As California’s temperatures get hotter and precipitation becomes increasingly variable with climate change , we expect a further systematic overestimation of suitable areas identified based on the past 30 years of weather data. For the suitability analysis we assigned temperature and soil texture to three categories that were each associated with a score: good , tolerable , and intolerable , while precipitation was divided into ranges that were suitable with no additional irrigation, suitable with additional irrigation, and unsuitable.For temperature, we considered the average maximum temperature in the three hottest months of the growing season , categorizing them separately with the scores described above . We then multiplied these three categorized scores together and took the cube root to get temperature suitability scores for the state, also excluding any areas whose monthly 30-year minimum temperature was above 59o F. We followed a similar procedure for soil texture, using SSURGO estimates of clay content averaged across soil horizons at a 90m resolution . Because farmers did not give numeric estimates of how much clay was needed in dry farm soils, we made sure our defined ‘tolerable’ range encompassed the full range of clay content observed in participating farms’ soils . To define the ‘good’ range , we excluded the farm with the lowest clay content, which was also the only farm where farmers stated that they could not grow tomatoes of a high enough quality to consistently market them as “dry farm.” This multiplication reflects the interaction between temperature and soil texture, grow rack in which good texture can compensate for higher temperatures by increasing soil water holding capacity, and lower temperatures can lessen the evapotranspirative demand that would be particularly problematic for plants growing in sandier soils with a lower soil water holding capacity.

We then separated the dataset into three areas based off of farmers’ understandings of where tomato dry farming could occur with no added irrigation and where it could occur with supplemental irrigation , and excluding areas that would not get enough winter rain to grow a suitable winter cover crop . The final map shows suitability scores in all areas that are categorized a ‘cropland’ in the 2019 National Land Cover Database . These areas are superimposed onto groundwater basins categorized as high priority in California’s Sustainable Groundwater Management Act . Crop totals on land that was deemed suitable for tomato dry farm management in these areas were calculated using the 2021 Cropland Data Layer .By focusing on the characteristics that limited water can give a tomato, these farmers highlight a recurring theme in understanding the functional definition of dry farming tomatoes. As the Central Coast faces increasingly limited water availability, the idea of dry farming has gained traction among policymakers purely by virtue of offering a means to continue farming while maintaining a restricted water budget. However, these farmers are quick to recognize that dry farming is only a management style that they can afford to choose for their operations insofar as it can excite customers and return a reasonable profit. In this way, the product that dry farming creates, which is valuable enough to consumers that they are willing to pay a significant premium for it, is the outcome that defines the management approaches farmers can use. Farmers know that they could alter the schedule for the minimal irrigation they do put on their dry farm tomatoes to increase yields . However, while defining the practice by some maximum threshold of water application, and then choosing to allocate irrigation water to maximize yields, may be appealing from a water savings perspective, farmers recognize that they must define the practice in terms of outcomes and not inputs.

Farmers must produce what consumers have come to expect from a dry farm tomato if they are going to make dry farming an economically viable choice for their operation.To better understand where tomatoes might conceivably be farmed in California given the environmental constraints identified above, we modeled dry farm suitability on California cropland as a function of precipitation, temperature, and percent clay in soil. The resulting map shows what lands could potentially support a dry farm crop, with and without supplemental irrigation, using constraints that are relaxed to encompass the least restrictive farmer-elicited constraints . The map therefore errs on the side of including land that is not an ideal candidate for dry farming, rather than leaving off land that may potentially be a good fit. With rising temperatures and less reliable rainfall, this map, which is based off of 30-year normals, likely also systematically overestimates what areas might fall into these thresholds when projecting into future climatic conditions. All areas in blue indicate land that meets a threshold where dry farming could be considered in a non-drought year without adding any irrigation. Areas in orange indicate that, while there is likely enough rain to sustain a winter cover crop, some amount of irrigation would often be needed to grow a successful dry farm crop. Areas in darker colors connote land that falls in conditions that are closer to ideal, whereas lighter colors indicate that more conditions are tolerable, rather than ideal, for dry farming.It is crucial to note that areas that show up as “suitable” on the map–including the most ideal locations–will likely require years of diversified management for soils to build the water holding capacity and fertility that allow for peak dry farm performance. These areas should therefore be considered candidates for long-term dry farm management, rather than ready-to-go dry farm fields. Because the constraints used to build the model were elicited specifically with regard to tomatoes, this of course is not a comprehensive map of everywhere that might be considered for dry farming non-tomato crops.

Particularly when it comes to grains and perennials , the range of possible locations is likely much broader. In the case of grains, winter varietals can be planted that take advantage of rain in winter months, while tree crops have far more extensive root systems that can reach water well beyond that which might be available to a tomato, in both cases relaxing the temperature and precipitation constraints that tomatoes need to survive without irrigation. Tomatoes are likely a better proxy for other vegetable crops , though each will have its unique requirements . As we imagine a shift towards dry farm agriculture in California, it is also important to consider how land that is suitable for dry farming is currently being used. Combining areas that are suitable for tomato dry farming with and without irrigation, we compiled a list of the top ten crops by area that are currently grown on these lands . Some of them are currently being dry farmed with some regularity in the state and could signal particularly easy targets for a shift to low-water practices. Others are dry farmed in other Mediterranean climates and suggest an important opportunity for management exploration in lands that might be particularly forgiving to experimentation. The remaining crops are some of the most water intensive in the state and would therefore lead to substantial water savings if the land could be repurposed. While unrealistic in the near future, calculating potential water savings from a complete conversion of suitable lands to dry farming allows for comparison with other water saving strategies. Even assuming that an acre-foot of irrigation is added to each acre of dry farm crops every year , vertical racks if all the land listed in Table 3 were converted to dry farming and irrigated to the statewide averages listed in the table , California would save 700 billion gallons of water per year, or nearly half the volume of Shasta Lake, the largest reservoir in the state. Given the overlap between suitable dry farm areas and high priority groundwater basins, these potential water savings are especially valuable as water districts scramble to balance their water budgets in light of SGMA. Perhaps the largest caveat to these potential water savings–and any analysis of dry farm suitability that relies solely on environmental constraints–is the economic reality in which conversions to dry farming currently occur. As discussed above, while a dramatic reduction in irrigation inputs might be feasible from a crop physiological perspective, whether farms can remain profitable through such a transition is an entirely different question. Given a dramatically increased supply of dry farm tomatoes, the profits that current dry farmers rely on could easily crumble. When considering other, less charismatic crops that could be good candidates for dry farming , customers’ likely hesitance to pay as steep a premium for high quality produce as they do for tomatoes also casts doubt on the viability of a large-scale dry farm transition given current profit structures for farmers.Our suitability map shows potential for vegetable dry farming to be practiced on California croplands that are currently irrigated, though its expansion is inherently limited.

Even if markets could be adapted to support an influx of dry farmed vegetables, our map indicates that climatic constraints will largely require dry farming to be practiced in coastal regions or other microclimates that can provide cool temperatures and sufficient rainfall. However, the Central Coast’s tomato dry farming offers principles–but not a blueprint–for low water agriculture in other regions. Based on themes from our interviews, these principles show a cycle of water savings that connect reduced inputs, management diversification, and market development . The cycle begins with lower irrigation , which can be accomplished in concert with soil health practices that build soil water holding capacity and increase long-term fertility. Reduced weed pressure and lower biomass production can then lead to reducing other inputs, such as labor and fertilizers, while also allowing for further water savings. The combination of reduced inputs and soil health practices then gives rise to a product that is unique in its water saving potential, and may also be of unusually high quality. By encouraging consumers to appreciate the products, or through novel policy support, farmers can develop markets that will provide a premium for these low-water products–or payment for the practice itself–which in turn creates an opportunity to expand the practice, further lowering inputs.As we ask how policies may impact dry farm production systems, we find a forking path in what types of expansion may result from different policies. An increase in production can be accomplished through both scaling size and scaling number . Both options can tap into the water saving cycle to decrease water usage; however, the search for just, agroecological transitions has pointed time and again to the need for scaling number . On the Central Coast, small, diversified farms have used this water saving cycle to both cut water use and develop a specialty product that allows growers to farm in areas with high land values by increasing their land access, profits, and resilience to local water shortages. Through these principles, small-scale operations have differentiated their management from both industrial farms and even other small farms in the region by creating a system based in localized knowledge, soil health practices, and thought-intensive management. However, it cannot be taken as a given that this water saving cycle will continue to uplift the small scale operations on which it started. Recent work highlights the potential for biophysical and sociopolitical conditions to combine to shrink–rather than grow–the use and viability of agroecological systems . In the case of dry farm tomatoes, socio-political attention is already beginning to target the biophysical need to decrease water consumption. If well-intentioned policy interventions designed to decrease irrigation water use build markets that value the fact of dry farming, rather than the high quality fruits it produces , growers will be able to scale the size of dry farm operations without needing to rely on the highly localized knowledge required to produce high quality fruits. As large grocers scale up dry farm produce sales without worrying about quality-based markets that may quickly saturate at industrial scales, the agroecological systems that originally produced dry farm tomatoes may be edged out of the market. On the other hand, if policies build guaranteed markets for small farms growing dry farm produce, dry farming may grow by scaling out to more small-scale operations. Policies focused on water savings may then favor industrial or small-scale farms, depending on how interventions shape the “Market Development” aspect of the cycle.