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A Growing Population, and a Falling Volume of Water

California's population has skyrocketed over the past hundred years; unfortunately, water runoff has not.
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For as long as California has been keeping track, its runoff—a measure of drought—has fluctuated widely. The last four years represent the driest such stretch on record, but extremely low points have occurred before: in 1928-31, when California also saw an influx of migrants from the Dust Bowl-affected states, and again in 1987-90.

Meanwhile, the state's population has only grown over the last century. That means that ever more people are directly affected by dry conditions in California, as you can see below:

California's population compared to its water runoff from 1901 to 2014. (Chart: Francie Diep/Pacific Standard)

California's population compared to its water runoff from 1901 to 2014. (Chart: Francie Diep/Pacific Standard)

Of course, population doesn't tell the whole picture for California's water demand. Over the past 20 years, the state's demand for water for urban uses—such as drinking, washing, and landscaping—has stayed steady despite population growth. That's because of Californians' adoption of water-savers such as low-flow showerheads and toilets, and drip irrigation for lawns and gardens. As you might have heard, 80 percent of California's freshwater use goes to agriculture, not home use. That's likely associated with population growth, but less directly.

In addition, the runoff datasets we used aren't a perfect indication of  a drought's severity. Runoff measures how much precipitation isn't absorbed by the land, with the idea that the extra runs into lakes and rivers. But the runoff data we used here come from gauges that take measurements downstream of some aqueducts, so the true runoff volume is likely higher. A more complete measure of drought might also take into account air temperatures in different regions of the state, which affect how much water evaporates.

Still, it's remarkable to see how the state's population has grown, in the face of a low, unreliable natural water supply.