How those reductions are allocated among Imperial Valley agriculture—the single largest user of Colorado River water— and the two largest southern California wholesale water agencies—Metropolitan Water District of Southern California , supplying 19 million people, and the San Diego County Water Authority , supplying 3.3 million people, will create challenges. While the reductions will create conflict, as evidenced by past lawsuits among these agencies, opportunities will also arise, including building upon past agricultural–urban water transfers. For example, in the early 1990s IID and MWD agreed to a long-term transfer of roughly 12,950 hm3 of water annually. In 2003, and in response to the USBR requiring California reduce its use of surplus Colorado River water due to demand growth in Arizona and Nevada, the largest agricultural-municipal water transfer in California was signed as part of the Quantification Settlement Agreement . Under the QSA, IID agreed to transfer up to 25,000 hm3 of water annually—generated through water conservation— to SDCWA, which due to lower priority water rights to the Colorado was to lose its allocation with the elimination of California’s surplus usage. Agriculture and environment interactions. A constant challenge confronting agricultural water transfers involving IID is the impact of those transfers on the Salton Sea. As a highly saline terminal lake dependent on IID irrigation runoff , square pots for planting its volume and surface area will fluctuate with changes in applied irrigation in IID.

Consequently, as water transfers from IID to MWD or SDCWA increase, or if water allocations to IID decrease, the Salton Sea will shrink and more play a will be exposed with significant negative externalities on the environment and local communities surrounding the Salton Sea . Such concerns were a primary reason IID withheld its support for the Drought Contingency Plan of 2019 involving California, Arizona, Nevada, and the USBR and, subsequently, sued MWD—under the California Environmental Quality Act—for signing the agreement and aimed to cover California’s share of the 2019 Plan reductions. While the lawsuit was dropped 2 y later, is emblematic of the environmental challenges irrigated agriculture will confront more regularly under climate change.Adaptation of California agriculture to climate change will proceed within the broader context of global climate change and global impacts on input and farm product markets. Climate change outside California influences the future of California agriculture by affecting economic prospects and hence choices of its farms. Any assessments of direct climate impacts on California productivity and water availability across farm commodities must consider these same impacts on agriculture supply conditions elsewhere, and hence the global market conditions for California grown commodities. Crops that face more intense competition in global markets may face more losses from increased production costs under climate change unless there are even larger negative climate impacts in competitive regions. Assessment of climate change–related impacts on the evolution of California agriculture will be inherently affected by demand and supply conditions in local and global markets that are affected by climate changes elsewhere.

Furthermore, impediments to global market access influence the functioning of markets and hence the impacts of climate change on California agriculture. Such impediments can positively or negatively affect agricultural production in California through their impacts on the demand for California exports . For example, Iran was the major competitor for California pistachios but lost market share in recent decades, in part due to sanctions and trade barriers, which created opportunities for California pistachios. Agricultural subsidies and trade measures favor certain commodities, farm practices, and growing regions, relative to others. U.S. farm subsidies tend to be low relative to global standards and have declined steadily until big jumps from ad hoc subsidies from 2019 through 2021. Because U.S. subsidies for the vegetables, fruits, and tree nuts grown in California tend to be relatively low, farm subsidies have had modest direct effects on production patterns in California in recent years, yet subsidies and trade barriers elsewhere do affect export market opportunities that are shifting with climate change. Climate change–related impacts affect costs and returns to crop insurance programs, which are controlled and subsidized by the USDA . Crop insurance is almost always highly subsidized. Farms pay less than half the costs of U.S. crop insurance. Farms generally enroll only if their insurance premium payments are far below expected farm payouts. Highly subsidized federal crop insurance programs may reduce incentives for adoption of climate-resilient farm practices, particularly if insurance payoffs exceed climate impacts on crop yields. By shifting climate-related costs away from farms, crop insurance subsidies may delay farm adaptations . At the same time, as climate change raises yield or price variability it may increase crop insurance subsidies . Nonetheless, even high crop insurance subsidy rates comprise a small fraction of farm revenue and thus have only modest effects on cropping patterns and, consequently, likely only small effects on adaptation to climate change.

Other farm subsidies and trade measures increasingly recognize climate change adaptation and GHG mitigation . The EU has begun to regulate farms to reduce GHG emission and propose farm import tariffs to impose parallel costs on imports. However, such policies may mask traditional protectionist barriers that could exacerbate losses from climate change . Finally, California agriculture could gain from foreign adoption of carbon taxes and other measures because it often has a lower carbon footprint than its competitors due to relatively high productivity per unit of output.California agriculture likely confronts a future defined by higher temperatures and both lower and less certain water supplies for irrigation. As such, the ability of California agriculture to thrive in the future will depend on its ability to develop mitigation and adaptation strategies to reduce vulnerabilities and increase resilience under this new climate and water regime. Farmers and other water users along with policy makers naturally consider a variety of approaches and possibilities to either reduce water scarcity itself or reduce the costs associated with it. These approaches can be categorized into three major groups: demand-side, supply-side, and institutional. Demand-Side Practices and Policies. Irrigated water demand reductions can ameliorate impacts of limited water access. In deciding whether to reduce their water use, users naturally consider costs of the competing options and potential gains from such reduced use. Over the past four decades, agricultural water use in California has decreased by nearly 15% while overall farm revenue has increased by nearly 40% . Reduction of applied irrigation water may be largely attributed to a combination of three factors—changes in irrigation practices, changes in crop mix, square pots plastic and irrigated land fallowing. Irrigation efficiency and scheduling. Changes in irrigation practices usually fall into two categories: a) increases in irrigation efficiency involving a higher ratio of irrigation-fulfilled crop evaporative demand to total applied water in the current season, and b) changes in irrigation timing and quantities so that a higher proportion of the applied water fulfills evaporative demand. Since the 1980s, the irrigated area in California using gravity-fed irrigation has decreased by about 25% from nearly 2.5 million ha down to approximately 1.9 million ha . Concurrently, the amount of acreage using sprinkler irrigation has slightly declined over that period while the amount of acreage with installed drip irrigation has increased from around 121,400 ha to nearly 1.2 million ha. Augmenting supply through recovery of conveyance losses may also contribute to increasing efficiency yet will reduce deep percolation. Much of this change has accompanied a change in the crop mix from annual crops to trees and vines. As pointed out increasingly , such forms of higher irrigation efficiency do not change crop evaporative requirements, but rather changes the amount of water applied by reducing the amount of return flows either as runoff, deep percolation, or both . Such reduced flows can have a variety of negative impacts, including i) if those deep percolation flows would otherwise recharge aquifers, ii) if the runoff had contributed to return flows for downstream users, and iii) if the runoff/ deep percolation flows contributed to environmental flows and ecosystems services. In these cases, applied water reductions may not increase what might be termed “system efficiency,” and/or may result in environmental damages. Without attention to the entire water balance, government programs intended to save water via increased irrigation efficiency, as defined around applied water or diversions, rarely save water on a system-wide basis. Improvements in irrigation timing to match soil water depletions may lead to significant reductions in non-beneficial water losses without any appreciable change in crop yield, or substantial infrastructure or production cost increases .

This strategy, which includes deficit irrigation, has seen a surge of research since the early 2000s and has found no substantial crop yield changes when appropriate phenological stage-scheduling and water yield response is considered . Significant systemwide potential water savings are limited with efficiency and timing strategies. Understanding whether and to what extent reductions in net water use have occurred under these strategies requires improved water accounting transparency and adoption of technologies—e.g., remote sensing—that can track water use patterns spatially and temporally. Water accounting that distinguishes between water withdrawals, consumptive use, and return flows are crucial . Changes in the crop mix. Over the past two decades, California has seen a significant increase in perennial crop area and a decrease in field crop area in response to expectations about long-term crop profitability and related factors . Crop net water requirements vary widely across California’s highly diversified agriculture with field crops using 58 cm/y consumptively , vegetables 45 cm/y, trees 66 cm/y, and alfalfa and pasture 94 cm/y on average. With typically lower net returns per unit of land and water relative to tree and vegetable crops, reduced field crop area, including alfalfa and pasture, provides a lower net cost means to respond to reduced water allocations. Even higher water savings could be achieved through a switch from irrigated crops to unirrigated winter cereal crop productions, as shown in ref. 47. These crops–or other traditionally rainfed crops that could be supported by additional marginal irrigation–may present an opportunity for maintaining agricultural lands while reducing the agricultural water footprint. Such evaluation would need to also consider the lower net returns of rainfed crops. While water use reductions may occur through changes in crop mix, we caution against a heavy regulatory approach in determining what crops are grown. Reductions in the availability of water would create incentives for growers to change their crop mix based on their business calculus. Pricing water at a rate that more accurately represents its scarcity value would also provide incentives for growers to shift their operations to less water-intensive crops . Current laws and regulation often require water prices to represent water delivery costs alone, which constitute a small fraction of crop production costs. As a result, only large increases in water prices from current rates would provide sufficient incentive for significant changes in crops. Land idling and repurposing. Reduced irrigation to address groundwater depletion and ongoing and future climate change impacts will likely lead to significant declines in irrigated cropland planting, with some repurposing to non-agricultural uses. In the SJV, average annual irrigation water supplies are likely to decline by 20% by 2040 from 2010 levels. Simulations indicate this would trigger a reduction in 180 to 350 thousand hectares of irrigated cropland–depending on the quantity of new water supplies that could be brought to the SJV . Reducing agricultural irrigated areas substantially comes with consequences that are not fully understood. First, to what extent will the local economy decline, and what options can mitigate such effects. Reductions in local farm and non-farm employment and income are a real concern without timely and effective transitions to other job-creating production . Second, such land use changes likely create downstream impacts on food prices and regional/global markets. California produces a significant share of U.S. consumption of many fruit, vegetable, and tree nut crops, but these high-revenue per irrigated acre crops are least likely to face cuts from water scarcity. Price impacts also depend on global competition and climate effects in other supply areas. Historically, specialty crop production in California has been relatively stable during droughts, and thus such price effects have been minimal . The nationally important SJV dairy supplies, which rely on locally irrigated forage crops, may also cause national price impacts. A widely cited case of the third-party effects of reduced water use through idling and transfers is the Colorado Big Thompson .