WSP09655

• File Number: 8276.400
• Description: McElmo Creek Unit - Colorado River Salinity Control Program
• State: CO
• Basin: Colorado Mainstem
• Water Division: 7
• Date: 6/1/1983
• Title: Evaluation Report on Onfarm Irrigation Improvements - McElmo Creek Unit Salinity Control Study
• Water Supply Pro - Doc Type: Report/Study

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WATER BUDGET

General

A sound ."present" , or "pre-project", water budget is the basis for future

~t load reduction calculation!!. However, the hydrogeology of this \D1it is so

coaplex that ",sound" is probably beyond achievement. The challenge is to

consider all of the appropriate factors and then consider the sensitivity of

the :IIOre important ones.

Much time has been spent trying to \D1derstand the water budget in the

McElIllO unit and in comparing data prepared by USBR and SCS. The difference

was first noticed in the anmal values for farm delivery and tailwater runoff sho~

by each agency. For farla delivery, the values are 77,040 acre feet (USBR 1I> and

105,000 acre feet (SCS g,1l. For tailwater runoff, these values were 9,660 and

~. _ 45,000, respectively.

~tP \" One of the problems in this comparison is that study area bO\D1daries are not
~\..j) L the same. The irrigated acreage in the SCS study area is 29 ,100 acres and that
for USBR is 2:1,8000 acres. The SCS study area includes a 1300 acre area south of
the Aztec Divide that drains into IIavajo \\lash, then to the Mancos River, then to
the San Juan River, and then to the Colorado River. Since the drainage from
this 1300 acre area is higbly saline, it is well that SCS included it. Possibly

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USBR should also have included the canals that service this area; they are

extensions of those included in their study.

USBR and SCS apparently approached their water budgets from differeDt directions.

This, no doubt, stems from their different primary interests; USBR - canal lining

and SCS - an onfarm program. USBR started with diversions from the Delores River

by the Montezuma Valley Irrigation Company (MVIC) and worked downstream, using

some inputs from SCS,

SCS appears to have used data fro.. sample farms studied and then

extrapolated this into a system onfarm budget. By doing this, it appears that

water reuse WIllI not adequately taken into acCO\D1t, nor WIllI the use of irrigation

water by pbreatophyt.. or the evaporation from farm ponds and onfarm and group

ditches quantifies. Also, tailwater runoff seepage and pond seepage was not

quantified. However, the effect of all this on salt load reduction calculations

is not of _jar proportions.

1/ McElmo Creek Unit, Colorado River \\later Quality
- Report, August 1982, Appendix B, Hydrosalinity,
Colorado Region. All future references to USBR
document.
2/ Onfarm Irrigation Improvements,
McElmo Creek Unit Salinity Control Study, Colorado River Basin Salinity
Control Program, Soil Conservation Se::vice, Denver, Colorado, Jan. 83.

Improvement Program, Planning
Bureau of Reclamation, Upper

report are to t()~ f~o 9