WSPC00025

• File Number: 8283.200
• Description: Colorado River Basin-Colorado River Computer Models-Colorado River Decision Support System-Ray
• State: CO
• Water Division: 5
• Date: 4/1/1994
• Title: Consumptive Use Modeling-Recommendations for Appropriate Changes to the USBR Methods Manual Used in Preparing the Consumptive Uses and Losses Report-1985-1990
• Water Supply Pro - Doc Type: Report/Study

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of meadow grass using Iysimeters. Hill et aI., 1989, and Pochop and Burman calibrated
the SCS-BC using Iysimeter measurements. Kruse and Walter derived monthly coefficients
for the Blaney-Criddle method (BC) using Iysimeter measured ET. The difference between
SCS-BC and BC is that a "Kt" coefficient is not included in BC. Therefore, SCS-BC's
"Kc*Kt" is equivalent to BC's "K". A listing of SCS-BC pasture grass monthly coefficients
and SCS-BC and BC monthly coefficients derived from the referenced Iysimeter studies is
provided in Table 3. The differences in the monthly value is readily apparent. Additionally,
it can be seen in Table 3 that when Hill et al., 1989, and Pochop and Burman calibrated
SCS-BC, they revised Kt.

Since pasture grass comprises 32.9% of the irrigated area in the Colorado River
basin, an analysis comparing SCS-BC TR-21 pasture grass monthly coefficients to
calibrated monthly coefficients for meadow grass derived by Hill et al., 1989, Pochop and
Burman, Kruse and Walter et al. was performed. At the Colorado River basin weather
station used in the June 1991 report, monthly values of Kc*Kt were calculated for each
set of coefficients listed in Table 3. These values were then compared to calculated
monthly values of SCS-BC Kc*Kt for pasture grass. The analysis, which is included in the
appendix, is summarized in Table 4. Based upon the ratios shown in Table 4, SCS-BC
pasture grass monthly coefficients for May through July, estimates of irrigated meadows
ET are 45% to 81 % of the actual ET.

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Comparisons of .August through October coefficients should be considered in two
groups. The first group is the studies of Pochop and Burman, Kruse and Haise and Walter
et al. The Iysimeters used to measure ET in these studies received a full water supply from
August through October. From Table 4, ET estimates using SCS-BC are 58% to 92% of
the estimated ET based upon Iysimeter studies. Hill et al., 1989, Iysimeters comprise the
second group of coefficients. Hill's Iysimeters were not irrigated after about July 15.
Thus, as soil moisture became limiting, the Iysimeters used less water. Thus, the
comparisons in Table 4 indicate that in meadows which are not irrigated in August through
October, estimates of ET using SCS-BC are 121 % to 321 % of the estimated ET based
upon Iysimeter studies.

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