Colorado Water Dec 2003

• Year: 2003
• Title: Colorado Water
• CWCB Section: Administration
• Author: Water Center of Colorado State University
• Description: December 2003 Issue
• Publications - Doc Type: Newsletter

COIO AN ALTERNATIVE PERSPECTIVE
„ "�ersity ON WATER USE IN COLORADO
C Experaave
Eitension
by Reagan Waskom
CWRRI Water Resource Specialist
Estimating consumptive use is a science that has been
inexactly practiced in Colorado for over a hundred
years. Precise knowledge of historic or season -long
consumptive use is usually unnecessary in times of plenty
However, during drought or in time of conflict over water
rights, in arguments about augmentation, or in court cases
with downstream states, this information becomes invalu-
able.
Farmers have estimated crop water needs by experience,
crop appearance, and soil moisture for as long as we've
been irrigating in Colorado. And in most day -to -day
operations, this approach is sufficient for surface irrigators
where adequate water is available. Daily estimates of crop
evapotranspiration (ET) that are in error by 10 — 20 percent
may represent only several hundredths of an inch and have
no practical significance to one farmer irrigating a field.
However, a 20 percent difference in the context of a basin -
wide court settlement multiplied over a number of years
and thousands of acres represents a significant quantity of
water.
Measuring water diverted from streams or pumped from
groundwater is relatively easy with today's technology.
Knowing how much water is evaporated from the soil
surface or transpired through growing plants requires a
more sophisticated science. Scientists at Colorado State
University, the U.S. Department of Agriculture and other
agencies have been working to devise accurate methods
and equations for determining crop ET or consumptive use
since the late 1800's. Today, most scientists agree that crop
ET can be estimated using the appropriate crop coefficient
and weather data as parameters in the appropriate equation.
Unfortunately, there are still disagreements as to the best
crop coefficients and equations for use in Colorado and
surrounding states.
Published water use information for the state of Colorado
is often confusing because it may or may not include both
surface and ground water components. Additionally, water
use is typically reported in two different ways. Diversion
(or withdrawal) is the removal of water from any body of
water by canal, pipe or other conduit. Consumptive use is a
diversion that results in a reduction of return flow. Non -
consumptive use is a diversion that eventually returns most
of the water to the stream system.
Agricultural water use is typically reported to account
for about 85 percent of all consumptive use in Colorado.
Agriculture uses the majority of the water delivered within
Colorado largely because the amount of land area devoted
to irrigated crops is larger than any other activity requir-
ing water. Additionally, growing plants use a lot of water.
Since agricultural consumption makes up such a large frac-
tion of the total, scientists and engineers are called upon by
society to accurately measure or calculate how much water
is used by each crop in every basin. Although obtaining
such information is critical to water management, improv-
ing ET estimates is hardly the type of cutting edge research
likely to be funded by federal agencies.
TABLE 1. Estimated average annual consumptive use in
in Colorado.
Water Consumed
Irrigation
5,505,272
Lovestock
50,082
Mining
25,326
Thermoelectric Power
47,203
Industrial
47,203
Municipal
172,782
Commercial
17,709
Reservoir Evaporation
707,330
Total 6,571,798
Source: U.S. Geological Survey Circular 1200, Estimated Use of
Water in the United States in 1995.
An alternative view of the water balance in our state is that
the natural environment, including the 22 million acres of
forest and over 30 million acres of rangeland, consumes the
vast majority of the water that falls on Colorado. Cropland,
by comparison, occupies 10.5 million acres of land in Colo-
rado, including 7 million acres of dryland crops plus the 3
million irrigated acres that utilize 5.5 million AF of irriga-
tion water annually to grow food.
Colorado receives on average approximately 95.5 million
acre feet of precipitation annually that falls in a spatially
and temporally non - uniform distribution on the State's 66
million acres of land (Grigg, 2003). Yet, most water ex-
perts tend to talk only of the 15.6 MAF of water that flows
in our streams and the 2 MAF of ground water pumped
annually, as the sum total of water used in Colorado. This
alternative view of the State's total water balance estimates