Where does our water come from?

It originates as rain and snow. Some falls in Oregon and drains into the Klamath River, and some falls in the vast drainage of the Colorado River. But most of it lands in California — about 200 million acre-feet on average per year. (An acre-foot is 326,000 gallons, what an average household consumes in between six months and two years.) 

To understand what that volume means, imagine a skyscraper 38,000 miles tall. Yes, miles tall, not feet. Now, fill it with water. That is California’s average annual precipitation. 

So, where does all this water go? More than half evaporates, leaving about 75 million acre-feet either frozen in mountains or filling rivers, lakes and reservoirs. Some sinks into the ground. 

About half of the 75 million is diverted to human uses, and half is left in watersheds, what is referred to as environmental “use” of water.  

Map of California showing the 4 coastal hydrologic regions (HRs), study sites and other locations referenced in the text, and average annual precipitation (1981–2010) for the State (https://earthworks.stanford.edu/catalog/stanford-td754wr4701). San Francisco Bay is outlined in blue within its HR.

Source: Daniel J. Hoover, et al. Map of California showing the average annual rainfall from 1981–2010. Used with permission.