Climate Change Projections

Climate change and increased climate variability may have a profound impact on the availability of water resources in the Sacramento Basin and could seriously affect the

Fig. 11.4. GCM grid overlay over the Sacramento watershed. The black dots represent weather stations.

multiple uses of water of the Sacramento, including domestic, industrial, agricultural and ecosystems. The importance of understanding the trade-offs and interactions among competing water uses will only increase with the added impact of climate change. Relevant to the Sacramento Basin, GCM projections estimate that: (i) average temperatures could increase by as much as 5°C; and (ii) mean annual precipitation may decrease over the period 1990-2100. However, at their current resolution, GCMs drastically smooth out most of California's complex topography. For example, current GCMs do not contain important terrain features such as the Coastal Range, the Central Valley and the Sierra Nevada Range. Comparisons of climate change patterns for California that emerged from an analysis of 21 GCMs showed that all models estimated warmer temperatures for the state under assumptions of greater radiative forcing from increased greenhouse gas emissions (Gutowski et al., 2000). For this project we evaluate impacts and adaptation strategies for the Hadley A2 andB2 scenarios. Figure 11.4 shows the 0.5° X 0.5° grid overlay for both climate scenarios over the Sacramento watershed.

Fig. 11.5. Upper and Lower Sacramento monthly precipitation and temperature (with 2 Xstandard deviation error bars) for the historical record (1961-1990) and future projections based on the HADCM2 GCM for 2070-2099.

The effects of climate change will vary across the Sacramento Basin. As an illustration, Fig. 11.5 shows the change in the monthly precipitation and temperatures for the periods 1961-1990 to 2070-2099 for two representative sub-basins, one in the Lower and one in the Upper Sacramento for the Hadley A2 GCM climate scenario. Two standard deviation error bars (standard deviation of monthly time series) are also shown and represent the monthly variation over the given time period (IPCC, 2001).

In both sub-basins, temperatures are expected to increase by about 5°C. On average, precipitation is expected to decrease, primarily during the winter months, contrary to the outcomes of the Canadian GCM but consistent with the Japanese GCM. The magnitude of this reduction is expected to be significantly larger for the Upper sub-basin than for the Lower Sacramento sub-basin. For the Lower Sacramento, the average total annual precipitation is 494 mm, 438 mm and 411 mm over the 1961-1990, 2010-2039 and 2070-2099 periods, respectively. For the Upper Sacramento, the average total annual precipitation is 1225 mm, 1062 mm and 1035 mm over these same periods, respectively.

Furthermore, examining the unadjusted precipitation time series, the coefficient of variation (CV) (ratio of standard deviation to mean) increases over the two time periods suggesting increased variability. The observed CV increases from 1.08 to 1.26 from the observed historical period to 2070-2099 for the Upper Sacramento. Similarly for the Lower Sacramento, the CV increases from 1.37 to 1.44. Lastly, analysis reveals that the persistence of anomalous climate events will also increase, the magnitude of which is larger for anomalous temperature events than precipitation events.

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