Irrigation Water Quality

Both irrigation water quality and proper irrigation management are critical to successful crop production. The quality of irrigation water might affect both crop yields and the physical condition of the soil, even if all other conditions and cultural practices are favorable. In addition, different crops require different irrigation water qualities.

This means that testing irrigation water prior to selecting a site and the crops to be grown is critical. The quality of some water sources might change significantly with time or during certain periods—like dry or rainy seasons—so it is recommended to have more than one sample taken, at different time periods.

The parameters that determine irrigation water quality are divided into three categories: chemical, physical and biological. In this article the chemical properties of irrigation water will be discussed.

The chemical characteristics of irrigation water refer to the content of salts in the water as well as to parameters derived from the composition of salts in the water—parameters such as EC/TDS (electrical conductivity/ total dissolved solids), SAR (sodium adsorption ratio), alkalinity and hardness.

The primary natural source of salts in irrigation water is weathering of rocks and minerals. Other secondary sources include atmospheric deposition of oceanic salts (salts in rainwater), saline water from rising groundwater and the intrusion of sea water into groundwater aquifers. Fertilizer chemicals that leach to water sources might also affect irrigation water quality.

Problems Related to Irrigation Water Quality

Irrigation Water Salinity

The main problem related to irrigation water quality is water salinity—which refers to the total amount of salts dissolved in the water, but it does not indicate which salts are present.

High levels of salts in irrigation water reduce water availability to the crop (because of osmotic pressure) and cause yield reduction. Above a certain threshold, reduction in crop yield is proportional to the increase in salinity level. Different crops vary in their tolerance to salinity and, therefore, have different thresholds and yield reduction rates.

The most common parameters used for determining irrigation water quality in relation to its salinity are EC and TDS. If irrigation water salinity exceeds the threshold for the crop, yield reduction will occur.

Sodium Hazard and Irrigation Water Infiltration

The parameter used to determine the sodium hazard is SAR, or sodium adsorption ratio. This factor indicates the amount of sodium in the irrigation water in relation to calcium and magnesium. Calcium and magnesium tend to counter the negative effect of sodium.

High SAR levels might result in a breakdown of soil structure and water infiltration problems. Soil tends to seal and become hard and compact when it’s too dry.

Ironically, higher salinity reduces the negative effect of sodium on soil structure. When sodium levels in the soil are high in relation to calcium and magnesium—in other words, when the SAR is high—flushing the soil with good-quality irrigation water will only exacerbate the problem.

Toxicity of Specific Ions

Irrigation water quality can be also determined by the toxicity of specific ions. The difference between a salinity problem and a toxicity problem is that toxicity occurs within the plant itself, as a result of the accumulation of a specific ion in the leaves. The most common ions that cause a toxicity problem are chloride, sodium and boron. As they do with salinity levels, crops differ in their sensitivity to these ions.

Special attention should be given to boron levels because toxicity occurs at very low concentrations, even though boron is an essential plant nutrient—toxic levels of even a single ion in irrigation water might make the water unsuitable.

There are some management practices that can help reduce the damage, however. These practices include proper leaching, increasing the frequency of irrigations, avoiding overhead irrigation, avoiding the use of fertilizers containing chloride or boron, and selecting the right crops.

Alkalinity and pH

Alkalinity is the sum of the amounts of bicarbonates (HCO₃-), carbonates (CO₃^2-) and hydroxide (OH-) in water. It is expressed as mg/l or meq/l CaCO₃. Alkalinity buffers the water against sudden changes in pH. If the alkalinity is too low, any addition of acidic fertilizers will immediately lower the pH. In container plants and hydroponics, ions released by plant roots may also rapidly change the pH if alkalinity is low.

Managing Irrigation Water Quality Problems

Infiltration Problems Resulting From Low Irrigation Water Quality

As mentioned earlier, SAR (sodium adsorption ratio) is an irrigation water parameter used to predict problems of water infiltration into soil. SAR is determined as:

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

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

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 one to four meq/l of dissolved calcium.

Other amendments

When lime (CaCO₃) is present in soil, some acids or acid-forming amendments can be used—these will cause calcium to be released into the soil solution. Some of these 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 irrigation water salinity or by reducing the SAR. By diluting the irrigation water source with water having a 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 via irrigation water and with applied fertilizers. Applying more water than is 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 is the minimum leaching requirement for the crop, ECw is the electrical conductivity of the irrigation water in ds/m (irrigation water salinity) and ECe is the soil EC tolerated by the crop, measured in a saturated soil extract.

The total irrigation water amount that has to be applied to meet both crop demand and your 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 appropriate irrigation intervals will depend not only on crop water demand, but also on factors such as the salinity threshold of the crop and the soil’s 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—this means that heavy soils require higher application amounts of irrigation water at larger intervals in order to prevent an accumulation of salts exceeding 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 the soil. The timing of leaching is not critical provided the salinity threshold of the crop is not exceeded.