BOARD01080

• Board Meeting Date: 1/27/2004
• Description: WSP Section - Native Species Trust Fund, Recommendations for FY 2004-05
• Board Meetings - Doc Type: Memo

4Water in the Balance 4

A nal)'sis of Historical Dry and Wet Periods in Colorado

Now that we have an underslanding of how we define and
assess droughls. how pr~ipitation falls in different pans
of Colorado. and how Coloradans us..: that precipitalion to
supply Water for different sons of demands. we can look al
the historical dry and wet periods in Colorado with a better
understanding of the significance of tho~ periods. and
how similar dry or wet periods would impact Colorado
today.

The historical record of precipitation. snowpack. and
streamflow data were analyzed for each river basin in
Colorado. The periods of above or t'lelow average precipi-
tation were idenlified both for individual basins and for a
majority of the slale as a whole. The table on the following
page shows the periods during which at least 60% of
Colorado was wet or dry. as detennined hy the Standard-
ized Precipitation Index (SPI) \ialues for ::!4-monlh periods,

The analysis perfonned in this study revealed several
important facts about dry and wet periods in Colorado:

1) Droughl is a vcry frequent visilor to Colorado

Single season droughts with precipilation of 75% or
less of average for one 10 three monlhs in a row occur
nearly e\'cT)' year in Colorado.

Based on the Standardized Precipitation Index. J.
month droughts with an index value of -lor lower
(equivalent to a moderale precipitation deficil with a
probability of occurrence of no more than 16% for any
conseculive 3-month period). occur approximalely 90
in 100 years al any gi\-en location (see graph below).

93'k of time allc3!>t 5% of Slate (based on percenl of
long tenn weather slations) is experiencing drought at
either a 3-. 6-. 12. or 24-month lime scale.

Whalthis means is that Colorado is almost always in
drought or near drought somewhere in the stale.

2) Drought rarely encompasses the entire state.

Only about five percent of the time (or approltimalely
one year in twenty) does moderate or greater drought
encompass at least half of the state at lhe same lime.

During recordcd hislory. moderate drought (as defined
by a standardized precipitation index value of -lor
lower) has never covered the entire stale al the ~me
rirn<.

Shon-teno droughts (3-month duration) have covered
as much a~ 80 percent of the state. Longer--duration
droughls (2-~ years) have reached to about 70 percent
of the stale,

What this means is that precipitation shortages rarely
occur in the entire Slate al the same time, Economic and
social impacts tied 10 a more "local" drought. however. can
in (Urn affect the economy of whole state.

31 The most common droughLS are short duration (6
months or less). They may be qui Ie localized (espe-
cially during the growing season) or more widespread
(especially during the fall. wimer. and early spring).
Depending on where and when they occur. they may
have little or no impact on our li\'Cs. More widespread
droughts do not necessarily have a tendency to
become more long lasting.

I

~) Multi-year droughts occur infrequently.

Precipitation time series for wealher stations across
Colorado show thaI periods of two or more consecu-
tive years with much below a\'Crage precipilation (less
than 80~ of average) have occurred a few times during
the 20'-" Century at most Colorado stations. Bdow
average precipitation laM.ing threc years in a row.
however. IS quile rare. ~Iany locations have nevcr had
thn:-e consecuti\'c vcry dT)' years. particularly over the
(continued 011 page 161

~

~

Fraction of Colorado in Drought
3-month SPI values

,
'" 0.8
15 0.6
c
.2 0.4
;;
. 0.2
-
..
0
0 ~ 0 g -
'" '" g -
00 00 '"

<0
'"

'"
'"
'"

~
'"
'"

'"
M
'"

<0
M
'"

M
~
'"

00
~
'"

~
~
'"

'"
~
'"

o
~
'"

~
~
'"

o
<0
'"

<0
00
'"

~
<0
'"

'"
'"

<0
'"
'"

Year

l~