Irrigating With High Salinity Water
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Irrigating With High Salinity Water

   

Irrigating With High Salinity Water 1

Dorota Z. Haman, John C. Capece, and Allen G. Smajstrla2

In humid areas such as Florida, salinity concerns are different than in arid areas since large amounts of rainfall will wash out salts concentrating in the soil profile. However, management may be required close to the coast where groundwater salt content is frequently high. Salinity management also may be required during extended drought periods.

In arid climates, where most of the crop water requirement is supplied through irrigation and the water often contains large amounts of dissolved salts, salinity control is frequently a major objective of irrigation management.

Irrigation with various types of waste water (municipal, industrial, etc.) can also create salinity hazards or toxicity problems. Above certain concentrations, sodium, chloride, boron, and other ions are toxic to many plants.

Salinity

Since water is a very good solvent, all irrigation waters contain some dissolved salts. Electrical conductivity is a reliable index of salt concentration in the water. A conductivity of 1 dS/m (decisiemens per meter) indicates a salt concentration of approximately 700 ppm (parts per million)(Soil and Container Media Electrical Conductivity -- IFAS Circular 1092). This value will vary to some extent with temperature and type of salts. Salinity is also frequently expressed in mg/l (milligrams per liter). The number of mg/l is equivalent to ppm. Decisiemens per meter is the SI unit for conductivity. The common English unit is millimhos per centimeter (mmho/cm). One dS/m is equal to one mmho/cm.

The salt concentration in the plant root zone is usually higher than that of irrigation water. Salts are concentrated due to evaporation and plant transpiration which selectively remove water leaving salts in the soil. These salts can be removed from the plant root zone by leaching.

Salinity Effects

Salinity restricts the availability of water to plants by lowering the total water potential in the soil. Salinity also has an impact on crop physiology and yield. Visible injury can occur at high salinity levels. Usually, crop yield is independent of salt concen-tration when salinity is below some threshold level, then yield gradually decreases to zero as the salt concentration increases to the level which cannot be tolerated by a given crop. This relationship is presented graphically in Figure 1 .

Figure 1.
Sensitivity to salinity level varies among crops. Some crops are much more tolerant than others. Plants are generally divided into four salinity rating groups: sensitive, moderately sensitive, moderately tolerant, and tolerant ( Table 1 ).

Table 2 lists examples of crops in each of those tolerance rating groups.

Salinity Control

In saline conditions, soil water availability to the crop can be accomplished through several strategies such as;

Leaching salts from the root zone

In arid climates irrigation must supply all water requirements of the crop for the growing season. Additional water must be applied to remove the salts from the root zone to avoid salt build-up above the threshold level for a given crop. The amount of additional water required is usually expressed as a leaching fraction ( Equation 1 ), which is a dimensionless number.

Equation 1.
where:

LF =leaching fraction (dimensionless)

D d =depth of water drained (inches or mm)

D i = depth of water applied through irrigation (inches or mm)

Ec i = electrical conductivity of irrigation water (mmho/cm or dS/m)

Ec d =electrical conductivity of drainage water (mmho/cm or dS/m)

In humid areas, rainfall reduces salinity problems due to irrigation with saline water. The total amount of water applied to the root zone must be taken under consideration, Equation 2 .

Equation 2.
where:

D i =depth of irrigation (inches or mm)

D r =depth of rainfall minus runoff (inches or mm)

D a =depth of the total water application (inches or mm)

The weighted average electrical conductivity of applied water can be calculated from Equation 3 :

Equation 3.
Pure water is an electrical insulator with zero conductivity. However, rain usually contains some salts that are present in the air. These levels can be slightly higher near the coast.

In humid climates such as Florida's, there are many large rainfall events. Most of the water infiltrates quickly due to the sandy texture of the soils. During the rainy season the depth of rainfall in Equation 3 is much larger than the depth of irrigation, and the electrical conductivity of the weighted average is low. As a result, salts and fertilizer nutrients are washed from the root zone before salinity concentration can significantly increase. However, salinity may be a problem during extended dry periods when water is applied only through irrigation. During this time, an additional amount of water should be applied with each irrigation event to assure salt removal from the plant root zone. The amount of additional irrigation water can be calculated using Equation 1.

Example:

Calculate the leaching requirement for tomato during an extended dry weather period in Florida (assume that all the water requirement is supplied through irrigation) knowing:

From Table 2, we know that tomato is a moderately sensitive crop that will be affected by a soil salinity level in the root zone higher than 3.0 dS/m (Table 1). To maintain an acceptable level of soil salinity, the conductivity of drainage water cannot be higher than 2.5 dS/m (Cir. 1092).

The total water applied through the irrigation system during each irrigation event (D i ) is the crop water requirement (CR) plus a drainage depth (D d ) due to the leaching requirement:

D i = CR + D d

Using Equation 1:

LF = EC i /ED d = 0.5/2.5 = 0.2

But LF is also equal to D d /D i , so that when LF = 0.2, D d = 0.2 D i . Also, D i = CR + D d = CR + 0.2 D i .

Solve for D i

D i = 0.4" + D d

butD d = 0.2 D i

thenD i = 0.4" + 0.2 D i

0.8 D i = 0.4

D i = 0.5 inches

and D d = 0.2 D i = 0.2 x 0.5 = 0.1 inches

A simple check of calculations can be performed using Equation 1:

D d /D i = EC i /EC d = LF

0.1/0.5 = 0.5/2.5 = 0.2

Answer:

During the dry season, the total amount of irrigation water that must be applied during each irrigation event to maintain a soil salinity level below 2.5 dS/m is 0.5 inches (13 mm). Of this total amount, 0.1 inches (3 mm) will drain due to the required leaching fraction LF = 0.2.

References

Jensen, M.E. 1980. Design and Operation of Farm Irrigation Systems. An ASAE Monograph. American Society of Agricultural Engineers, 2950 Niles Road, St. Joseph, MI 49085.

James, L.G. 1988. Principles of Farm Irrigation System Design. John Wiley & Sons, Inc. New York.

Hanlon, E.A., B.L. McNeal and G. Kidder. 1993. Soil and Container Media Electrical Conductivity Interpretation. Florida Cooperative Extension Service, IFAS, University of Florida, Gainesville, FL. Circular 1092.

Tables

Table 1.

Table 1. Threshold and zero yield salinity levels for four salinity groups.
Salinity Rating
 Threshold Salinity
 Zero Yield Level

dS/m
 dS/m
Sensitive
1.4
8.0
Moderately Sensitive


 3.0
16.0
Moderately Tolerant


 6.0
24.0
Tolerant
10.0
32.0
(adopted from Jensen, 1980)

Table 2.

Table 2. Example of crops in four salinity rating groups.
Sensitive
 Moderately sensitive
 Moderately tolerant
 Tolerant
almond
alfalfa
red beet
sugarbeet
apple
broccoli
safflower
cotton
avocado
cabbage
olive
date palm
bean
tomato
soybean
bermudagrass
carrot
lettuce
wheat

grapefruit
corn
ryegrass

orange
cucumber
wheatgrass

lemon
grape
wildrye

okra
peanut


onion
potato


strawberry
radish


peach
rice


plum
sugarcane


(adopted from Jensen, 1980)

Footnotes

1. This document is Bulletin 322, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. Publication date: February 1997. Please visit the EDIS Web site at http://edis.ifas.ufl.edu.

2. Dorota Z. Haman, associate professor, Agricultural and Biological Engineering, Gainesville, FL; John C. Capece, assistant professor, Southwest Florida Research and Education Center, Immokalee, FL; Allen G. Smajstrla, professor, Agricultural and Biological Engineering, Gainesville, FL; Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida, Gainesville, 32611.

The Institute of Food and Agricultural Sciences (IFAS) is an Equal Opportunity Institution authorized to provide research, educational information and other services only to individuals and institutions that function with non-discrimination with respect to race, creed, color, religion, age, disability, sex, sexual orientation, marital status, national origin, political opinions or affiliations. For more information on obtaining other extension publications, contact your county Cooperative Extension service.

U.S. Department of Agriculture, Cooperative Extension Service, University of Florida, IFAS, Florida A. & M. University Cooperative Extension Program, and Boards of County Commissioners Cooperating. Larry Arrington, Dean.


Copyright Information

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