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Shasta Trinity National Forest Watershed Vulnerability Assessment, Pacific Southwest Region (R5)<br />

Precipitation<br />

Differences in winter precipitation throughout California appear to have increased from 0.1 to 7.9 inches<br />

with the greatest increases in the north (Figures 3 and 4). California shows great variability in growing<br />

season precipitation, compared to the rest of the nation. Northern California received more precipitation<br />

(0.1 to 2.1 inches) while southern California has received less (0.1 to 1.3 inches).<br />

Figure 4. Winter and growing season changes in precipitation (PRISM Data: 1961-1990 vs.1991-2007)<br />

RECENT CLIMATE TRENDS ON FOREST (Summarized from Butz and Safford 2010)<br />

Mean Annual Temperatures<br />

Most of the Forest has had an increase of about 2 degrees Fahrenheit in mean annual temperature over the<br />

last 75 years, driven primarily by nighttime temperature increases (Figure 5). No changes in temperature<br />

have occurred at the Mt Shasta weather station (northern most portions of the forest in the Southern<br />

Cascade Ecoregion). PRISM data suggest mean annual temperature increases are slightly less at lower<br />

elevations (1ºC, 1.8º F).<br />

189 Assessing the Vulnerability of Watersheds to Climate Change

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