Bulletin 132-78 The California State Water Project -- 1977 Activities and Future Management Plans

California Department of Water Resources (DWR) | October 31st, 1978

1977 was a bittersweet year for the State Water Project. The worst drought in our State's history limited our ability to serve our 31 water service contractors; yet at th

1977 was a bittersweet year for the State Water Project. The worst drought in our State's history limited our ability to serve our 31 water service contractors; yet at the same time the largest water exchange in the history of the State took place between urban users in the San Francisco Bay area, agricultural users in the San Joaquin Valley, and The Metropolitan Water District of Southern California with San Bernardino Valley Municipal Water District, Coachella Valley County Water District, and Desert Water Agency. This greatly helped to significantly reduce the magnitude of the shortage.

Bulletin No. 6, Irrigation Requirements of California Lands, being Appendix "B" Report to the Legislature of 1923 of the Water Resources of California

California Department of Water Resources (DWR) | January 1st, 1923

California Agricultural Production and Irrigated Water Use

Congressional Research Service (CRS) | June 30th, 2015

California ranks as the leading agricultural state in the United States in terms of farm-level sales.  In 2012, California’s farm-level sales totaled nearly $45 billio

California ranks as the leading agricultural state in the United States in terms of farm-level sales.  In 2012, California’s farm-level sales totaled nearly $45 billion and accounted for 11% of total U.S. agricultural sales. Five counties—Tulare, Kern, Fresno, Monterey, and Merced—rank among the leading agricultural counties in the nation. Given current drought conditions in California, however, there has been much attention on the use of water to grow agricultural crops in the state. Depending on the data source, irrigated agriculture accounts for roughly 40% to 80% of total water supplies. Such discrepancies are largely based on different survey methods and assumptions, including the baseline amount of water estimated for use (e.g., what constitutes “available” supplies). Two primary data sources are the U.S. Geological Survey (USGS) and the California Department of Water Resources (DWR).  USGS estimates water use for agricultural irrigation in California at 25.8 million acre-feet (MAF), accounting for 61% of USGS’s estimates of total withdrawals. DWR estimates water use withdrawals for agricultural irrigation at 33 MAF, or about 41% of total use. Both of these estimates are based on available data for 2010. These estimates differ from other widely cited estimates indicating that agricultural use accounts for 80% of California’s available water supplies, as reported in media and news reports. Attention has also focused on trends in California toward growing more permanent orchard crops, such as fruit and nut trees and vineyard crops, as well as production of grain and pasture crops, much of which is used to support the state’s meat and dairy industries. Orchard crops refer to tree or vineyard crops that are planted once, require continuous watering to reach maturation, and cannot be fallowed during dry years without loss of investment. In contrast, most vegetables and other row crops (including grain and pasture crops) are annual crops that are sown and harvested during the same production year, sometimes more than once, and may be fallowed in dry years. Between 2004 and 2013, overall harvested acres in California increased for almonds, walnuts, pistachios, raisins, grapes, berries, cherries, pomegranates, and olives, but also for corn. During the same period, overall harvested acreage decreased for some field crops (cotton, alfalfa, rice, wheat), but also for certain orchard crops (wine grapes and some citrus and tree fruits). This shift to growing more permanent crops, especially tree nuts, appears to be largely market-driven. The availability of irrigation water has been a major factor in the development of California’s agricultural production. California has the largest number of irrigated farmed acres compared to other states and accounts for about one-fourth of total applied acre-feet of irrigated water in the United States. Water use per acre in California is also high compared to other states. Available data for 2013 indicate that, of total irrigated acres harvested in California, about 31% of irrigated acres were land in orchards and 18% were land in vegetables. Another 46% of irrigated acres harvested were land in alfalfa, hay, pastureland, rice, corn, and cotton. Congressional interest in California agriculture and water use centers largely on the Bureau of Reclamation’s Central Valley Project (CVP), which supplies water to numerous agricultural and municipal contractors. In recent years, the CVP has cut back water deliveries due to drought and environmental factors. Congress also authorizes and oversees U.S. Department of Agriculture support for individual crops and farmers, and some Members have expressed concern over the broader implications of decreased agricultural production and/or lack of water availability throughout the state.

California Simulation of Evapotranspiration of Applied Water and Agricultural Energy Use in California

Journal of Integrative Agriculture (Elsevier) | August 5th, 2013

The California Simulation of Evapotranspiration of Applied Water (Cal-SIMETAW) model is a new tool developed by the California Department of Water Resources and t

The California Simulation of Evapotranspiration of Applied Water (Cal-SIMETAW) model is a new tool developed by the California Department of Water Resources and the University of California, Davis to perform daily soil water balance and determine crop evapotranspiration (ETc), evapotranspiration of applied water (ETaw), and applied water (AW) for use in California water resources planning. ETaw is a seasonal estimate of the water needed to irrigate a crop assuming 100% irrigation efficiency. The model accounts for soils, crop coefficients, rooting depths, seepage, etc. that influence crop water balance. It provides spatial soil and climate information and it uses historical crop and land-use category information to provide seasonal water balance estimates by combinations of detailed analysis unit and county (DAU/County) over California. The result is a large data base of ETc and ETaw that will be used to update information in the new California Water Plan (CWP). The application uses the daily climate data, i.e., maximum (Tx) and minimum (Tn) temperature and precipitation (Pcp), which were derived from monthly USDA-NRCS PRISM data (PRISM Group 2011) and daily US National Climate Data Center (NCDC) climate station data to cover California on a 4 km×4 km change grid spacing. The application uses daily weather data to determine reference evapotranspiration (ETo), using the Hargreaves-Samani (HS) equation (Hargreaves and Samani 1982, 1985). Because the HS equation is based on temperature only, ETo from the HS equation were compared with CIMIS ETo at the same locations using available CIMIS data to determine correction factors to estimate CIMIS ETo from the HS ETo to account for spatial climate differences. Cal-SIMETAW also employs near real-time reference evapotranspiration (ETo) information from Spatial CIMIS, which is a model that combines weather station data and remote sensing to provide a grid of ETo information. A second database containing the available soil water holding capacity and soil depth information for all of California was also developed from the USDA-NRCS SSURGO database. The Cal-SIMETAW program also has the ability to generate daily weather data from monthly mean values for use in studying climate change scenarios and their possible impacts on water demand in the state. The key objective of this project is to improve the accuracy of water use estimates for the California Water Plan (CWP), which provides a comprehensive report on water supply, demand, and management in California. In this paper, we will discuss the model and how it determines ETaw for use in water resources planning.

California’s Next Million Acre-Feet: Saving Water, Energy, and Money

Pacific Institute | September 26th, 2010

A new analysis from the Pacific Institute recommends specific actions that can annually save a million acre-feet of water quickly and at a lower economic and ecological c

A new analysis from the Pacific Institute recommends specific actions that can annually save a million acre-feet of water quickly and at a lower economic and ecological cost than developing new supplies. The assessment notes that new actions are immediately needed to reduce the growing tensions over the state’s water resources and to address California’s persistent water supply challenges. This is a key time for California water: the California Water Bond has been tabled for at least two years and may be scrapped altogether. New reviews from around the state are calling for prompt efforts to use technology, economics, and institutional reform to address the state’s water crisis. All parties seem to agree that the state will need a diverse portfolio of solutions – but it makes the most sense to do the most effective things first. The Pacific Institute’s new report, California’s Next Million Acre-Feet: Saving Water, Energy, and Money, quantifies more than one million acre-feet of water that can be conserved through improved efficiency, with savings coming from the urban and industrial sectors and improvements in agriculture. “There is vast untapped potential to reduce our demand for water without affecting the services and benefits that water provides,” said Heather Cooley, co-director of the Pacific Institute’s Water Program and lead author of the report. “We identified how the next million acre-feet of water can be conserved in California, with approximately 30% of the savings from the urban sector and 70% from the agricultural sector. Both sectors have savings potential far exceeding this amount, and while we could have identified one million acre-feet of water savings annually in either sector alone, we’re showing how all sectors can benefit from these improvements.” In the urban sector, the report identifies water savings from replacing old, inefficient water-using devices with high-efficiency models in our homes and businesses, as well as replacing some lawns with low-water-use plants. In the agricultural sector, best water management practices include weather-based irrigation scheduling, regulated deficit irrigation, and switching from gravity or flood irrigation to sprinkler or drip irrigation systems. The efficiency improvements identified in California’s Next Million Acre-Feet require an upfront investment of less than $1.9 billion, costs that could be borne by a mix of state, federal, and local agencies and individuals – all at a small fraction of the cost of the proposed water bond. The cost of the conserved water is $185 per acre-foot for the agricultural sector and a net savings of $99 per acre-foot for the urban sector, over the lifetime of the efficiency improvement. Conserving water also conserves energy, saves money, and reduces the need for new water and energy projects.

Climate-smart management of soil water storage: statewide analysis of California perennial crops

Environmental Research Letters (IOP) | April 9th, 2019

The FAO-56 dual crop coefficient model was used to simulate site-specific irrigation management to quantify the green water resource (rainfall stored in soil) in five Cal

The FAO-56 dual crop coefficient model was used to simulate site-specific irrigation management to quantify the green water resource (rainfall stored in soil) in five California perennial crops (alfalfa, almonds, grapes, pistachios, and walnuts), considering local soil water holding capacity and climate data from 2003 to 2018. We tested different rooting depths and irrigation management thresholds (allowable depletion) across 1.46 million hectares of cropland to determine how the size of the soil water reservoir affects green water utilization and, consequently, blue water demand (irrigation). The 13-year cumulative green water utilization ranged from 17 to 36 million km3 out of a 57 km3 rainfall input and 162–263 km3cumulative blue water demand. For a deep scenario (2 m rooting; 50% allowable depletion), green water met 12% of cumulative crop water demand. However, green water use was not uniform: 20% of the landscape met over 20% of its annual crop water demand. Deeper rooting or greater allowable depletion reduced blue water demand more than the increase in green water utilization, due to less frequent irrigations, which reduced soil evaporative loss. Compared to a 'business-as-usual' shallow irrigation management scenario (0.5 m rooting; 30% allowable depletion), a moderate scenario (1.0 m rooting; 50% allowable depletion) saved 30 km3 blue water evaporation and increased green water use by 7 km3through 13 years. Such savings would fill California's largest reservoir, Shasta Lake, 6.6 times. This study demonstrates an opportunity for climate-smart management of soil water storage, by delayed spring irrigation, applying deeper irrigations less often, and ending fall irrigation early.

Colorado River Basin August 2022 24-Month Study

U.S. Bureau of Reclamation (USBR) | August 16th, 2022

Cover cropping and no-tillage improve soil health in an arid irrigated cropping system in California’s San Joaquin Valley, USA

Soil and Tillage Research (Elsevier) | January 3rd, 2017

The concept of soil health has attracted considerable attention during the past two decades, but few studies have focused on the effects on soil health of long-term soil

The concept of soil health has attracted considerable attention during the past two decades, but few studies have focused on the effects on soil health of long-term soil management in arid irrigated environments. We investigated the effects of cover cropping and no-till management on soil physical and chemical properties during a 15-year experiment in California’s San Joaquin Valley (SJV) USA. Our objective was to determine if soil health could be improved by these practices in an annual crop rotation. The impact of long-term no-tillage (NT) and cover cropping (CC) practices, alone and in combination, was measured and compared with standard tillage (ST) with and without cover crops (NO) in irrigated row crops after 15 years of management. Soil aggregation, rates of water infiltration, content of carbon, nitrogen, water extractable organic carbon (WEOC) and organic nitrogen (WEON), residue cover, and biological activity were all increased by NT and CC practices relative to STNO. However, effects varied by depth with NT increasing soil bulk density by 12% in the 0–15 cm depth and 10% in the 15–30 cm depth. Higher levels of WEOC were found in the CC surface (0–5 cm) depth in both spring and fall samplings in 2014. Surface layer (0–15 cm) WEON was higher in the CC systems for both samplings. Tillage did not affect WEON in the spring, but WEON was increased in the NT surface soil layer in the fall. Sampling depth, CC, and tillage affected 1-day soil respiration and a soil health index assessment, however the effects were seasonal, with higher levels found in the fall sampling than in the spring. Both respiration and the soil health index were increased by CC with higher levels found in the 0–5 cm depth than in the 5–15 and 15–30 cm depths. Results indicated that adoption of NT and CC in arid, irrigated cropping systems could benefit soil health by improving chemical, physical, and biological indicators of soil functions while maintaining similar crop yields as the ST system.

Cultivating climate resilience in California agriculture: Adaptations to an increasingly volatile water future

National Academy of Sciences (NAS) | July 29th, 2024

Deepening water scarcity in breadbasket nations

Nature Portfolio (Springer Nature) | January 28th, 2025

Water is crucial for meeting sustainability targets, but its unsustainable use threatens human wellbeing and the environment. Past assessments of water scarcity (i.e., wa

Water is crucial for meeting sustainability targets, but its unsustainable use threatens human wellbeing and the environment. Past assessments of water scarcity (i.e., water demand in exceedance of availability) have often been spatially coarse and temporally limited, reducing their utility for targeting interventions. Here we perform a detailed monthly sub-basin assessment of the evolution of blue (i.e., surface and ground) water scarcity (years 1980-2015) for the world’s three most populous countries – China, India, and the USA. Disaggregating by specific crops and sectors, we find that blue water demand rose by 60% (China), 71% (India), and 27% (USA), dominated by irrigation for a few key crops (alfalfa, maize, rice, wheat). We also find that unsustainable demand during peak months of use has increased by 101% (China), 82% (India), and 49% (USA) and that 32% (China), 61% (India), and 27% (US) of sub-basins experience at least 4 months of scarcity. These findings demonstrate that rising water demands are disproportionately being met by water resources in already stressed regions and provide a basis for targeting potential solutions that better balance the water needs of humanity and nature.