Wheat is successfully adapted to conditions in the southern Central Valley where it is generally warmer in the winter than it is in Yolo County. For alfalfa production, a warmer winter is expected to provide favorable conditions, particularly since alfalfa varieties commonly planted in northern California are either semi‐dormant or non‐dormant . In the Sacramento Valley, alfalfa is harvested six to seven times a year, with its first harvest beginning in April, thus warmer conditions in the spring would increase production and income. For orchard crops, the prune and grape acreage equations have significant own price effects . For grapes, this may be due to increased wine consumption and demand in the United States. Precipitation variables show some differences between annual and orchard crops. Orchard crops may be more resilient to water availability, since they tend to use drip irrigation and use a smaller share in total costs for irrigation than annual crops. Finally, no one plants orchards without already securing access to water, and this is a very long‐term consideration, not dependent on short‐run fluctuations. The data also indicate that winter chill hours have statistically significant relationships with acres of prunes and miscellaneous fruits , and for walnuts . These effects indicate that an increase in winter temperatures is associated with a decrease in acreage for these crops. Walnuts and prunes are among the fruits that require significant numbers of chill hours . Further calculation indicates that 1 percent change in chill hours induces also about 1 percent change in acreage for prunes and walnuts, but about 1.7 percent change for miscellaneous other fruits.Based on the regression results provided above, projections of GDD summer, GDD winter, winter chill hours,wholesale grow bags and precipitation can be used as drivers for acreage changes over the next four decades .

This assumes that the same general types of production systems and markets occur as at present. Down scaled climate projections from two IPCC emissions scenarios were used to until 2050, a time‐frame consistent with using past crop‐climate relationships to guide future decision‐ making. All independent variables other than Yolo County climate variables are held constant at the value in the last year of actual data . By holding all other variables except for climate variables, we focus on the acreage effects of the changes in temperature and precipitation in the A2 and B1 scenarios. These are projections of plausible scenarios that are based solely on past responses of growers. No attempt is made to forecast relative prices, technical changes, new markets, or other factors that will also surely affect how much of each crop is planted. Notice also that we do not consider the direct or indirect effects of climate elsewhere on acreage in Yolo County. So, for example, we do not incorporate potential impacts of a smaller snow pack on irrigation water availability. Nor do we consider indirect price effects of global and national supply adjustments in response to climate change in other parts of California or in other regions that may produce crops that compete with crops in Yolo County.Using daily GFDL projections for 2010–2050 under A2 and B1 scenarios, we estimated the values of our climate indices following the procedures used for the historical data above. These same data are used in Section 3 for climate change projections for hydrological modeling. Use of the climate indices with GFDL data for 2010–2050 produce acreage projections that are generally consistent with the historical trends of the past century .

The temperature patterns for the A2 and B1 scenarios are remarkably similar for the period from 2009–2050, except for an unexpected decrease in the A2 scenario after 2035, with a concomitant increase in B1 . This is mainly due to winter temperatures, as is evident from lower GDD winter and higher winter chill hours in A2 versus B1 in this time period . Note that GDD summer and precipitation show very similar patterns for the scenarios. The greater winter warming in B1 derives directly from the climate data in the GFDL‐BCCA output runs. It appears to be an artifact of the climate down scaling or and clearly does not reflect the long‐term pattern of greater warming in A2 by the end of the century . Statewide projections from several other GCMs show similar trajectories for A2 and B1 until mid‐ century reflecting the expectation that our current actions to mitigate GHG emissions may have little effect in the near future.The estimated acreage tracks the actual acreage well over the historical period for the field crops , indicating that our regression results do fit the data well . The acreage projections into the future use the estimates obtained in our regressions, which are based on historical data, and do not include unanticipated shocks, such as new pests or changes in relative prices. Therefore, compared to the previous 50 years, our acreage projections vary less from year to year than do the historical data or the fitted values over the historical period. This smoothing follows because future acreage was projected by varying only future climate, holding all other variables constant, and seeking broad patterns of change consistent with growers’ expectations about future climate. Compared to current acreage, future rice acreage tends to be slightly higher, but with no clear trend . Recall from our regression results that the rice own price is a more significant determinant of rice acreage than climate. Wheat acreage decreases significantly under the warming of the B1 scenario in the final 15 years of the projection period .

The opposite is true for alfalfa . Increasing GDD in winter is favorable for alfalfa and alfalfa acreage increases significantly during this warming period.Safflower acreage tends to fluctuate over a wider range than other crops during the projection period . Our regressions showed that safflower acreage increases when water availability is restricted. Safflower is one of the least irrigated crops, and the fluctuations in safflower acreage are related to precipitation. Corn acreage also fluctuates around the current level with no major trend . Corn acreage may well increase in Yolo County if prices are much higher , but such a scenario is not incorporated in this study. Irrigated pasture acreage also varies little from the current level over the projection period . Displaying all field crops together allows us to readily compare across crops . Under both scenarios, rice, alfalfa, and wheat remain as major field crops throughout the period to 2050, although wheat acreage is much lower in 2040 and 2050 compared to earlier decades in response to warming. A concomitant projected increase in alfalfa acreage presents an interesting implication for water use. Wheat is one of the least water‐using crops because much of its growing season coincides with the rainy season in California. Alfalfa, on the other hand, is one of the more intense water users. Thus, any significant decline in wheat acreage combined with an increase in alfalfa acreage is expected to increase regional irrigation water demand. Winter warming between 2035–2050 reduced field crop acreage, and was mainly related to loss of wheat acreage. In 2008 wheat covered the second most acreage among the field crops, following alfalfa . Even though the warmer winter under the B1 scenario increases alfalfa acreage,grow bags for gardening the decline in wheat acreage caused by warmer winter is larger, leading to a decline in overall field crop acreage. Tomato acreage is projected to increase compared to the current level, mainly in the latter half of the projection period under the B1 scenario . This projected increase is related to the increase in the GDD in the winter months. A warmer climate in the late winter has a positive effect on tomato production because it allows early planting and provides favorable conditions for establishment. In Yolo County, growers plant tomatoes every week during the late winter and spring. Projected acreage for other vegetables is small and changes little . Since tomatoes dominate vegetable acreage in Yolo County, any change in other vegetable acreage, even if it were significant, would have little effect on total vegetable acreage.Projected prune acreage shows a downward trend under both scenarios, which is a result of reduction in winter chill hours . However, prune acreage fluctuates more under the B1 scenario than under A2. Grape acreage is almost constant over the projected period ; changes in grape acreage are induced by changes in factors other than climate. Almond acreage, unlike the rest of orchard crops, increases in the latter half of the projected period when winter warming occurs under the B1 scenario . Almond has relatively a low winter chill hour requirement, and appears to be little affected by the lower chill hours that are projected. For walnut and miscellaneous fruit , reduced chill hours are found to be a contributing factor, and acreage tends to decrease more under the B1 scenarios. Figures 2.21a and 2.21b present crop specific orchard crop acreage for selected years under A2 and B1 scenarios, respectively.

Only small differences exist between the two scenarios and the acreage variations over the projection period are small overall. Climate change is of second order importance in determining orchard crop acreage. For the entire projection period, total orchard crop acreage changes little, and there is little difference in acreage exists between B1 and A2 scenarios. In the second half of the projection period, the acreage reduction for prunes, walnuts, and miscellaneous fruits under the B1 scenario is offset by the acreage increase for almond and grapes. Thus, climate‐induced changes in composition of tree and vine crop species are more likely than loss or gain of acreage. Reduction in the 10‐year moving average of winter chill hours does not appear to be a major factor that will contribute to acreages at least until 2050, reinforcing the point that climate change is of second‐order importance in determining orchard crop acreage. Once again, it is important to keep in mind that our projections in acreage changes are driven solely by forecasts of climate indices with no other drivers of acreage change included in the model. Further, the effect of this future climate change was evaluated based on farmers’ past history on how climate expectations may have affected their acreage decisions. Thus, as with any analysis, these projection results should be understood within the model framework.Recall that projection of climate‐induced acreage changes use the relationships between climate change and acreage that were established over the past 60 years. Based on this observed behavior the projected climate‐induced changes in acreage shares among crops in 2050 differ from the shares in 2008 . Significant changes in acreage shares are found mainly with annual crops; particularly rice, alfalfa, wheat, and tomatoes. Under both climate scenarios, acreages of tomatoes and rice increase. The tomato share increases from 15.5 percent to up to 20.6 percent by 2050. Rice acreage increases from 12.4 percent to as much as 16.2 percent. The alfalfa acreage share increases from 23.4 percent to up to 26.1 percent. Wheat now accounts for a major share of Yolo Country crop acreage, but climate change would induce significant declines in wheat acreage by 2050. Again, these projections do not include anticipated changes in relative prices, pests, climate variability or other drivers that are not themselves driven by climate indices in Yolo County. These econometric models are based on continuing into the future with the same patterns of decisions that have influenced farmers’ past history, using the 2008 starting point for crop acreages. Therefore, these projections do not include anticipated changes in relative prices, pests, climate variability, short‐term extreme events in climate, or other drivers that are not themselves driven by climate indices in Yolo County.Our research developed unique sets of data on agricultural climate aggregates and crop acreages to establish statistical relationships between climate and the pattern of crops planted over the past six decades. Estimates of parameters that characterize these relationships guide projections about how alternative climate changes forecasted under the two GHG emission scenarios affect acreage patterns in Yolo County from 2010 to 2050. Growing degree days have risen, especially for the winter season, while the number of chill hours has declined. The specific crop mix has changed over the decades, while these overall patterns have continued.