Mike Cohen of the Pacific Institute, in some conversations last week sparked by my post about the risk of what we often call the “first ever” shortage in the Colorado River Basin, points out that shortages in fact are routine in the river’s Upper Basin.

This is a result of hydrology. The Lower Basin – Nevada, Arizona, California, and Mexico – has two big storage reservoirs upstream of its water users that fill in wet years and therefore allow continued water use downstream in dry years. But if you’re in Utah or Colorado, without a big reservoir above you, you’re dependent on the snowpack in the mountains above you and the water flowing down your river. If it’s a dry year, you don’t have water to irrigate.

The U.S. Bureau of Reclamation attempts to calculate the shortages that result – the amount of water that would have been used but is not available simply because it’s a dry year. Here’s Mike’s graph of the results:

Courtesy Michael Cohen, Pacific Institute; data from USBR

2002 is particularly interesting, and was apparently particularly galling for folks in the Upper Basin. It was a horribly dry year, with big shortages to Upper Basin water users. But the big downstream reservoirs were relatively flush, and the Bureau declared a “surplus” on the Lower Colorado, giving California a big slug of extra water.

Typically in a big river system, the people upstream have the advantage, and those downstream are left behind because of upstream use. But on the Colorado, it’s the reverse.