You call this a drought? Researchers find evidence of far longer dry spells

Ventura County Star – 9/20/07
By John Krist, Star columnist

To say the past year has been dry in Southern California would be an understatement of almost ridiculous proportions. Totals since last Oct. 1, the date marking the start of the official rainfall year, have averaged around 4 inches across much of the region. That pushes the heavily populated south coast out of the “semi-arid” category and straight on into full-blown “arid.” As in Sonoran Desert arid, Death Valley arid. Heck, even Sahara-Gobi-Kalahari arid.

Low rainfall in Southern California does not, by and large, have much to do with whether local water agencies and public-works departments caution residents to be frugal with their irrigation, showers and laundry.

For more than half a century, it’s been more important to pay attention to snowfall in the Rocky Mountains, and in the northern and central Sierra Nevada. Those are the sources of most of the region’s water, conveyed here by gigantic state and federal projects, and by regional entities such as the Los Angeles Department of Water and Power, and the Metropolitan Water District of Southern California.

The geographic diversity of those major sources has always served as a sort of anti-drought insurance policy. Odds were thought to be pretty long against simultaneous subnormal snowpacks across a region encompassing a third of the continental United States.

But it turns out there is a lot we don’t know about what’s possible when it comes to drought in the West. And for water managers starting to get nervous as the current dry weather continues, there’s plenty more reason for worry waiting in the wings.

Imagine, for example, that this drought continues not for a few years or even a decade but for 60 years. There’s not a water system anywhere in the West that’s equipped to keep the taps flowing during six decades of drought.

It’s not clear that building such a system would even be possible. Where, for example, would you store a 60-year supply for California’s 36 million inhabitants?

But, according to a recent report published in the journal Geophysical Research Letters, the upper Colorado River basin underwent just that sort of drought in the middle of the 12th century.

To reconstruct prehistoric rainfall patterns, scientists gathered core samples from living trees and cross-sections from stumps and fallen logs to develop a climate chronology based on tree-ring width. Using those annual growth patterns to gauge seasonal precipitation, the researchers then calculated Colorado River flows dating back as far as 762 A.D. They compared those to flows recorded between 1906 and 2004.

There’s nothing particularly startling about the researchers’ conclusions. Climate scientists have known for a long time that the past century, the period widely regarded as “normal” from a hydrological standpoint, has been anything but.

The natural variability of precipitation patterns in the West over the past thousand years encompasses several megadroughts unlike any in living memory, more successive years of sparse rainfall and runoff than the designers of dams, reservoirs and aqueducts ever imagined. And scientists are finding increasing evidence that those megadroughts are associated with the sort of higher-than-average temperatures that are predicted to become more common over the next few decades as the buildup of greenhouse gases warms the planet.

Yet, most patterns of settlement in the modern West are predicated on what turns out to have been an anomaly – an oddly wet series of years in the early 20th century, when plans were laid for the water projects that would make the region’s subsequent population boom possible.

Building more dams and enlarging existing reservoirs – the object of a full-court publicity push launched this month by California’s governor, several lawmakers and a troop of industry lobbyists – may help in the short term. But not much, not for long, and probably not at a cost commensurate with the benefits. Taking the truly long view, something more fundamental than new plumbing is needed if westerners are to reach a sustainable equilibrium with the natural forces that have shaped the landscape they inhabit.

The price of failure may also be read in the prehistoric record. Right around the time of the multidecade Colorado Basin drought detected by the tree-ring researchers, an early experiment in urban living and irrigated farming came to an abrupt end in the region. The silent remnants of those cities, and the sand-buried outlines of their long-fallow fields, can still be seen in the region’s canyons and washes. Tourists come from far away to look at them and wonder who built them.

Read the artcle at the Ventura County Star