WSP03798

• File Number: 8273.600.30
• Description: Colorado River Basin Salinity Control - Federal Agencies - USDA
• Basin: Colorado Mainstem
• Water Division: 5
• Date: 1/1/1991
• Author: JD Rhoades
• Title: Blending Saline and Non-Saline Waters Reduces Water Usable for Crop Production
• Water Supply Pro - Doc Type: Report/Study

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root zone (Ingvalson, et al. 1976). The water-uptake
distribution within the irrigated root zone is assumed to
be 40:30:20:10 by successive quarter-depth fractions;
steady-state chemistry and "piston-displacement-type"
water flow are also assumed. Each of these assumptions
is sufficiently true that the results are reasonable
(Rhoades and Merrill, 1976). The required leaching
fraction, Lr, is taken to be that value of L required to
keep the average salinity of the rootzone from exceeding
the tolerance level of the crop (the maximum level that
the crop can tolerate without loss of yield, ECe. a
higher value can be used, if some loss of yield can be
tolerated) .
The average level of soil salinity (expressed as the
electrical conductivity of the saturation-paste extract,

ECe = Fc . ECiw'

[ 2 ]

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I,
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ECe) within the crop rootzone resulting from the long-
term irrigation with a water of ECiw is predicted from

where ECiw is the electrical conductivity of the
irrigation water and Fc is the relating concentration
factor appropriate for L. A calculable relationship
exists between Fc and L; it is the same as that existing
between Fe and Lr, which is depicted in Figure 1 after
Rhoades (1982, 1984a).
The following example illustrates how the prediction

of ECe is made for the case of conventional irrigation
management, an ECiw of 0.5 dS/m and L equal to 0.15. Fc
1S determ1ned to be 1.51 (see Figure 1) and hence the
average level of soil salinity within the active
root zone (ECe basis) is predicted (from Equation. [2])
to be 0.75 dS/m \=0.5 x 1.51).
ECe is taken as the maximum tolerable level of ECe in
all cases herein. Thus, the maximum degree to which the
irrigation water can be concentrated before salinity
begins to reduce crop yield is given by Fe:

maximum permissible salinity, EC~
Fe=

. [31

electrical conductivity of irrigation water, ECiw

The values of ECe used were taken from the crop
tolerance tables of Maas, (1986). The leaching
requirement is calculated using ECe, the value of F~
obta1ned from Equat10n 3 and Figure 1.
The fract10n of the 1rr1gation water that can be
consumed in evapotranspiration (i.e., Vet/Viw) without
yield loss is related to Lr as
Vet/Viw = (l-Lr) [4]

4

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