GunnisonComments13

• Basin: Gunnison
• Title: Comments 13
• Date: 6/15/2004
• SWSI - Doc Type: Comments

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May 19,2004

Special report to:

Statewide Water Supply Initiative

Submitted by:

Dan H. Smith
Professor. Dept of Soil and Crop Sciences
.Colorado State University
and
Principal Investigator
Upper Gunnison River Basin Irrigation Water Consumptive Use Studies

The Department of Soil and Crop Sciences at Colorado State University recently
completed studies to determine consumptive water use depletions by inigated meadows in the
upper Gunnison River basin. This research was jointly sponsored by the Colorado State
University Agricultural Experiment Station and the Upper Gunnison River Water Conservancy
District. The field investigations included direct measures of actual evapotranspiration (or
consumptive water use) using compensating Iysimeters under fully inigated conditions. This
work was conducted over a period of five consecutive growing seasons (1999 to 2003). A
description of the location and characteristics of each site used for these studies is presented in
Table 1"

A summary of total consumptive water use and irrigation requirement (obtained by
subtracting effective precipitation). from all sites over the five-year period is presented in Table
2. The seasonal inigation requirement (also referred to as inigation depletions) of 22.68 inches
generaUy would not be realized because of the actual production systems used in the basin and
the lack of water available for inigation under existing water rights decrees. Table 3 provides a
comparison between the season-long irrigation dt?pletions and those that would be expected
under two different management scenarios that are commonly observed in the basin. Both of
these scenarios assume that irrigation begins on May 1 and continues through July 15. when
water is removed to allow for hay harvest. The first alternative. labeled "No late irrigation,"
would be applicable for meadows where no additional irrigation occurs for the remainder of the
season. This situation is observed in instances of either limited water rights decrees or lack of
sufficient water because of inadequate water late-season water supplies, such as what was
observed during the extreme drought conditions of 2002. The second alternative management
scenario, labeled "Late irrigation." provides an estimate of the inigation depletions that would be
expected where irrigation was returned to a meadow after hay harvest. Both of these alternati ve
management scenarios account for irrigation depletions of well over 1.0 acre-ft per acre of
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