Our econometric models related acreages of each major crop to relative crop prices and key climate variables that are expressed as 10‐year moving averages to represent the recent memory of growers’ decision making. In general, the data indicate significant influences of prices and only moderate influences of expected growing degree days, chill hours and precipitation on acreage of individual crops. Overall the data indicate that Yolo County climate change has played a moderate role in the evolution of crop acreage in Yolo County in recent decades. The models did not investigate many other factors that affect Yolo County crop acreages, such as irrigation water effects of climate change outside Yolo County, extreme events, or the potential influence of statewide or global climate change on relative prices,. We applied the estimated parameter values to down scaled GFDL climate projections to assess how future climate change in Yolo County may affect crop acreage patterns from 2010 to 2050. The results should not be interpreted as acreage forecasts. For example, we took no account of recent trends or expected changes in prices, technology, or other factors in projecting acreage change. Instead, we invesitgated the acreage impacts of two paths for climate change , holding constant the relative prices and other relevant drivers of crop acreage. An underlying assumption in our approach was that the basic relationships between climate and acreage that were estimated using the data from 1950 to 2008 apply to projected climate effects on acreage from 2010 to 2050. Average temperature is projected to rise in Yolo County under both scenarios, associated with winter temperature increases and the reductions in winter chill hours. The two climate scenarios diverge for the period after 2035 with the A2 scenario cooler during this period,garden grow bags despite a long‐term increase in temperature compared to B1. Among field crops, warmer winter temperatures are projected to cause wheat acreage to decline and alfalfa acreage to rise. This led to a small projected decline in total field crop acreage and projected increase in tomato acreage.

The largest impact of warmer winter temperatures is for projected wheat acreage. Using the historical relationships, climate change induces a decline in projected wheat acreage share from about 17.5 percent of crop acreage in 2008 to as low as 4 percent of acreage in 2050. Even though the projected change was significant for the acreage of certain crops, the overall impact on total crop acreage has been moderate. Some care must be exercised in interpreting our results. Our projections focused exclusively on the using historical patterns to project relationships between acreage change and climate change. They are not year‐to‐year forecasts. Further, our projections were based on the statistical estimates derived solely from historic data, meaning that factors other than climate do not change from their historic values. In terms of adaptation to climate change, however, these results indicate that farmer decisions may now need to be based more on uncertainty of climate than in the past, which is not incorporated in our projection .In California, demand for water from agriculture, industry, urban areas and the environment has meant that most watersheds in the state are consistently over‐allocated . In the near term, projections suggest that by 2020 demand for water will exceed the available supply by >2.4 million acre‐feet in average rainfall years and up to 6.2 million acre‐feet in dry years . In the long term, climate change and population growth will place additional demands on the state’s water resources .While there is uncertainty regarding the extent to which climate will change in any given location, there is a growing consensus that the impacts on California’s water resources will be outside the range of past experience . Consequently, state agencies such as the Department of Water Resources and the California Energy Commission have urged water managers at the regional, district, and local levels to examine the potential impacts of and responses to climate change as a part of their planning efforts . Past climate and hydrologic records provide ample evidence that climate change is already having a measurable effect on California’s water supply . For instance, statewide weather records show that mean annual temperatures have increased by roughly 0.6– 1.0°C during the past century, with the largest increases seen in higher elevations .

This warming trend has contributed to a 10 percent decline in average spring snow pack in the Sierra Nevada over the same period, which equates to a loss of approximately 1.5 million acre‐feet of snow water storage . Global climate models suggest that this warming trend will accelerate, with temperatures expected to increase by 2 to 6°C by the end of this century . While there tends to be less agreement among the climate models as to whether mean annual precipitation in California will increase or decrease, inter‐annual variability is already on the rise and projected to increase further during the latter half of this century . Since the relationship between precipitation and surface runoff is non‐linear, a minor decrease or increase in precipitation could have disproportionate effects on the state’s water supply . Some of the water supply vulnerabilities for agriculture and other sectors can be mediated through traditional infrastructure improvements or alternative water policies; for instance by expanding water storage, updating levies and aqueducts, interstate transfers, modifying the existing operating rules, expanding conjunctive use or groundwater banking . Many of these supply side adaptations also have important trade offs, namely high capital costs and/or significant environmental impacts . Shifts in temperature and precipitation are also projected to have significant implications for the demand side of California’s water balance. Higher temperatures will increase the demand for water from agriculture, as well as the losses associated with water storage, delivery and irrigation. Since agriculture accounts for approximately 80 percent of California’s water use, methods to manage and minimize agricultural water demand are seen as an important way to adapt to climate change . Local conservation strategies implemented by water managers and agricultural users tend to also be more economical than developing new supplies . Demand management options may include water pricing and markets, allocation limits, improved water use efficiency, public and private incentives for irrigation technology adoption, reuse of tail‐water, shifting to less water‐intensive crops, and fallowing . The degree to which climate change will impact both water resources and agriculture is likely to vary considerably throughout California . Thus, for climate impact assessments to be useful they must be conducted at a scale which is fine enough for regional and local water managers to integrate research findings into their planning and adaption efforts. One tool that has helped water resource managers integrate climate change projections into their decision making process is the Water Evaluation And Planning system .

WEAP is a modeling platform that enables integrated assessment of a watershed’s climate, hydrology, land use, infrastructure, and water management priorities. In California, WEAP has been used to model the impact of various climate change, land‐use and adaptation scenarios on the Sacramento and San Joaquin River Basins . Likewise, Mehta et al. used WEAP to evaluate potential climate warming impacts on hydropower generation in the Sierra Nevada. Joyce et al. combined these regional models into a statewide WEAP application that is being used for integrated scenario analysis by the California Department of Water Resource. While these large‐scale hydro‐climatic models have proven useful for state and regional water managers,tomato grow bags their spatial resolution is often too coarse to be of immediate value to local irrigation districts. The WEAP framework has the potential to address this limitation by developing local applications that use more refined input data and greater spatial disaggregation. Models developed at the district scale would also provide an opportunity to improve communication between water managers and climate scientists, cultivate a better understanding of the risks and uncertainties, and ultimately enhance the community’s capacity to adapt . In this study we use WEAP to build a hydrologic model of the Cache Creek watershed and to assess the potential effects of climate change and adaptive management on the water resources dispensed by Yolo County Flood Control and Water Conservation District. This district was chosen for several reasons. First, most studies examining climate impacts on the state’s water resources have focused on watersheds fed by the Sierra Nevada, while those originating in the Coast Range have received little attention. Examining the Cache Creek watershed therefore provides an opportunity to investigate how watersheds that are not reliant on Sierra Nevada snowmelt may be affected by climate change. A second reason is that Yolo County is the site of an ongoing interdisciplinary case study on agricultural adaptation to climate change carried out by the University of California at Davis and the California Energy Commission. As such, the hydro‐climatic analysis is further informed by locally relevant agronomic and socioeconomic data. While several integrated water management plans have been formulated for the District over the past decade , our work adds value in several ways. Unlike past studies, we simulate the hydrology of the catchments in Lake County which form the headwaters of Cache Creek. Since this analysis is conducted at the district scale, we are also able to capture the explicit operating rules and legal decrees which govern local water management decisions. We then use down scaled climate projections from two IPCC emissions scenarios to simulate the District’s future water supply and projected demand under one baseline and three hypothetical adaptation scenarios.Between 1970 and 2008, total irrigated agricultural area in the county averaged 332,000 acres, varying between a maximum of 395,000 acres in 1980 and a low of 280,000 in 1982 . As indicated in the economics section above, there has been an overall downward trend in total agricultural area.

The county covers a portion of two geomorphic provinces: the Coastal Range and Central Valley. Surface water supply comes from a number of drainages: the eastern and northern parts of the county depend on the Sacramento River, Colusa Basin Drain, and Yolo Bypass, while the western part depends on Cache Creek . Most of the water in Putah Creek supplies neighboring Solano County. Agriculture accounts for almost 95 percent of the approximately 1 million acre feet of the county’s total water demand. About 70 percent of that water is estimated to be supplied by surface water; the remaining is pumped from groundwater . The Yolo County Flood Control and Water Conservation District service area covers 41 percent of the county’s irrigated area and is located in the western and central portion of the county . The District was established in 1951 and supplies surface water for irrigation from Cache Creek. The upstream reaches of the Cache Creek watershed are wetter and cooler than the valley floor. For example, average annual rainfall and temperature in areas upstream of Clear Lake are 988 millimeters and 13.3 °C respectively, compared to 560 mm of precipitation and 16.5 °C respectively in the valley. Snow does not occur in the watershed, except intermittently in high elevations. Upland soils to the west are well drained but shallow to bedrock composed of marine shales, silt stones, and sand stones. Lowland soils are part of alluvial fans, underlain by the Tehama formation . In the District, alfalfa, tomatoes, wheat, almonds, walnuts, wine grapes, and rice are the dominant crops.Two reservoirs located upstream in neighboring Lake County are critical for District water deliveries: Clear Lake and Indian Valley. The District purchased water rights from Lake County in 1967, amounting to a maximum of 150,000 acre feet annually. The actual amount available for District release in any given year is strictly controlled by the stipulations of the Solano Decree . In 1976, the Indian Valley reservoir was completed. Since it is owned and operated by the District, it allows greater flexibility in supplying water to its downstream customers. Water is delivered to customers via a network of canals and ditches downstream of Capay diversion dam. The District does not own or operate any groundwater wells for the purpose of meeting customer demands. However, many privately owned wells exist throughout the District, and landowners rely on these wells for domestic purposes and to add flexibility to their farming operations. The groundwater basin experienced some depletion of storage in the 1960s and early 1970s. The increased storage and provision of surface by Indian Valley Reservoir has been identified as a key factor in the recovery of groundwater levels in Yolo County in recent decades .