WSP11706

• File Number: 8271.300
• Description: Colorado River Basin Salinity Control Program - General Information and Publications-Reports
• Basin: Colorado Mainstem
• Water Division: 5
• Date: 6/1/1980
• Title: Development of Procedures to Evaluate Salinity Management Strategies in Irrigation Return Flows
• Water Supply Pro - Doc Type: Report/Study

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the root zone, as a result of silicate
mineral weathering and dissolution of soil
lime (Rhoades et al. 1974). Depending on the
drainage of the soil-water system, the
above processes may redistribute the storage
of salts within the soil profile and develop,
over long periods of time, a high capacity
for Ilbuffering" salts. For soil types such
as those encountered in Vernal, Utah (King
and Hanks 1975 and Melamed 1975), a charac-
teristic soil solution concentration profile
emerged. A unique salt concentration devel-
oped at each position in the soil profile,
and it remained nearly unchanged as waters of
various concentrations passed through.

Salt balance studies based on outdoor
experiments with lysimeters (Rhoades et aI.
1973 and 1974) indicated that, depending on
the LF, there could be either a net gain of
salts (by dissolution), and a net loss of
salts (by precipitation). Both calcarious
(with 1% GaG03) and noncalcarious Pachappa
sandy loam soils were investigated (Figures
2.1 through 2.4). The fact that these lines
representing drainage water salinity tend
to become horizontal with large leaching
fractions suggests that for these soils a
threshold leaching fraction (LFT) could be
identified beyond which there is neither a
net gain nor a net loss of salt relative to
the applied water. The estimated drainage
water composition, "Threshold Salt Concentra-
tion" erSe), corresponding to the LFT for
several irrigation waters is shown in Table
2.2. Since the data represent tile drain
water or the drainage waters as they emerge
from the root zone, the above estimates of
ECT values correspond to the concentration

of the soil-solution at the bottom of the
root zone. The estimated values of TSC for
noncalcarious (without lime) soils are
higher than the corresponding TSC values for
calcarious soils.

TSC values are highly site specific
and depend on the quality of irrigation
water, soil type, and the leaching status of
the soil at the time of determination. For
example, soils recently brought into irriga-
tion may have a higher TSC than do soils
irrigated for many years. Field experiments
of the type described in Rhoades et al.
(1973) provide in situ TSC for the usual soil
types encountered in most parts of the
western states where the soils provide
a buffering capacity with respect to CaC03.
Further investigations are necessary to
extend this concept to other soils. Never-
theless, the TSC concept is useful for
deriving a simple but reasonably reliable
hydrosalinity model as described in a
subsequent chapter.

Spatial and temporal variability of
drainage water concentratIons. Observed
salinity concentrations in drainage waters
from various irrigated areas change with
the time and location of individual drains.
The results of a number of research projects
shed light on these patterns.

1. The eight studies of drainage water
salinity concentrations reported in Table 2.3
indicate that concentrations remained similar
throughout the year, suggesting that salt
outflow was proportional to the corresponding
water flow. Those observations appear,

Table 2.2. Estimated threshold salt concentrations (TSC) for Pachappa sandy loam.
Figures 2.1, 2,2. 2,3, and 2.4.)

River type

Estimated LF at which
no salt precipitation
or dissolution occurred

1 Feather
2 Grand

0.285

3 Missouri

0,17
0,185
0,185
0,145
0,235
0.225

4 Salt

5 Colorado

6 Sevier
7 Gila

0.230
0.235

8 Pecos

(Based on

Estimated Threshold Salt Concentration, TSC (mnhos/cm)

calcarious lysimeters
(with 1% eaQ)3)

noncalcarious lysimeters

2,75

3.75

4.25

7.2

10.5

4.25

4.10

11.75

12.25

5

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