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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Other pr inciples or assumptions used in the
more sophisticated models were chemical
equilibrium, spatial homogeneity, complete
mixing of the solutes, insignificant lateral
dispersion and diffusion, presence of soil
lime, constant pH at each depth increment,
constant GEG for a given soil, insignificant
tempera-ture effects, Bnd no salinity change
from chemical reactions or ion exchange in
the s atur ated zone. The above ass umpt ions
were common to many of the models, but there
was considerable variation in the level
of detail used in representing the soil-
water-chemistry of the system.

A few general assumptions are inherent
in the one-dimensional instream salinity
models based on statistical analyses. Con-
s tant salinity concentration in the source
groundwater was assumed by Pinder and Jones
(1969). Fairly constsnt proportions of
constituent ions were assumed by MacKinhan
and Stuthmann (1969) and Jensen (1976).
These models did not allow for the possibil-
ity of salt pickup.

Jensen (1976) made the two assumptions
on p. 10 in stating that stream salinity
concentration c can be represented by:

c = Cg (QT/Qo)B

for QT > Qo

(3,1)

and
c ~ Cg
in which
Cg
QT
Qo
B

for QT :S Qo

(3.2)

groundwater
total runoff.
river flow at which the ground-
water component is greatest.

TDS concentration.

an exponent varied between -1.0
and 0.0 for the Colorado River
system.

Because data were lacking for estimating
natural TOS concentration, Jensen (1976)
assumed that the increase in salinity concen-
tration over the period of record was the
result of water depletions rather than in-
creases in salt loading. As this assumption
has not been verified, his results should
be interpreted carefully.

Surface groundwater interrelationships

As water moves slowly in the groundwater
aquifers, a long time might be required for
the deep percolating (OP) water to emerge as
effluent flow. Since the percolating water
carries dissolved salts, mixes with the
groundwater, and eventually joins the surface
rli'noff from the basin, it is important to
represent the surface-groundwater quality
interactions accurately. Two major param-
eters are 1) the delay time of the subsurface
flow to emerge as outflow, and 2) the propor-

t ion of subsurface flow that joins the
stream. Hystt et a1. (1970), Thomas et a1.
(1971) and Hill et a1. (1973) assumed both
parameters to be constants. They estimated
values for both as part of their model
calibration procedure.

Since the runoff in winter months is
predominantly from subsurface sources, an
assumption that effluent groundwater is a
constant proportion of the runoff from sub-
surface sources is not realistic. Narasimhan
(1975) successfully modeled the subsurface
contribution to total runoff by time variant
parametric representation.

Salt pickup phenomena

The mechanisms governing salt pickup
sre highly complex. Reliable field dsta are
a prerequisite to identify and quantify the
salt pickup by the percolating waters. if
total dissolved solids (TOS) is considered
the salinity indicator, then the possible
assumptions are that the rate of salt pickup
is 1) proportional to the quantity of perco-
lating water, 2) uniform over time, or 3)
follows some more complicated relationships
that needs to be derived for the agricultural
drainage system from reliable field data.

The models reviewed in this study con-
tsined explicit parametric relationships for
salt pickup. Hill et a1. (1973) sccounted
fdr increases in salt flow in the surface and
subsurface return flows separately. The
parameter CF determined the proportional
increase in agricultural surface return flow
salt content, while the parameter SWS as-
signed a soil weathering rate in tons/acre/
month. Hyatt et a1. (19"(0) assigned a
fsrameter Cga to indicate the average sa-
inity concentration within the 80il solution
beneath the agricultural lands. The value of
CS!;B was estimated during model calibration.
Tne natural sslinity contribution was esti-
mated by sssuming that within each basin
substantial interchanges occur between
surface and subsurface waters. The rate of
salt flow resulting from the interchange
process was estimated by the equation:

s~S kp Qr Cg . , (3.3)

in which

SNS
r -

rate of salt flow contributed
from natural sources within the
basin.

kp

= percentage of the surface flow
recirculating through the stream
alluvium or groundwater basin.

monthly average of inflow and
outflow to a subbasin.

Qr

Cg

average water salinity level
within the groundwater basin or
stream alluvium. This quantity,
assumed to be constant through-

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