PUB00013

• Year: 1985
• Title: Estimated Use of Water in Colorado 1985
• CWCB Section: Water Conservation & Drought Planning
• Author: David W. Litke and Cynthia L. Appel
• Description: Water-Resources Investigations Report 88-4101
• Publications - Doc Type: Brochure

Irrigation water use was calculated based on irrigated acreage for
alfalfa, dry beans, corn-grain, corn-sileage, orchards, potatoes, sorghum,
spring grain (barley, oats and spring wheat), sugar beets, and winter wheat as
reported for each county by the Colorado Agricultural Statistics Service
(Colorado Agricultural Statistics Service, 1985, 1986). Pastureland and
vegetable acreages are not included in these estimates; county data for these
were taken from the 1982 Census of Agriculture (U.S. Bureau of the Census,
1984). County ratios for irrigation-system type (sprinkler or flood) and for
water-source type (ground water or surface water) also were obtained from
Census publications (U.S. Bureau of the Census, 1976, 1984). The county crop
acreages were subdivided into county cataloging unit crop acreages. This was
done by using county land-use ratios developed from county land use maps
published by the U.S. Soil Conservation Service. The net irrigation require-
ment for each crop within each county cataloging unit was then calculated
using the modified Blaney-Criddle method (U.S. Soil Conservation Service,
1970). A computer program (XCONS) for performing the modified Blaney-Criddle
calculation was obtained from the U.S. Bureau of Reclamation. Inputs to this
program include average monthly temperature, total monthly precipitation,
frost dates, crop growth-stage water-requirement curves, and crop growing
season parameters. Modifications for water-short lands, carry-over winter
moisture, and elevation of crops also were made. The output from this program
(net irrigation requirement) agreed well with similar estimates calculated for
the various crop types and climatic zones in Colorado that are published in
the Colorado Irrigation Guide (U.S. Soil Conservation Service, no date). From
net irrigation requirement, conveyance losses and withdrawals were calculated
using estimated conveyance and farm efficiencies. Efficiency estimates for
major basins in Colorado were obtained from previous studies (U.S. Soil
Conservation Service, 1976, p. 14-19 of Appendix; Whittlesey, 1977, p. 8).
More specific efficiency estimates for the Arkansas River basin (Colorado
Water Conservation Board and U.S. Soil Conservation Service, no date, table B4
of Appendi~) and South Platte River basin (U.S. Army Corps of Engineers, 1977,
p. 2-17 to 2-20) also were examined. Surface-water withdrawal data for
irrigation for the hydrologic subregions were obtained from the State
Engineer's Office, and conveyance and farm-efficiency estimates were adjusted
until the calculated withdrawal estimates agreed with the data from the State
Engineer's Office. No withdrawal information was available for ground water
for 1985; therefore, assumed system efficiencies could not be adjusted.

Four independent estimates of irrigation consumptive use for basins of
Colorado (~hree sets of estimates pcesented in Whittlesey, 1977, p. 13; one
set of estimates from Colorado Depactment of Natural Resources, 1981) compare
fairly well with each other and with estimates derived by this study.
Estimates of total irrigation consumptive use for the State cange from 4,125
to 4,939 thousand acre-feet, while ~his study estimated 5,120 thousand
acre-feet. Agreement was poorest for the Rio Grande Headwaters subregion
(range of 597 to 828 thousand acre-feet). This poor agreement may be due to
differences in defining the boundary of this area, which may include the
headwaters of the Rio Grande, the closed-basin part of Saguache County, or the
San Luis valley structural basin.

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