Rice yields are expected to increase by almost 50% for the A2 and 20% for the B2 scenario, mainly as a result of enhanced CO2 levels (Fig. 3.7). However, to maintain this level of yield an increased irrigation application is necessary from an average 900 mm/year in the period 1961-1990, 1000 mm/year in 2010-2039, to 1150 mm/year in the period 2070-2099 for the A2 scenario. From a water productivity point of view, it is interesting that it is still worthwhile to do so (Fig. 3.10).

Since irrigation is a key issue in Sacramento, we have defined two adaptation strategies. One is that irrigation is restricted to use on average maximum 900 mm/year and the second is that irrigation is reduced by about 10% (deficit irrigation). The first option has no impact for the period 1961-1999, since this 900 mm is the actual amount used. For the near and distant future such a restriction would have a dramatic impact on yields (Fig. 3.11) and water productivity. From a basin perspective this implies that if no more water will become available for irrigation, some rice areas should be taken out of production. An overall reduction in rice irrigation of 10% will induce a substantial loss in production of about 30%. Note that with this strategy an increase in irrigation over time is still assumed, but less than during the baseline.

Tomato production provides the highest water productivity of all the cases considered in this study. It should be taken into account that we have defined WP here in terms of gross production, while production costs of tomato are known to be high. Somewhat remarkable is that variation in yield reduces over time for the baseline as well as for the increased irrigation strategy.However, variation is still high in comparison to the rice crop.

Increasing the amount of irrigation (adaptation strategy 2) is not boosting the production substantially. To stop irrigation completely (strategy 1) does not have a severe impact on yields for the period 1961-1990, but is dramatic for the future, especially for the period 2070-2099. For this period the projected rainfall decreases by almost 15% for the A2 and B2 scenario, while temperatures and thus crop water requirements are higher. The low impact of stopping irrigation for the 1961-1990 period can be explained by the favourable soil characteristics in terms of soil waterholding capacity.

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