Water Arithmetic

Much of California has a semi-arid climate, and fresh water is a valuable resource. Sadly, California and much of the US Southwest experiences drought, sometimes for years at a time. The past year has been one of the driest on record, so we have been praying for a wetter than normal winter.

This evening, as I write this blog post, it has been raining in Livermore since midday, and is supposed to continue raining until tomorrow morning. We have already received over 1.5 inches of rain today, and hope to have at least that much more before the storm moves on. Maybe that does not sound like much to my friends in other parts of the country, but bear in mind that Livermore averages only about 15 inches of rain per year. To get a tenth of our yearly total in the first storm of the season is a big deal, and we thank God!

While we enjoy God’s blessing of rain, let’s do some simple arithmetic. Please follow along, and tell me if I make a mistake. Here goes:

California is a big state, about 163,695 square miles in size. Only Alaska and Texas are larger. By the time it is over, this storm will cover at least a third of the state, which amounts to 54,565 square miles. This particular storm is dropping several inches of rain over the entire area, but let’s assume that one inch of rain is available for extra (unneeded) runoff.

How many cubic feet of water would one inch of rain create? Let’s multiply 54,656 square miles by 5280 ft/mile, multiply again by 5280 ft/mile, and then multiply by 1/12 of a foot (our one inch of rain). If my calculation is correct, that amounts to 126,765,408,000 cubic feet of water. A really big number. But how many acre-feet is that?

An acre-foot is an acre covered with water one foot deep, and it is a common measurement for the amount of water stored in a reservoir. To get to acre-feet, let’s divide our really big number above by 43,560 square feet, which is the size of one acre. That gives us about 2,910,133 acre-feet. In other words, one inch of rain creating excess runoff from one-third of California amounts to about 2.9 million acre-feet of water. By comparison, Lake Oroville, one of the largest reservoirs in Northern California, has a total capacity of about 3.5 million acre-feet. So the runoff created by one inch of rain could go a long way in filling a reservoir for future use. (Remember to check my math!)

By the way, knowing that we are subject to periodic dry years, over the past two decades California voters have passed at least two large bond measures to improve state water management to relieve the effects of drought. How much of the runoff from today’s storm was saved in new reservoirs built with money from those bond measures? None. Zero, Zilch. Nada. That’s because the state has built no new reservoirs with the billions of dollars raised by the bond measures. What did the state do with the money? Good question.