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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9

Major assumption for planning the impact of improvements:

One. The groundwater outflow salinity concentration( mg/L) will
~) not change as volume decreases from improvements in onfarm and
~ off farm efficiencies. continuous weathering of shale seabeds and
~ lacustrine lake beds underlying the irrigated area provide a
~ constant source of salt.

Two. The decrease in salt loading is directly proportional to I
the reduction in seepage and deep percolation due to irrigation.
improvements

Methodology for evaluating existing conditions:

1. Water quality data collection includes:

a. Inflow water quality (TDS or mg/L) of the water diverted
from the river and applied to the field.

b. Ground water outflow water quality (TDS or mg/L).

salinity measurements of drainage return flows are needed and
should be made when there is no surface water entering the drain,
which is usually during the late fall or winter season.

Hundreds of drains and wells have been monitored on a regular
basis for the salinity control program since the early 1970's.
Most canal, lateral, and ditch seepage and onfarm deep
percolation intermingle and return to the stream system
inseparable.

2. Ground water salt pickup(tons) is estimated by use of a water
and salt budget which accounts for the inflow and outflow of
water and salt from irrigation. The logic of the water budget is
a mass balance of inflow and outflow water. However, the salt
budget requires an evaluation of salt inflow and salt pickup from
the underlying aquifer.

Methodology for evaluating future conditions:

1. The methodology is the same as for existing conditions.

However, with improved efficiency and reduced seepage and deep
percolation, phreatophyte use may be reduced and water shortages
to crops may be satisfied and changes have to be estimated.

The resultant volume of ground water outflow is calculated by
mass balance, since the inflows, crop use, and surface outflows
for irrigation are known. The inflow salt quality is known and
the salinity concentration of the outflow is known - they have
not changed from the existing condition. The salt pickup can be
computed using the change in quantity of subsurface return flow.