PUB00106

• Year: 2000
• Title: A History of Drought in Colorado: Lessons Learned and What Lies Ahead
• CWCB Section: Water Conservation & Drought Planning
• Author: Colorado State University
• Description: Lessons learned and what lies ahead
• Publications - Doc Type: Tech Report

~olorado Water Resources Research ln~titute.

mountains, and other areas with more reliable precipi-
tation, accumulated deficits only reach about 1.2 years
of average precipitation. For example. if Breckenridge
averages 22 inches of precipitation per year, a very
severe multi.year drought might result in an accumu-
lated deficit of more than 22 inches over a few years
time before above average precipitation reduces those
deficits.

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7) Timing of drought - When does it start? When does
it end?

Short duration droughts can begin and end in any
season.

Major droughts with durations of one year or longer
tend to begin in and end in the season that is locally
the wet season. This varies from place to place in the
state. For example:

Major droughts in the mountains tend to begin or end
during the winter or spring months. In extreme
southwestern Colorado, significant droughts have
both begun and ended in the fall. The more dramatic
the seasonal precipitation cycle is, the more difficult
it is to end a major drought during the time of year
that is the climatological dry season for that area.
For example. over eastern Colorado, rhe months of
December, January and February are typically dry.
The three months combined produce, on average, less
than 10% of the annual precipitation. Rarely does
enough precipitation fall during this time of year to
significantly alleviate longer-term moisture deficits.
However, a week or two a/very wet weather during
late spring east of the mountains can bring a major
drought to an end. For example, the spring of 1995
and the last few days of April 1999 delivered enough
precipitation to compensate for large deficits.

There are several extUhples of droughts ending abruptly
with the appearance of widespread excess precipitation.
For example, the severe drought of the I 950s was
followed by the wettest year in Colorado's history in
1957. There are other examples of droughts that have
ended more subtly, however.

8) Does a dry winter foretell a wet summer?

There is endless folklore concerning drought. Even before
"El Nino" found its way into climatological jargon, people
have talked ways to predict drought. Does the climate of
one season foretell the next? Much folklore would suggest
that. Our analyses. howevet, did not bear that out. We
looked at a number of combinations. What happens after a
very dry winter in the mountains? What happens after a
very dry autumn at lower elevations (such as fall 1999 in
Colorado)? Our analyses showed that sometimes dry
winters in the mountains were followed by wet summers (like
1999), but other years they weren't. Sometimes dry springs
along the Front Range were folJawed by hot dry summers
(like 1954), but other years they weren't. Further analysis
wO'uld be required to determine how and if precipitation
during one seasO'n helps foretell the next here in ColoradO'.
Less than 10% of the variance in summer precipitation is
explained anywhere in Colorado by the variations in winter
precipitation.

9) Are there drought cycles?

People don't ask climatologists whether there are drought
cycles in CO'lorado; most people are positive that there are.
Some say there is a 3 year cycle, while others claim 7. The
sunspot cycle of II years has caught some people's
attention, while many strongly believe that a 22 year
drought cycle (double sunspot cycle) controls Colorado's
drought patterns.

We examined our rainfall records in ColoradO' in search of
drought cycles. There is some evidence af a twO' to' three
year cycle over portions of southern and eastern Colorado.
The dry periods in the I 890s. 1930s, 1950s and again in the
1970s have convinced some observers that the double
sunspot cycle really does affect drought patterns in
Colorado. That theory doesn't explain why the 19105 were
so wet, why parts of the 1960s were very dry, and why we
have been wet for the better part of 18 years in a row, but
many still believe it. As far a seven or eleven-year cycle,
there isn't much supparting evidence for that. It is true that
dry periods are foIlowed by wet, and wet followed by dry?
That makes a cycle. doesn't it? The problem is that those
cycles just aren't very reli.ble. As such, they don't help us
much if at all in predicting what will happen next year or the
year after that. Even throwing in the irregular cycle of the El
Nino Southern Oscillation, we are stin left with a great deal
of unexplained variability in our precipitation.

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