Precipitation in Yosemite Valley, 1894-2013

California's severe, state-wide drought owes a significant chunk of its origins to reduced snowfall in the Sierra Nevadas. Much of the state is semi-arid, and over the last century an extensive series of dams, canals, and pipes was put into place to service the water needs of both agriculture and urban centers. The Carbon Dioxide Information Analysis Center maintains and hosts a great database of temperature and precipitation data from stations across the United States. Many of these stations have records which go back over a hundred years, and it's a phenomenal resource to quickly eyeball trends or confirm/deny a Saturday afternoon's idle theorizing.

First, here's an 18-month rolling average of precipitation from 1894-2013.

yosemite rolling average precip


From glancing at that, you might notice that the right-side tail looks a bit odd, let's zoom in on the records since 1980 and add a linear trendline (auto-calculated by Excel).

yosemite rolling average precip 30 years


This provides a more detailed picture of the reduced recent snowpack, but it's important to remember that this is only a single location and barely scratches the surface of the complete story.

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