WSP04560

• File Number: 8271.300
• Description: Colorado River Basin Salinity Control Program - General Information and Publications - Reports
• Basin: Colorado Mainstem
• Water Division: 5
• Date: 2/9/1994
• Author: John Hedlund
• Title: Salt Primer - Water and Salt Budgets
• Water Supply Pro - Doc Type: Report/Study

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anyone year on the the mainstem of the Colorado River to get a
reliable estimate of water use or sa1tt1oading from irrigation in
a reach of the river. Refer to Figure ES5 for a display of
monthly and annual variation in salinity concentration. A three
percent reduction in the salinity contributed due to the USDA
program would be hard to monitor when the monthly and annual
variation is so great. Most of the time the irrigation component
is less than the error of measurement of flow or salinity.
Therefore, it is imperative that water and salt budgets be
prepared for the irrigation component based on data collected on
acreage, diversions, estimated water use, monitored irrigation
management, and other irrigation parameters. There is fairly
good agreement between the 40 year average of gaged data and
computations of annual salt loading using irrigation budgets.

Diversion records from irrigation companies in the valley were
compiled and evaluated by the Colorado Water Conservation Board.
Crop consumptive use using acreage and standard
evapotranspiration methods gave a fairly reliable estimate of
water use. Field irrigation evaluations done over the entire
season in the area provide a good estimate of onfarm efficiency
and an estimation of deep percolation. In Grand Valley,
irrigation information was gathered by SCS on each farm and field
and validated by contacting over 900 farmers. Onfarm
application efficiency was estimated to average about 25 percent.
Diversions to the farm averaged more than eight acre feet per
acre or four times the water consumed by the crop. It does not
require sophisticated evaluations to make reliable determinations
at these low irrigation efficiencies. Therefore, the current SCS
monitoring is not of unimproved systems but of the management of
recently installed irrigation systems.

ARS collected several years of data on irrigation water
management and salinity. ARS evaluated salinity impacts,
irrigation water management, and drainage options. ARS\SCS data
dating back to 1954 is available. Salinity data published for
1917, 1928, 1954, 1973, and 1988 indicates that salinity
concentration of subsurface return flow, the ground water
aquifer, and other characteristics have remained constant over
the years.

ARS conducted a special research study in 1975 to negate the
notion that salts can be flushed off the surface of an irrigated
field. It once again substantiated that an increase in salinity
of surface return flow is not measurable and leaching through the
soil profile is required to remove salts. Conclusion; "Note that
there is no statistical difference in Electrical conductivity,
chloride concentration, or nitrate concentration between the
delivered irrigation water and tailwater runoff." Ions involved
are quite soluble and are readily moved beneath the soil surface
by the advancing irrigation front and are not removed by the
surface runoff water. ARS did collect data on the amount of
sediment leaving the fields, 3 tons/acre. They also found a 9
fold increase in phosphorous in the surface runoff.