Statewide regulations mandating conserving practices have also been put in place, most notably were in 2004, a law requiring water suppliers to install water meters on all customer connections by 2025 ; the imposition of water budgets and water-efficient landscaping on most new large landscapes ; and the requirement that water suppliers and local governments improve the coordination between land and water use planning through preparation of Urban Water Management Plans and Urban Water Shortage Contingency Analyses . In 2001, Orang et al. conducted an irrigation method survey throughout California. That analysis shows that for all crops combined, the use of gravity/flood irrigation and sprinklers has declined, while micro/drip and sub-irrigation use has increased . Using historical data on irrigation methods by crop type between 1972 and 2001. Gleick et al 2005 used a linear trend to calculated and project the irrigation method for 2030. The result of their estimate is shown in Figure 2-20. Water footprint assessment has become as a useful quantitative tool for assessing water consumption of goods and services. Following the introduction of the virtual water footprint concept, various studies were conducted to quantify global virtual water flows between nations as have been carried out by Hoekstra and Hung , Chapagain & Hoekstra , Zimmer and Renault and Oki et al . Virtual water flows and water footprint assessments became important elements in evaluating global and national water budgets as noted by Chen and Chen , Duarte et al., , Guan and Hubacek , Hubacek et al. , Velazquez , Yang et al. , Yu et al. , Zhao et al., .
The majority of water footprint studies thus far have been limited to quantifying the overall virtual water requirement as a cumulative measure to reach a final crop product. No study to date has focused specifically on quantifying the physical water content contained in agricultural products being exported and relating it to the water exportation induced through evapotranspiration. The total exported water in agricultural products will physically leave a geographical boundary, whether in the crops or in a form of evapotranspiration or combination of both,big plastic pots whereas a significant portion of the virtual water footprint may still remain within the same spatial domain. Hence, from a hydrological perspective the exported water content causes a spatial imbalance in the water supply cycle, irreversibly for the portion contained in the crops. It is therefore warranted that research be conducted to quantify the exported water content to better assess water use efficiency and practices in agricultural irrigation. It is also warranted to analyze the associated energy intensity and carbon footprint assessment of water supply sources used in irrigation to assess and determine effective uses of California’s water and energy resources. Table 2-7 and 2-8 illustrate existing water footprint studies currently available in the literature.As pressures on water resources intensify globally, there is a growing interest in evaluating the complex ways in which human activities impact the world’s water resources . Globally, the majority of water consumption is used in the production of agricultural products . As a result, the agriculture industry is by far, the most dominant water-using sector. To assess the amount of water used throughout the production and distribution process to produce a final product, researchers have used the term ‘water footprint’, to describe this quantity .
Water footprint assessments have emerged as a tool for quantifying consumption of everyday goods and services in one location and the cumulated water use associated with the production of those goods and services in other distant locations . Originated from an ecological standpoint , the concept of ‘virtual water’ was first introduced by Allan to describe the transfer of water resulting from exports of water-intensive commodities from comparatively water-abundant regions to water scare regions. Allan argued that international agricultural food trades were equivalent to exporting water in its virtual form and observed the trend in national governments becoming more dependent on other countries for their water security. Hoekstra and Hung then introduced the concept of ‘water footprint,’ as a quantitative measure to assess virtual water content consumed by an individual or individuals of a nation for goods and services.Following the introduction of the water footprint concept, various studies were conducted to quantify global virtual water footprints and assessed virtual water flows between nations , , and . Virtual water flows and water footprint assessments became important elements in evaluating local, national, and global water budgets as reported by Chen and Chen , Duarte et al., , Guan and Hubacek , Hubacek et al. , Velazquez , Yang et al. , Yu et al. , Zhao et al., . In addition to assessing virtual water footprints in food products, virtual water footprints in non-food products were also examined. Hoekstra and Chapagain , assessed water footprints of different countries using calculated data from Chapagain and Hoekstra and Hoekstra and Hung . Subsequently, Mekonnen and Hoekstra improved upon data presented by Hoekstra and Chapagain and provided a more complete and detailed global virtual footprint assessment.
By categorizing global water resources into green, blue, and gray water, Mekonnen and Hoekstra showed that the international virtual water trade in agricultural and industrial products were 2320 billion cubic meter per year in the period 1996-2005, equivalent to 26% of the global water footprint of 9087 Gm3. noted that although practically, every country participates in the global virtual water trade, few governments explicitly consider assessing virtual water footprint and its impact in their management policies. The majority of water footprint studies have examined international virtual water footprints between nations . Few have also analysed the virtual water footprints at a sub-national or state level such as regions within Australia , China , India , and Spain . Within the United States, two studies have also been conducted. Fulton et al., reported that California imported more than twice virtual water as it exported and that more than 90% of its water footprint is associated with agricultural products. Mubako et al., quantified virtual water for California and Illinois, and reported that the two states were net virtual exporters in agricultural water trades. Previous studies on virtual water footprints at global, national and sub-national levels including the ones conducted by Fulton et al., and Mubako et al., only aimed to quantify the cumulative water footprint required to produce a final product. None however, has focused specifically on quantifying the physical water content contained in agricultural products being exported. The total exported water in agricultural products is distinctively different than the overall virtual water footprint in that the former is physically exported outside of a geographical boundary,growing berries in containers whereas parts of the water used in quantifying virtual water footprint may still remain within the local geographical boundary and may be absorbed or reused in some ways. Thus, from a hydrological perspective, the exported water content in crops is permanently lost and is no longer available for local regeneration from where it originated. We make a distinction to differentiate this quantity of exported water content in agricultural products because previous virtual water studies only focused on quantifying the cumulative virtual water flows or the total water footprint. In addition to the physical water content contained in agricultural products, irrigation associated with agricultural activities has also been shown to have a direct contribution on local and regional climate by increasing surface evapotranspiration.
Our research hypothesis is that for each unit of crop exported, there is a greater loss of water induced through evapotranspiration than the physical water content contained in the crops. The aim of this research is to test our hypothesis by using California as a case study to assess the overall exported water associated with agriculture by quantifying the contained water content in the exported crops and quantifying the direct induced evapotranspiration associated with those exported crops. Due to availability of public data and the extensive number of crops produced in California, we focus our research on the top 50 crops which make up the majority of water consumption and gross receipt for California by examining the latest available data from the years 2000-2012. The results show that from 2000 to 2012, of the 50 commodities we have analyzed in this study, the annual average crop production was 1.57 x 109 metric tons in California, of which an estimated 17% was exported internationally for a total of 2.63 x 108 metric tons. The contained water content associated with the exported crops was calculated at 2.32 x 108 m3 yr-1 representing 0.6% of the total annual water used in irrigation for California. In contrast with the water uptake by plant tissues, typically reported at 5% of irrigation water use , the contained water content represents 11% of the total water absorbed by plant tissues calculated at 2.1 x 109 m3 yr-1 . The induced evapotranspiration water associated with irrigation for the exported crop was calculated at 2.85 x 1010 m3 yr-1 equivalent to 67.7 % of the total annual water used in crop irrigation. The total exported water associated with the exported commodities altogether was calculated at 2.88 x 1010 m3 yr-1 which represents 68.3% of the total water used in irrigation annually. Our results show that the actual water contained in the crop is significantly less relative to the associated exported water in the form of induced evapotranspiration. The results indicate that the actual physical water contained in crops only represent 0.81% of the total induced evapotranspiration water resulted from irrigating and growing of the exported crops. When comparing the results of the calculated induced evapotranspiration to the Field Capacity Model recently reported by Sorooshian et al., , our results represented 97% of the model prediction. This finding suggests that irrigation of agricultural products play a significant role in consuming the total water used in crop production. The results also confirm that agricultural irrigation does have a direct impact in contributing to the increase in surface evapotranspiration as previously reported by Wei et al., , and Lo and Famiglietti, . When computing for the overall total exported water for each crop , we find that hay, grapes, almonds, rice, cotton, wheat, walnuts, tomatoes, oranges, lettuce, pistachios, plums, peaches and nectarines, broccoli and avocados were the leading 15 crops consuming most of the water used in irrigation. A detailed list of annual exported water consumed by each of the 50 crops is shown in Figure 3-8. When computing for the dimensionless ratios , we find that cotton, almonds, pistachios, walnuts, plums, rice, olives, cherries, beans, and figs were the top 10 with the highest overall exported water indexes. This result suggests that compared to other crops grown in equivalent planting area and exported percentages, the top ten crops with the highest exported water indexes would consume the majority of the water. However, when taking into account of the actual export percentages and planning areas, the total exported water for each of crop from the average 13 years , the results are shown in Figure 3-8. As an index reference, we report the three dimensionless ratios below in Table 3-1 to indicate the actual amounts of water contained in each crop and their respective water equivalents as a result of direct evapotranspiration and overall resultant exported water content. These indexes may be used to compute future exported water and associated induced evapotranspiration contribution as the outcome of agricultural irrigation. The study found that from 2000 to 2012, California exports approximately 2.63 x 108 metric tons of crops and commodities, resulting in approximately 2.32 x 108 m3 of water contained in the crops, equivalent to 0.6% of the total irrigation water use. The associated induced evapotranspiration from the exported crops was calculated at 2.85 x 1010 m3 yr-1 equivalent to a total of 67.7% of the overall irrigation usage. The calculated ETc is approximately 123 times greater than the physical water contained in the crops, confirming our hypothesis that for each unit of water exported, there is a greater loss of water through evapotranspiration as a result of irrigation. As a comparison, Nguyen et al., estimated that the average annual urban water consumption in California was 1.01 x 1010 m3 , thus the average induced evapotranspiration as a result of agricultural irrigation is 2.8 times greater than the total annual water consumed by municipalities. Of the 50 crops analyzed, alfalfa hay, grapes, almonds, rice, cotton, wheat, walnuts, tomatoes, oranges, lettuce, pistachios, plums, peaches and nectarines, broccoli and avocados were the 15 leading crops consuming most of the exported water.