Impacts of climate change food security

The current system of priorities in the basin generally is first to supply ecosystem requirements, second urban needs, and finally agricultural needs. Both agricultural

— Und-use and dimale charge (2010-2039)

-Land-use and climale change (2070-20991

-Historical (1961-1999)

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Fig. 11.6. Average monthly flows in the Upper Sacramento according to different scenarios and baseline situation (1961-1999).

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Fig. 11.6. Average monthly flows in the Upper Sacramento according to different scenarios and baseline situation (1961-1999).

and urban demands will increase under climate change, in spite of a shift of land from agriculture to urban use. This is due to the increased temperatures under climate change affecting evapotranspiration, which in turn leads to increased crops water requirements that outweigh the loss of agricultural area. Future average water demands are shown in Table 11.2. Environmental demands are assumed not to change in the future. Agricultural demand in all periods is in the order of 90% of the total water demand in the Sacramento system. Interestingly, the largest demand growth is in the urban sector (i.e. a factor of 4 over the next 100 years compared to a factor of 1.2 for agriculture). Still, agricultural demands clearly dominate the system. Agricultural demands are slightly larger under the B2 climate scenario due to less overall precipitation, offsetting the smaller projected temperature increases.

With this system of priorities, environmental demands are essentially satisfied under both the no-climate change and with-climate change scenarios. With land use changes only, both agriculture and urban areas face shortfalls in the order of about 5%. This unmet urban demand is entirely from Placer and El Dorado counties due to the higher priority downstream American River AFRP flow requirements. With climate change unmet demands increase over time for both agricultural and urban users.

Average unmet agricultural demand with the 'land use changes only' scenario (i.e. using the historical climate) is approximately 482 X 106 m3 (standard deviation = 329 X 106 m3) for the period 2010-2039. With climate change, the average unmet agricultural demand increases to 786 X 106 m3 (standard deviation = 359 X 106 m3) in 2010-2039 (see Fig. 11.7 below). On a percentage basis, unmet agricultural demand increases from 5% to 7% with climate change in 2010-2039, and to 12% in 2070-2099. This baseline 5% unmet demand reflects predominantly unmet demands for irrigated pastures and orchards in the upper watersheds of the Sacramento (e.g.

Table 11.2. Average sector demands for historical period and two future projected climate periods 2010-2039 and 2070-2099 (both are HA2 GCM scenarios). Ranges are given in brackets.


Historical period (1961-1999)

Projected period (2010-2039)

Projected period (2070-2099)

Agriculture (106 m3)







Urban (106 m3)







Environment (106 m3)








a Excludes winter rice flooding requirements of 123x10® m3.

a Excludes winter rice flooding requirements of 123x10® m3.

Fig. 11.7. Annual unmet agricultural demand for the historical climate and climate change 2010-2039, with land use changing under both climate regimes.

Upper Pit, Upper Yuba, Upper Feather). The sources of available water for these demands are limited to the rain- and snowmelt-fed tributaries from which they draw. The majority of these unmet demands occur during the summer months of June-September (see Fig. 11.8) - the most critical months in terms of production. The coefficient of variation (defined here as the ratio of the standard deviation to the mean) also increases over these two climate change periods from 0.46 in 2010-2039 to 0.65 in 2070-2099. This has important implications as incomes for farmers may become less certain on an inter-annual basis.

Fig. 11.8. Average monthly agricultural demand supplied and unmet for 2010-2039 (HA2 GCM climate scenario).

Note, as pointed out earlier, the agricultural demands under the climate change scenario are significantly higher due to increased temperatures. The direct impact on the crop water requirements for agriculture are a potential increase in the total water needed of 10% by 2010-2039, and more than 20% by 2070-2099. These changes are validated by the field-scale model, SWAP, for both rice and tomatoes - two of the major crops in the basin. However, the SWAP model also shows increased productivity for these two crops (van Diepen et al., 1989). Rice yields may increase by almost 50% for the A2 and 20% for the B2 scenario, while tomatoes may increase by as much as 20%. This increase in productivity is related to photosynthetically active radiation (PAR), which is used by the plant as energy in the photosynthesis process to convert CO2 into biomass. Crop production is therefore affected by the air's CO2 level and in many high-input farming systems the CO2 levels are the limiting factor in crop production. Important in this process is to make a distinction between C3 and C4 plants. Examples of C3 plants are potato, sugarbeet, wheat, barley, rice, and most trees except mangrove. C4 plants are mainly found in the tropical regions and some examples are millet, maize and sugarcane. The difference between C3 and C4 plants is the way the carbon fixation takes place. C4 plants are more efficient in this, with the loss of carbon during the photorespiration process nearly negligible for C4 plants. Alternatively, C3 plant may lose up to 50% of their recently fixed carbon through photorespiration. This difference has suggested that C3 plants will respond less positively to rising levels of atmospheric CO2. However, it has been shown that atmospheric CO2 enrichment can, and does, elicit substantial photosynthetic enhancements in C4 species (Wand et al., 1999).

Demand increases in a period of lower flows with climate change. One would therefore expect more severe levels of unmet demand. However, effects are buffered by groundwater supplies, as discussed in the following section.

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- - ■ Land use and climate change -Historical (1961-1990)

25 Year

Fig. 11.9. Total groundwater storage using the Hadley A2 scenario 2010-2039 for climate change.

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