Managing Irrigation Water Quality Problems

Several management practices affect the use of marginal-quality irrigation water.

Infiltration problems resulted from low irrigation water quality

SAR (Sodium Adsorption Ratio) is an irrigation water parameter, which is used to predict problems of water infiltration into soil.  SAR is determined as:

Apart from water shortage, which is a result of of water infiltration problems, some other related problems might occur as well. For example weed growth, diseases, poor aeration, poor germination of seeds, root rot etc.

Various measures can be taken to overcome water infiltration problems, which are water-quality related. Such measures include soil or water amendments, reducing the SAR of the water supply, cultivation and tillage, addition of organic residues and irrigation management.

Soil amendments and irrigation water quality

The purpose of soil amendments is to counter the effect of sodium, by increasing the soluble calcium content or by increasing the salinity of the irrigation water.

Gypsum and other calcium supplying materials - Gypsum is the most commonly used soil amendment. Since water infiltration problems caused by sodium affect mainly the upper few centimeters of soil, repeated small applications of gypsum, incorporated at lower rates into a shallow depth, are preferred over a single large application.

If the salinity of the irrigation water is low (EC<0.5 ds/m), gypsum can be added to the irrigation water at rates of 1-4 meq/l of dissolved calcium.

Other amendments - when lime (CaCO3) is present in soil, some acids or acid-forming amendments can be used. These amendments cause calcium to be released to soil solution. Examples for such amendments are elemental sulfur, sulfuric acid and ferric sulfate.

Organic residues - these amendments improve soil structure and water infiltration, by keeping the soil porous.

Blending Irrigation Water Sources

Water infiltration can be improved either by increasing the irrigation water salinity or reducing the SAR.

By diluting the irrigation water source with water of lower sodium concentration, the SAR of the irrigation water is reduced, even if calcium and magnesium concentrations are higher.

Management of Soil and Irrigation Water Salinity

When salts build up in soil, or in the growing medium, their concentration might become excessive.

Salts are added to soil with the irrigation water and with applied fertilizers. Applying more water than that needed by the crop leaches the salt below the root zone, deeper into the soil, or out of the growing medium when growing container plants.

It is important to know how much to leach and when. The leaching requirement can be estimated from the following equation:

LR=ECw/ [5*ECe - ECw)]

Where:

LR -  the minimum leaching requirement for the crop.

ECw - the electrical conductivity of the irrigation water in ds/m (irrigation water salinity)

              
ECe - the soil EC tolerated by the crop, measured in a soil saturated extract.

The total irrigation water amount that has to be applied to meet both crop demand and leaching requirement can be estimated from the equation:

AW = ET/ (1-LR)

Where AW  is the amount of irrigation water that has to be applied, ET is the crop water demand and  LR is the calculated leaching requirement.

For example:

Crop water demand - 30m3/ha/day

ECe = 2.5 ds/m, ECw = 1.2 ds/m

LR=1.2/(5*2.5-1.2)=0.1

AW=30/(1-0.1)=33.33 m3/ha/day.

Irrigation Intervals

Knowing the total amount of irrigation water to apply is not enough for managing salinity. Irrigation intervals must also be considered.

The irrigation intervals depend not only on crop water demand but also on factors such as the salinity threshold of the crop and the soil capacity to hold water.

Applying the same amount of water to two soils with different characteristics will result in different wetting patterns and depths. Irrigation depth in a heavy soil is lower than in a sandy soil, since heavy soils hold more water than sandy soils.
 

Therefore, compared to sandy soils, heavy soils require higher application amounts of irrigation water, at larger intervals, in order to prevent the accumulation of salts which exceed the salinity threshold of the crop.

It must be noted though, that salinity in the root zone increases between irrigations as a result of crop water uptake and water evaporation from soil. The timing of leaching is not critical, provided the salinity threshold of the crop is not exceeded.