BOARD02080

• Board Meeting Date: 7/25/2006
• Description: WSP Section - Consumptive Uses and Losses and High-Altitude Crop Coefficients
• Board Meetings - Doc Type: Memo

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Agenda Item 10
July 24-26, 2006 Board Meeting
Page 2 of4

For the South Platte Decision Support System (SPDSS), Leonard Rice Engineers (LRE) investigated
using locally calibrated Blaney-Criddle crop coefficients, as derived from lysimeters, for calculating
consumptive use. The following table is adapted from their SPDSS Task 59.1 memo from February
2005 and compares potential consumptive use (PCU) calculated with Iysimeter-derived crop
coefficients and with modified Blaney-Criddle coefficients with an elevation adjustment from TR-21.

Table t
Average Annual (1950 - 2003) PCU for Pasture Grass using Standardized Coefficients with
Elevation Adjustment vs. Lysimeter-Derived Crop Coefficients

Climate Modified Blaney- Lysimeter-Derived Ratio
Station Criddle w/Elev. Adj. Crop Coefficients (Mod. B-C/Lysimeter)
Antero 1.3 3 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 Iysimeter data. Consequently,
Leonard Rice Engineers recommended t,1-je use of Iysimeter-derived crop coefficients with the Blaney-
Criddle method for high-altitude areas in SPDSS, as well as in other parts of the state.

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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 and Losses

The USBR calculates consumptive uses and losses for the Colorado River Basin every five years,
including those from irrigated agriculture, reservoir evaporation, stock ponds and livestock, mineral
resources, thermal-electric, M&I, and exports. Irrigated 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 coefficients in the Colorado River Decision Support System
(CROSS), the USBR's calculations of agricultural CU were generally higher than those from CROSS.
This changed after the incorporation of the high-altitude coefficients into CRDSS. Figure I below shows
the results for the average annual water supply limited crop CU for 1996-2000. The CROSS 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 I and 2 show

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