Sacramento Valley, along with high Sierra watersheds, has the most water physically available (7 MAF) for extended periods (50 days) from November to April and the highest inter-annual recurrence frequency (64% of years between 1970 and 2014). In contrast, physical surface water availability in the San Joaquin Basin is marginal (2 MAF over 53 days during only 36% of years between 1989 and 2014) from November to April, while the Tulare Basin has a limited potential for in situ surface water for groundwater recharge. The STARR was further utilized to develop the Optimal Month Rating (OMR) which identifies the month in the December to February and November to April periods that has the greatest water availability and highest reliability. For the two periods, December to February and November to April, the OMR identifies the early spring months (February and March, respectively) as the potentially optimal months for recharge in the Sacramento Basin. For the San Joaquin -- Tulare Basins, the OMR identifies January (for both periods) as the potentially optimal month for recharge. These results suggest that groundwater recharge projects could consider transporting the early available winter water (November to February) from the Sacramento Valley towards the south, where the growing season starts earlier, and utilizing the physically available water during spring (February to April) in situ within the Sacramento Valley as leaf-out tends to occur later in the season in northern California. Given future projections on climate change and the overall growth of California agriculture, this research further evaluated long-term trends in the surface water availability metrics as a means to understand future surface water availability and found limited evidence for significant trends in all surface water availability metrics. A thorough comparison of trend statistics estimated over the full historic record and the post-impairment record (i.e., the record of data since the most recent impairment including dams and diversions) indicates that many of the significant trends may result from long-term alteration of streamflow due to dams, diversions, and water management practices. Finally, this study evaluated the physically available surface water in the context of existing surface water allocations and environmental use and found that surface water is potentially available for groundwater recharge projects based on both existing surface water allocations and reported demand. However, despite the seemingly excessive over-allocation across the Central Valley, this study identified under-utilized water. This under-utilized water is potentially available for short-term appropriation, which would suit the particular needs of groundwater recharge projects, which are ideally conducted over short periods throughout the winter.In conclusion, flows above the 90th percentile potentially provide an untapped source of surface water during the winter months, which could be utilized for groundwater recharge projects to mitigate groundwater overdraft.