WSPC04493

• File Number: 8283.200
• Description: Colorado River Basin-Colorado River Computer Models-Colorado River Decision Support System-Ray
• State: CO
• Water Division: 5
• Date: 8/25/1995
• Title: Consumptive Use Modeling-Task Memorandum-Phase III-Analysis of the Bear Report Blaney-Criddle Crop Coefficients
• Water Supply Pro - Doc Type: Report/Study

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NW8

TAll.E I
GROWING 8IA.ON 'Oil COLOllADO NWS

AVERAGE DATU AVERAGE DATU
'OR ITART or 'OR END or
GROWING SIA.ON GROWING 8EASON

Gunn180n
8toamboat Springe
Moybed
aolta

MoyS
April SO
Ap,1111
April'

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O.t..... .
O.t..... 12

Oot.be, II

LINGTH Of
GROWING SIASON

1S4
lit
17.
201

The growing 8eason at Steamboat Springs and Maybell are .Imirar to the Bear River sites.
Gunnison's growing season Is ahorter and Datta's growing season la much longer.

The possible utilization of the Hill SCS-BC coefflclent8 was also analyzed using estimated Et similar
to thl methodology used to derive the Bear River coefficients. 0811y m8xlmum and minimum
temperatures at Gunnison, Steamboat Springs, Maybell and Oehl for the years 1971 through
1986 were used In the analysis. The analysis of evapotranspiration waa as followS:

1 . Monthly vlluea of potential crop evapotrenlplr8tlon using scs-ac was calculated
using crop coefficients for pasture grass from TR-21. (ETP _SCSBCI

2. Monthly values of potential crop evepotransplratlon u8ing HlII'a Kcm coefficients
and Etr as cllculated using the Iqultlon Etr - . a TMIN + b 0 TMAX + c 0 ElEV
+ d, which is dascribed in REFERENCE EVAPOTRANSPIRATION MAPS FOR
COLORADO. The coefficients a, b, c, and d are listed in Table 3 of the report.
(ETP _ETRI

3. Average monthly ratios of ETP_SCSBC divided by ETP_ETR were calculated.'

4.

The monthly ratio I were IdJusted upward by 8% to account for the use of NWS
temperature clatalnstead of temperature from agricultural altes.

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5. A second adjustment of 7% waa made to account for the use of the Penman
Monteith Instead of the 1972 Kimberly Penmln.

8. Corrected monthly SCS-BC coefficients calculated by dividing TR-21 pasture grasa
coefflcienta by the monthly ratios found In step 5.

7. The corrected SCS-8C coefficient. were compared to HIII'8 SCS-8C coefflclent8.
....'i
Tha basis for ste~ls that the temperature8 at a NWS.re commonly 3 to !5 degrees higher
than temperatures In Irrigated fields. Temperaturea In agricultur81 areaa were used to determine
the relationship describe in step 2~ Analysla of the Steamboat data found th8t if the temperatures
are lower by 3 or 5 decrees, Etr Is reduced by 8 to 11 % respectively. The conservative value of
6% waa used in this analysts. '"\ "'- ~;;c>
Cl(\,\'-'\

The basia for step 4 Is that ASCE PRACTICE MANUAL 70, EVAPOTRANSPlRA TION AND
IRRIGA TION WA TER REQUIREMENT, Tabla 7.13, shows that In arid region8, leasonal estimltea

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