BOARD00039 ‎(2)‎

• Board Meeting Date: 9/20/2006
• Description: WSP Section - Consumptive Uses and Losses and High-Altitude Crop Coefficients
• Board Meetings - Doc Type: Memo

Agenda Item 21
September 19-20, 2006 Board Meeting
Page 4 of6

2005 and compares potential consumptive Jse (PCD) calculated with lysimeter-derived crop .
coefficients and with modified Blaney-Criddle cdefficients with an elevation adjustment from TR-21.
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Ifable 1
Average Annual (1950 - 2003) PCU for pJsture Grass using Standardized Coefficients with
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Elevation Adjustment vs. Lysimeter-Derived Crop Coefficients
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Climate Modified Blaney- I Lysimeter-Derived Ratio
Station Criddle w/Elev. Adj. I Crop Coefficients (Mod. B-C/Lysimeter)
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Antero 1.33 feet 1.86 feet 72%
Bailey 1.66 feet 2.34 feet 71%

The above comparison shows that the modified Blaney-Criddle method with standard coefficients, even
with an elevation adjustment, underestimates potential consumptive use of pasture grass by about 30
percent when compared to calculations with coefficients derived from lysimeter data. Consequently,
Leonard Rice Engineers recommended the use of lysimeter-derived crop coefficients with the Blaney-
Criddle method for high-altitude areas in SPDSS, as well as in other parts of the state.

Leonard Rice's recommendation led to a discussion in April 2005 with the State Engineer's Office on the
use of high-altitude pasture grass coefficients for the entire Western Slope, where most of the high-
altitude irrigated acreage is pasture grass. The State Engineer concurred with the use of high-altitude
coefficients, and CWCB has since been incorporating them into their decision support systems. .

Comparison with USBR Consumptive Uses an~ Losses

The USBR calculates consumptive uses and losses for the Colorado River Basin every five years,
including those from irrigated agriculture, rese.rvoir evaporation, stock ponds and livestock, mineral
resources, thermal-electric, M&I, and exports. lITigated agriculture accounts for about 70 percent of the
total.

CWCB has an ongoing dialog with USBR staff regarding the methodologies used in these calculations.
Before the use of high-altitude crop coefficie,nts in the Colorado River Decision Support System
(CRDSS), the USBR's calculations of agricultural CU were generally higher than those from CRDSS.
This changed after the incorporation of the high-altitude coefficients into CRDSS. Figure 1 below shows
the results for the average annual water supply limited crop CU for 1996-2000. The CRDSS calculation
went from being about six percent under the USBR's to about six percent over with the inclusion of high-
altitude coefficients.

Figure 2 illustrates a longer range annual comparison between the USBR calculated crop CU and the
CROSS calculated CU using high-altitude crop coefficients. It should be noted that Figures 1 and 2 show
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calculated agricultural CU as limited by water supply. Also, Figure 2 includes incidental consumptive use
of water associated with irrigation.

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Flood Protection. Water Project PlaI~ing and Finance. Stream and Lake Protection
Water Supply Prote~tion. Conservation Planning
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