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

.Water in the Balance.

Precipitation:
Colorado's Direct Source of Water

Of all the elements that make up our climate, precipitation-
how it forms and when and where it falls - is perhaps the
most interesting and significant. Here are a few important
traits of precipitation.

1) Unlike temperature, humidity and wind. which are
continuous variables, predpitation comes in episodes.
Depending on where you are in Colorado. precipitation
falls only two to six percent of the time, on average.

2) Precipitation is highly variable both in time and
space. No two years are ever alike. Within the state
boundaries. it is extremely rare to have all parts of the
state experiencing above or below average precipita-
tion at the same time, even for a month. There is
always diversity, with some areas being wet while
others are dry. At any given point, annual precipita-
tion totals can vary from only about half of the long
term average in a very dry year to close to double the
average in a very wet year. In relative terms, the
variability is even greater for seasons or single
months.

3) Rain versus snow - the fraction of annual precipitation
that falls as snow varies greatly across Colorado from
less than 15% over southeast Colorado to more than
70% at high elevations in Colorado's northern
mountains. This has a major bearing on Ollr water
balance and helps determine how much moisture
evaporates, soaks into the ground, or runs off to
become stream flow and reservoir supplies.

4) A few big storms contribute the majority of the
precipitation that falls each year. More than half of
the total annual precipitation falls in only 20% of the
days when precipitation occurs. The other half of
annual precipitation comes during the remaining 80%
of days when it rains. snows, or hails. What this
means is that the difference between a very wet and a
very dry year may come down to the presence or
absence of just a few major storm systems.

5) Precipitation is highly seasonal. Most areas have
well defined times of year that are distinctly wetter
than others. This is not unusual; it is typical for many
parts of the world to have pronounced and repeatable
wet and dry seasons. For example, throughout all of
California most precipitation falls during the winter
with very little precipitation falling during the summer.
What is unique about Colorado, however, is the
amazing diversity of seasonality. Some parts of the
state experience their annual wet season at the
identical time that others are in their dry season.

Any of the four seasons will be "the wettest time of
year" in some part of Colorado. Just the sbort
distance from a mountain peak to a nearby valley
bottom can result in wet and dry seasons being out of
phase. For example. Denver's wettest period is in the
spring, but just 30 miles to the west, Berthoud Pass
has its maximum precipitation in mid-winter. This
complexity is very important for understanding
drought development in Colorado.

6) Long range prediction (weeks, months, or seasons in
advance) is very difficult. While considerable
progress is being made in long-term climate prediction,
it remains extremely difficult to accurately predict when
and where precipitation will fall with confidence and
reliability.

To help understand drought history and potential for
future drought, a brief discussion of precipitation mecha-
nisms and moisture sources is appropriate.

There are two essential ingredients for
precipitation anywbere in the world:

Water vapor in the atmosphere

Upward vertical motion, which cools the
air so that the water vapor condenses.

The complexities of precipitation patterns and seasonality
in Colorado are largely a result of our interior continental
location. far removed from primary oceanic moisture
sources. in combination with Colorado's high elevation and
rugged terrain. Three quarters of all land in the continental
U.S. at elevations exceeding 10,000 feet is found here in
Colorado. The Continental Divide that follows the crest of
the Rockies through Colorado acts as an effective barrier to
moisture transport, most of which occurs in the lower few
thousand feet of the atmosphere.

In Colorado, the primary sources of water vapor in the
atmosphere are:

the Pacific Ocean

Gulf of Mexico and recirculated Gulfmoisture from
evapotranspiration off of the land and vegetation
throughout the Mississippi Valley

Gulf of California and Gulf of Mexico moisture that
reaches Colorado after first crossing Mexico

Mechanisms for producing rising air (upward vertical
motion) in Colorado include convection (warm air rising),
orographic (mountain-induced) lifting and large-scale storm
systems that produce colliding air masses (storm-induced).

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