WSPC01228

• File Number: 8281.300
• Description: Colorado River Basin-Colorado River Studies and Investigations-Consumptive Uses and Losses Reports
• State: CO
• Water Division: 5
• Date: 1/1/1984
• Title: Blaney Criddle Estimates-Elevation-A Bias Error in SCS Blaney Criddle ET Estimates
• Water Supply Pro - Doc Type: Report/Study

., -
'''I ~"'.' . ~~. :, ;-, '1-' .~. . . . ,- - . ,. ~ "t-. .

. f
l

IJU2823
TABLE 3. COMPARISONS OF ESTIMATED (U) VS MEASURED (ET) CONSUMPTIVE USE. MM, FOR KENTUCKY BLUEGRASS.

Laramie. Wr WheaUand. WY Fort Comns, CO Northglenn, CO Santa Ana, CA
Month u ET u ET u ET u ET u ET
Ap> 69 68 88 100 106 10.
May 101 11' 126 12' 136 103
'un 137 166 161 146 13' 172 160 156 16' 131
July 157 163 181 168 178 152 . 18. 190 168 169
Au, 128 13. 146 12' 1.6 "6 161 153 154 145
s,p 91 93 106 113 111 109 117 136 123 108
0<1 56 59 68 70 66 60 97 77
Season 739 806 873 856 569 679 678 715 937 837
8% Underestimate 2% Overestimate 2% Underestimate 5% Underestimate 12% Overestimate
. Estimated values eidculated ustng'kt of equation (e).

TABLE 4. STATISTICS COMPARING DIFFERENCES. MM, BETWEEN MONTHLY MEASURED ET AND CALCULATED ET FOR
KENTUCKY BLUEGRASS USING (1) THE ORIGINAL SCS-BC USING ke VALUES FOR PASTURE GRASS; (2) THE SCS.BC
METHOD WITH CALIBRATED ke AND kt VALUES AND (3) AS IN 2 ABOVE BUT WITH AN ELEVATION ADJUSTMENT OF
9.1% PER 1000 M ELEVATION INCLUDED (FOR THE FIVE LOCATIONS LARAMIE. WHEATLAND.
FORT COLLINS. NORTHGLENN. AND SANTA ANA).

Meuul't!d V1 SCS-BC
Measured vs SCS-BC with caUbrated
Measured vs original with calibrated kt and kc
SCS.BC kt and kc and elev adj
Mean St. dv. Mean St. dv. Mean St. dv.
Month diff oCdiff diU oCdifC dlff of dlff
Ap, 36.3 13.6 9.0 6.2 13.3 2,6
M.y 33.3 29.9 15.3 15.6 7.3 6.8
'un 30.2 28.0 19.6 14.8 14.4 13.1
'u1 19.8 20.3 14.4 11.4 16.4 9.8
j Au, 14.2 12.2 7_8 8.7 7.8 9.3
s,p 22.0 11.2 9.0 7.7 6.' 6.3
0" 27.3 15.2 9.8 8.7 6.8 6.8

I

Crop growth stage coefficients (k,) were then
recalculated to account for the loop effect between
monthly temperatures and monthly consumptive use
coefficients as discussed by the SCS (1.967). Crop growth
stage coefficients - averaged for the five sites - are
given in Table 2. The seasonal variation of these
coefficients form the typical bell-shaped curves depicted
by the SCS.
The calibrated crop growth stage. coefficients of
Table 2 were used to estimate consumptive use of
Kentucky bluegrass at the five sites for which the original
measurements were obtained. The estimated
consumptive use values (u) are compared with the actual
measured values (ET) in Table 3. Assuming a perfect fit
of the calibrated data, the estimated consumptive use
values should perfectly match the measured values.
However, at the higher elevation site (Laramie) an
underestimation occurs while at the lowest elevation site
(Santa Ana) an overestimation occurs. At the
intermediate elevation sites (Wheatland, Fort Collins
and Northglenn) the estimated and measured
consumptive use values compare rather closely. Basically
lhe results support the premise that SCS Blaney Criddle
estimates of reference crop ET values should be adjusted
upward with increasing elevation above sea level.
However, in this case the adjustment is not with respect
to sea level. An upward or downward adjustment should
be made depending on whether the location at which
estimates are being calculated has an elevation which is
greater or lower, respectively. than the location at which
lhe formula is. calibrated. For example, the average
coefficients of Table 2 apply without elevation
adjustment at an elevation of 1350 m. For Kentucky
bluegrass. for the season (April through October) our

1

I
1

1984- TRANSACTtONS of the ASAE

estimates give a 9.4% adjustment per 1000 m elevation
and for the summer months (June through August) this
adjustment is 7.6%. Thus if seasonal ET estimates are
being made at an location having an elevation of 2350 m
and using the coefficients of Table 2, then an upward
adjustment of 9.4% should be applied. Likewise,
estimates at 350 m elevation should be adjusted
downward 9.4 0/., etc. f
The authors strongly urge local calibration when using
the SCS Blaney Criddle. As shown in Table 4,
calibration of the SCS.Blaney Criddle for Kentucky
bluegrass improved the ET estimates for the five
locations considered herein. Application of the elevation
adj,!stments for estimating consumptive use of lawn
grass in Wyoming will use the SCS Blaney Criddle
method calibrated at Laramie and Whealland.
Estimates at other locations will use the calibrated values
for either Laramie or Wheatland and elevation
adjustments based on the differences between the
elevation of Laramie or WheatJand and the elevation of
the site in question.

Alfalfa
Alfalfa consumptive use data were obtained from the
literature for the nine sites listed in Table 5. These sites
were selected based on availability of data to give a
comparision and/or confirmation of the elevation
adjustments which were determined for bluegrass. The
main criteria was to have sites with a wide elevation
range. Unlike the bluegrass data, the alfalfa data for the
different sites were not collected during the same years or
even using the same procedures.
The procedure used for bluegrass was applied to the
alfalfa data. The computed temperature coefficient was

127