Levee breaches on deeply subsided islands draw brackish water into the Delta during rapid flooding, temporarily degrading water quality over a large region. However, long-term degradation of water quality may result from flooding of subsided islands, which affects tidal prism dynamics within the Delta . In addition to sea level rise, the effects of a warmer climate such as reduced snow pack storage, higher flood peaks during the rainy season, and reduced warm-season flows after April increases the risk of contamination of California’s freshwater supplies by salinity intrusion . California has an extensive network of water storage, transfer, distribution, and use systems. The primary infrastructure for agriculture water includes the California State Water Project, the Federal Central Valley Project, and the Colorado River Project. These systems, together with local water districts, are expected to become more coordinated using water transfers as one of means for providing water where it is most desired . Water transfers are described in the Water Code as a transfer or exchange of water or water rights that result in a temporary or long-term change in the place of diversion, place of use, or purpose of use. Water transfers have increased over the past 20 years and are expected to accelerate in the future. Transfers came to prominence with the drought in the 1980s when 80,000 acre–feet were transferred in 1985, and by 2001, 1,250,000 acre–feet were transferred . The transfers are mostly local,hydroponic grow kit with 25% within the same county, almost 50% within the same the same basin, and the remaining 25–30% of the transfers between regions .
Most transfers come from agriculture and go to either environmental or urban use. With higher water values, there are concerns about a net agricultural loss. Concerns about these losses have prompted requests for a balanced approach to regulating transfers.The California value integration network model is an engineering optimization of surface water, ground water and water infrastructure below 1000 feet elevation . Most of California’s agricultural land is covered in the model, with the exception of the coastal regions. Coupled with the CALVIN model is the Statewide Water and Agricultural Production model . The CALVIN model uses monthly estimates of water for 25 regions of the SWAP to determine the statewide allocation of water for 72 years of variable hydrology. It assumes a freely operating water market. Climate change is incorporated into SWAP by modifying crop yields and amount of irrigated water used by the predominant 17 crops in several regions, based on quadratic response functions for yield and ET that examined the effect of elevated CO2 , technological advances , and a range of temperature and precipitation scenarios . SWAP uses economic rules for its optimization solution. The rules include endogenous principles that are beyond the producers’ reach such as world grain prices, exogenous principles, and technological factors such as on farm water delivery and plant breeding developments in water use efficiency optimizing the assimilation to transpiration ratio. Through quadratic crop yield function expressions, yield is related to seasonal temperature, precipitation, land quality measures and technology progress. The quadratic expressions account for both crop gains such as an increase from cold to warm, and to crop declines such as from warm to hot . This is a simple modeling approach that does not take into consideration crop-specific developmental responses to temperature, or effects of extreme events .
This crop modeling effort is the most comprehensive to date in California, and suggests a low impact on crop productivity in response to climate change, through adaptation via technological innovations and land use changes in response to a large range of climate change scenarios.The combined CALVIN and SWAP modeling tools found that although agriculture adjusts to climate change, there will be less land under cultivation and growers will switch to higher value crops. Currently, due to their geographic location, current climatic conditions and the commodities grown in the region, agricultural water users in the Central Valley are the most vulnerable to climate warming and could be devastated by severely dry forms of climate warming. Some Central Valley regions would lose or sell about half their desired water use. According to Tanaka and colleagues , climate change could reduce Valley agricultural water deliveries by 37% from current deliveries in the dry PCM2100 scenario and raises Valley water scarcity costs by $1.7 billion. With a shift to higher value crops, agriculture income only falls 6% while sustaining about a 24% decrease in agriculture water deliveries on 2100 urbanization adjusted water demands . Currently, agriculture uses 70% of the state’s water and results in 10% of the state’s gross domestic product Lund, pers. comm., 2005. If climate change results in an increase in water availability at appropriate times, farmers may benefit. However, if water availability decreases, farmers are likely to be affected more than urban and industrial users, who can pay more for water.Due to growing constraints on new water supply, California is exploring improved water use efficiency in all sectors. Improvements in both agricultural and urban WUE may offer sources of new water supply by reducing overall demands for water in every sector , although this needs to be balanced against the projected increases in the population of California. Wastewater reuse, seawater desalination and water conservation show promise as water supply sources, in particular in southern California where cheaper alternatives may not be available . Under a drier climate scenario, about 1.35 million acre feet year-1 comes from wastewater reuse and about 0.24 maf/year comes from seawater desalination .
Increased winter rainfall could result in increased groundwater recharge; however, higher evaporation and a shorter season of rainfall may reduce recharge to deep aquifers .Agricultural WUE improved considerably from 1980 to 2000 in terms of agricultural production per unit of applied water by 38 percent for 32 important California crops . However, major WUE improvements are still possible in the agricultural sector, particularly through implementation of more efficient irrigation practices. Micro-irrigation can achieve a WUE of up to 95% versus 60% or less for flood irrigation, one of the simplest, yet most inefficient methods. More efficient sprinkler systems, such as the low-energy, precision application system, reduce evaporative losses by discharging water just above the soil surface. Laser leveling of fields can decrease water use and by allowing water to be distributed more uniformly. Precision irrigation is one of the most effective water conservation techniques but also the most expensive,hydroponic indoor growing system and perhaps best suited for high value crops, such as vegetables . Other benefits of precision irrigation are increased fertilizer application efficiency through fertigation, and decreased potential for leaching of nitrate, some pesticides and other soluble compounds. Case studies of successful use of drip irrigation in row crops showed mixed economic results. Drip irrigation in cotton was sometimes less profitable than furrow irrigation . However, a study of buried drip irrigation of pepper compared with historical sprinkler irrigation in two locations in California resulted in higher yields , lower water use , higher energy use , but higher energy use efficiency , and higher net returns by $4,288 ha-1 and $2,265 ha-1 . Using the above examples, Hanson et al. also suggest that drip irrigation is most suited to higher cash value crops. History has shown that as a drought becomes more severe, farmers increasingly adopt water conservation technologies in California , fallow land with relatively low-value crops, and increase ground water pumping. Previous droughts also caused decision makers to introduce mechanisms that rely on market forces and financial incentives to encourage water conservation . The U.S. Department of Agriculture Natural Resources Conservation Service provides growers with incentives to conserve water through the Conservation Security Program, and the California Ground and Surface Water Initiative . Though some economic incentives exist at the state level, increased state support of local agencies in developing incentives for water conservation may improve water use efficiency in California . Because higher water costs may encourage conservation, the state may influence water use by instituting rates that support better water management .Given California’s projected population increase , promoting urban water conservation may reduce the amount of water diverted from agriculture. In addition to augmenting the water supply, water recycling may offer several benefits to farmers such as providing a more secure water supply during droughts, and supplying more reliable local sources of water, nutrients, and organic matter for agricultural soils.
Currently, California recycles about 500,000 acre-feet of wastewater annually, of which approximately 250,000 acre-feet year-1 is used by California farmers on 52 different crops . There is a potential to obtain 0.9 million to 1.4 million acre-feet annually of additional water supply from recycled water by the year 2030. Recycled water is also used for groundwater recharge, with 15% of all recycled water in 2002 used for groundwater recharge . Some water resource management strategies, aside from new water-supply infrastructure, are: wastewater reclamation and reuse, water marketing and transfers, and desalination; however, none of these alternatives are likely to alter the trend toward higher water costs . The increased cost of traditional water sources , and the reduced costs of desalinization due to improvements in technology, have resulted in greater consideration of this option as a water source in some areas of California and elsewhere. Under a wet climate change scenario, Central Valley flooding will become a serious problem. Widening the lower American River flood way, raising levee heights, and potential levee relocation due to increased urbanization are possible strategies for dealing with increased flooding. In a recent study , the most extreme case considering urbanization and the increase in land value, as well as flood damage costs and frequency due to climate change, the height of levees and their setback would need to be 65 feet and 500 feet respectively; which has implications for other infrastructure projects.Given the large discrepancy in current precipitation prediction models, an immediate need is improved prediction of precipitation amounts and spatio-temporal patterns. Refinement of nested regional models that can assess change and responses at a scale that matters to agricultural producers are also needed. These models will provide a better analysis of evapotranspiration demand, and water management at the landscape scale. Precipitation and wind speed are not expected to be well-described in any models in the near future, yet these models are important both for understanding extreme weather events and for optimizing crop breeding and agronomic technologies to make best use of available water resources . Results from improved regional models can enhance the precision of input data sets for the water infrastructure operational optimization models such as CALSIM, a California DWR water resource planning tool that provides input to CALVIN, as well as contributing directly to the CALVIN and SWAP models. Currently plant physiological processes, such as drought tolerance at different plant water potentials, are of limited use because quantitative descriptions are not available in a form compatible with the quadratic response functions for individual crops in the SWAP model. Evapotranspiration in the model is treated as single static value rather than a dynamic variable that responds to changes in climate. In addition, there are many developmental processes in plants that are affected by temperature that are not included in this approach . New models with the capacity to integrate physiological and developmental processes for high-cash value specialty crops are a high priority.Water conservation in agricultural production results in a direct mitigation for greenhouse gases because of the decrease in fossil energy used to pump and deliver water to crops. Specifically, there is a need to focus on high value water efficient agricultural commodities. For example, Pimental et al., described commodity water use using the volume of water required to produce a particular mass of product. Alfalfa, which is a C3 plant that prefers a milder climate, was found to require 1100 liters per kilogram produced of yield. Wheat is a C3 plant that evolved in a semiarid environment and prefers a relatively dry climate. It was reported to require 900 liters per kilogram. Corn, a C4 plant that originated in a hot dry climate requires 650 liters per kilogram. Rice, which is a C3 plant that evolved in a tropical environment, requires 1600 liters of water per kilogram of rice.