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Comparing droughts

ALBUQUERQUE, N.M. — One of the questions raised by my stories this week on drought (worst two years in history and forecast to continue) is how the current drought compares to past droughts in New Mexico.

The definition of drought is a difficult thing, because people mean different things when they use the word, but the differences are often obscured. So to call this, as I did in Wednesday’s story, the driest and warmest 24 months in recorded history is both useful as a benchmark, but also has the potential to mislead.

Writing a decade ago in the Bulletin of the American Meteorological Society, Kelly Redmond offered up this simple definition of drought (pdf, well worth reading in full if you’re interested in the subject):

Most concepts of drought involve a water balance. This implies that both supply and demand must be considered, as well as the question of whether there is “enough” (and, enough for what?). Thus, through time I have come to favor a simple definition; that is, insufficient water to meet needs. This covers a broad range of situations, from an asteroid to a Pacific Ocean beach, if need be. By intention, it highlights the importance of both the supply and the demand sides of the issue. From this standpoint, a system is “in drought” when supply does not meet demand.

So given that definition, how does the current drought stack up? Here’s the US Bureau of Reclamation’s graph of water storage in Elephant Butte Reservoir over time – one of the places where New Mexico’s water supplies and demand converge:

Elephant Butte water storage, courtesy US Bureau of Reclamation

Elephant Butte water storage, courtesy US Bureau of Reclamation

You can clearly see the current drought, represented by a long steady decline in the Butte beginning in the late 1990s, has nothin’ on the “Drought of the ’50s”, which really stretched from the mid-1940s, which a few breaks, into the 1970s. Here’s another measure, the annual cumulative Palmer Drought Severity Index. Down-pointing lines are droughty, up-pointing lines are wet. Apologies for the quirky graph, you can barely see the current year on the right – it’s minus 4, the worst single-year number since the 1950s. But look at that deeply stacked drought in the ’50s:

New Mexico Palmer Drought Index

New Mexico Palmer Drought Index

By both measures, we’re still a long way off from the trouble of the 1950s.


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