These regions, largely Northern California and Colorado, are likely to experience changes in precipitation , temperature, and thus altered snow pack and runoff patterns .Water from Northern California, specifically snow pack in the Sierra Nevada Mountains, is expected to decrease due to higher rain and snow elevations, and earlier snow melt and spring runoff. Snow pack will be reduced by more than 60% by the middle of the century, with positive feed backs further exacerbating these warming and snow melt trends . Increased evapotranspiration and decreased snow pack will also cause decreases in water supply in the Colorado Basin . Over the next fifty years, droughts lasting five or more years are projected to occur fifty percent of the time. These impacts will reduce the water in these areas which supply San Diego, as well as counties across California and along the Colorado Basin, resulting in worsened water resource challenges. Thus, it is imperative that San Diego consider alternative sources of water and infrastructure developments, in addition to enhanced water-use efficiency across all sectors.As a sector that is greatly dependent on climate and highly sensitive to environmental conditions , agriculture is exceptionally vulnerable to the effects of a warming world. With ongoing shifts in natural processes that dictate agricultural practices, productivity, and costs, the future of agriculture is one with distinct and palpable challenges. Because the effects of climate change on agriculture are highly dependent on variables such as climate, geography, soils, and customary agricultural practices, the net impact felt by regions will vary greatly. In some areas, it is projected that climate change may result in beneficial consequences for agriculture, while in others, consequences could be detrimental. Therefore, it is necessary to develop regionally and locally unique solutions for these changes . Most Mediterranean regions, such as San Diego, will feel the greatest impacts from increasing variability in precipitation compounded with increasing temperatures. Precipitation and temperature are deeply embedded within the hydrologic cycle, and thus, as these climate variables continue to shift, they will alter many hydrologic processes. Furthermore, increased climate variability making adaptation increasingly difficult for the farming community .

Ongoing changes, such as the timing and frequency of precipitation, reduced snow pack levels, and earlier snow melt, present several challenges for the region’s water resources . These water-related challenges are inextricably linked to the overall functioning and viability of agriculture, and are thus paramount in determining the persistence and growth of San Diego agriculture. There are several key hydrologic variables that play a role in the overall functioning of a landscape, and as these hydrologic variables change, agricultural lands are impacted. Table 2 outlines these hydrologic variables, their impact on a landscape, and projections for future climate scenarios. These hydrologic variables include: CWD, AET, runoff and recharge. One of the major hydrologic variables San Diego’s landscape is climatic water deficit . CWD is the amount of additional water that would have evaporated/transpired if soil water was not limiting, combining the effects of evapotranspiration, solar radiation, and air temperature on watersheds, given the soil moisture level from precipitation . CWD can be translated to direct impacts for agriculture. In Mediterranean climates, it is considered a proxy for water demand based on irrigation needs . Another important hydrologic variable that heavily impacts landscapes is actual evapotranspiration , which is the amount of water actually lost by the vegetated surface. For the farming community, AET translates to above ground net primary productivity and is used as a proxy for productivity of a landscape . Changes in AET and CWD can be used in quantifying the additional water necessary to maintain vegetation or crops in a landscape , effectively identifying the amount of irrigation demand needed to cover seasonal deficit . Critical to the relationship between climate change and natural landscapes is understanding the contribution of agriculture to increasing GHGs and in turn, climate change. Degrading and eroding soil from intense grazing, plowing, and clear-cutting, has throughout time, played a significant role in the increasing concentration of atmospheric GHGs . Long-term degradation of important features of natural lands, such as soils, forests, and wetlands, is one of the key drivers of a warming world . Relative emissions and impact, however, vary with region depending on soil properties and agricultural practices. In the San Diego region, agriculture contributes approximately five percent of total unincorporated county emissions . In general,plastic planters bulk most farm-related carbon dioxide emissions result from a variety of soil, livestock, and manure management practices, including soil tillage, overgrazing, farm equipment, livestock and fertilizer use .

The world’s soils play a critical role in food production, water resources, both quantity and quality, and increased net primary productivity. Enriched with soil organic matter , soil has the ability to recycle dead matter into mineral-rich nutrients vital for plants and other organisms. Additionally, soil provides the distinct and critical service of removing gases from the atmosphere. Through the biological process of carbon sequestration, carbon dioxide is removed from the atmosphere and stored as sinks in soils. This service helps keep terrestrial and atmospheric carbon levels in a balance . Carbon is the primary component of SOM and provides soil with defining characteristics such as water-retention capacity, filtering capabilities, structure, and fertility . Because pools of soil organic carbon aggregates are stable and robust, they provide the largest store of terrestrial carbon and have the ability to be sequestered for up to a millennia . The length of time and amount of carbon that remains in the soil is largely influenced by ecosystem and environmental processes, depending on vegetation, soil properties, water drainage, and climatic conditions. Thus, levels of SOC varies on large-scale global patterns and on smaller-scale regional and sub-regional basis . The unique capability of soil to nourish vegetation and capture carbon long-term helps buffer the implications of climate change for both society and ecosystems alike. Additionally, with the likelihood of increased flood events, agriculturally managed lands could play a role in retaining flood waters for flood risk reduction as well as possible groundwater recharge. However, the ability of soils to provide these services is contingent on its overall quality. The length of time and amount of carbon that remains in the soil is largely influenced by management practices, in addition to ecosystem and environmental processes . If soils are poorly managed with unsustainable agricultural practices, soils can release CO2, contributing to atmospheric concentrations. Alternatively, if healthy, soils can enhance sequestration and continue to play an essential role in climate change mitigation. Thus, promoting healthy soil is critical to ensuring the resilience of landscapes, agriculture, and society. Recently research has focused on the potential of enhancing SOC sequestration to help moderate high levels of atmospheric carbon. On a large scale, SOC sequestration could hypothetically sequester all current annual GHG emissions globally, at approximately 52 gigatonnes of CO2 equivalent . This research highlights the ability of soils to offset increasing atmospheric CO2, where restored SOC pools could promote productivity, fertility, and resiliency to a variety of climate extremes. There are a variety of carbon farming practices that farmers can adopt in order to achieve these benefits .

Table 6 outlines some of the common on-farm conservation practices recognized by the Natural Resource Conservation Service to improve soil health, sequestration rates, and associated co-benefits . From permanent crops, compost and mulch application, windbreak renovation, no-till row crops, to cover cropping, the agricultural community has several options when it comes to implementation . Overall effectiveness, in terms of sequestration rate, GHG reduction, and benefits will depend on various climatic and environmental factors. Thus, suitability of practices vary by region and individual agricultural context. Many studies have shown that compost application is one of the most impactful practices for carbon sequestration rates. Field and model results from a report within California’s Fourth Climate Change Assessment indicate that a one-time ¼ “ application of compost to California’s range and croplands can lead to increased carbon sequestration and net primary production rates in soils maximized after 15 years . In another study conducted by the Marin Carbon Project, it was shown that a one-time application of a ½ layer of compost on grazed range land was able to increase carbon storage by 1 ton of carbon per hectare. This resulted in both increased forage production and water holding capacity . This study uses down scaled statewide modeling data to analyze hydrologic response in San Diego as it relates to agricultural land evaluation, based on a report for California’s Fourth Climate Change Assessment by Flint et al. 2018. The Basin Characteristization Model is a grid-based model that combines climate inputs, watershed, and landscape characteristics to calculate the water balance. By combining fine-scale data, the BCM can generate detailed assessments of coupled climate and hydrologic response . Precipitated water can act in various ways as it enters into a landscape, from evaporation and transpiration, recharge, or runoff. Given climate data, governed by latitude, longitude, elevation, slope, and aspect, in addition to soil properties, and characteristics of deep soil materials, the BCM can effectively model the response of these hydrologic factors . Flint et al. 2018 utilized a revised version of the BCM to include SOM percentage for calculations of WHC. Using this modified version of the BCM, Flint et al. calculate how increases in SOM changes hydrologic response to climate . The study assesses changes in WHC as a result of additional SOM, and the impact that changes in WHC have for hydrologic variables such as recharge, runoff, AET, and CWD throughout the state. Table 6 outlines the predicted hydrologic response to these changes, however, response is dependent on several factors, including precipitation. Figure 10 shows the high variability of potential hydrologic benefits from increases in SOM for the period of 1981-2010 across the state’s working lands . These results showcase the diversity of climates and soil properties throughout the state’s landscape, and the impacts these factors have for potential benefits in forage production, landscape stress, irrigation demand, and water supply .Hydrologic benefit is calculated using a hydrologic index from changes in these variables, specifically increases in AET and recharge, and decreases in CWD . The hydrologic index is binned into three classifications of benefit based on index value, including “no benefit”, “minimum benefit”, “moderate benefit”, and “maximum benefit”. Hydrologic benefit is mapped for the entire county , the unincorporated county excluding the incorporated areas , and the incorporated county .