Crop irrigation management using saline water

Water for agricultural use is normally considered to be in one of five salinity classes. These classes are outlined in Table 7.8, their boundaries being defined by the total dissolved

solids and electrical conductivity of the water.

Crop responses to salinity vary with crop species and, to a lesser degree, with the crop variety. The tolerance of crops to salinity is generally classified into four to six groups, from the sensitive (or non tolerant), where most horticultural and fruit crops are included, to the tolerant, which includes barley, cotton, jojoba, sugarbeet, several grass crops, asparagus and date palm. Full lists of crop tolerance classes are given by Ayers and Westcot (1985) and Rhoades et al. (1992) among others. A summary of limitations to use of water of different classes and of impacts of salinity on yields in the main crop tolerance groups is given in Table 7.9.

T^ABLE 7.7. Water quality for irrigation and required restrictions in use (Ayers and Westcot, 1985).

Problems Water characteristic No restrictions Slight to moderate restrictions

Severe restrictions Salinity effects on

water availability EC (dS/m) < 0.7 0.7 to 3,0 > 3.0

TDS (mg/l) < 450 450 to 2000 > 2000

Surface irrigation (me/l) <4 4 to 10 > 10

Sprinkle/spray (me/l) <3 > 3

Boron (mg/l) < 0.7 0.7 to 3.0 > 3.0

Trace elements Variable

Sprinkle/spray

irrigation Bicarbonate (me/l) < 1.5 1.5 to 8.5 > 8.5

Plant nutrition pH Normal 6.5 - 8.5

T^ABLE 7.8. Salinity classification of water

Non saline Slightly

(mg/l) < 500 500 to 2000 2000 to 4000 4000 to 9000 > 9000

Electrical conductivity, EC (dS/m)

< 0.7 0.7 to 3.0 3.0 to 6.0 6.0 to 14.0 > 14.0

The behaviour of various crops under irrigation with water of different degrees of salinity varies with species, varieties and the crop growth stages. As irrigation water salinity increases, germination is delayed. Germination is adversely affected for most field crops when the EC of the irrigation water or the soil saturation extract reaches a threshold of 2.4 dS/m. Adverse effects occur at lower values (< 1 dS/m) for non-tolerant crops, and at

higher values for tolerant crops (generally not exceeding 4 dS/m). The germination and seedling stages are the most sensitive to saline water irrigation. Any adverse effects at such stages will lead to a reduction in crop production proportional to the degree of plant loss during germination and plant establishment. At this stage, water of good quality should be used, especially if plants are sensitive. If fresh water is lacking at this stage, irrigation during seedling development, after germination must be carried out with fresh water to avoid dramatic effects on yields (Hamdy, 1996). Besides germination and crop establishment, another growth stage where most crops are more sensitive to salinity is the reproductive phase. Other critical stages vary from crop to crop. Several case studies are reported in the literature (e.g. Hamdy, 1994; Hamdy and Karajeh, 1999).

T^ABLE 7.9. Summary guidelines for use of saline waters.

Crop tolerance to salinity Non

saline Slightly saline Medium saline Highly saline Very highly saline I sensitive Limitations to use None Slight to

medium

Restricted Not usable Not usable

Yield reduction None Up to 50 % > 50% ― ―

II moderately

sensitive Limitations to use None Slight Medium Restricted Not usable Yield reduction None Up to 20 % Up to 50 % > 50% ― III moderately

tolerant

Limitations to use None None Slight to medium

Medium Very

Restricted Yield reduction None None 20 to 40 % 40 to 50 % > 50%

IV tolerant Limitations to use None None Very slight Slight to

medium Restricted Yield reduction None None Practically

none 20 to 40 % > 50%

Halophytes Limitations to use ― ― ― ― Suitable

The suitability of water for irrigation based on salinity, leaching and drainage requirements, and crop tolerance to salinity must be related to irrigation management.

Several approaches can be adopted in water and crop management to minimise the accumulation of salts in the active root zone and to eliminate salt stress, especially during the critical growing stages of the plants. These include:

ƒ Appropriate selection of irrigation methods.

ƒ Efficient leaching management, including volumes and frequency, and respective drainage of the salty water away from the cropped land.

ƒ Proper irrigation scheduling, in agreement with the available irrigation system.

ƒ Crop rotations adapted to the prevailing conditions. Considerations must include irrigation water quality, soil salinity levels, chemical and physical properties of the soils, and climatic conditions.

The selection of the irrigation method must consider the quality of the water and the potential for both the water and the irrigation method to produce negative impacts. A summary of the more relevant considerations is presented in Table 7.10. These refer to the capabilities for controlling:

T^ABLE 7.10. Evaluation of the irrigation methods for use with saline water.

above As for flat basins but depending on avoiding salt stress at plant emergence and crop establishment

Border

irrigation As for basin irrigation but infiltration control is more difficult, as is the control of the leaching fraction

As for flat basins Because water infiltrates while

irrigation Salts tend to accumulate on the tops of the ridges.

irrigation Not likely to occur when set systems are used except for the under-irrigated parts of the field due to low uniformity. Instead, parts of the field due to low uniformity, including that resulting from nozzle clogging when water filtration is poor

Not likely to occur Problems generally do not occur except parts of the field due to low uniformity and clogging;

ƒ soil salinity hazards due to salt accumulation in the root zone,

ƒ toxicity hazards caused by direct contact of the salty water with the plant leaves and fruits,

ƒ soil infiltration and permeability hazards caused by the modification of the soil physical properties, mainly due to the Na ion, and

ƒ yield hazards which may occur when the irrigation system does not allow adoption of appropriate irrigation management, the frequency and volumes of irrigation.

One of the most important factors in crop management when using saline irrigation is the irrigation frequency. Saline water requires more frequent irrigation than for fresh water because salts in the water and the soil increase the osmotic potential of the soil water, which makes water uptake by the crop roots more difficult. However, increasing the frequency implies reducing the depth of water applied at each irrigation to avoid gross accumulation of salts in the soil. The irrigation application depths that can be used depend on the irrigation method and the off-farm system delivering water to the fields.

Surface irrigation methods make it extremely difficult to apply small irrigation depths, as outlined in Section 8.8. When water is delivered to the farms through surface canal systems, the delivery schedules are generally of the rotation type, and are rigid, delivering large irrigation volumes at long intervals. These systems are inappropriate for irrigation of less tolerant crops.