Irrigation Water Quality

Both irrigation water quality and proper irrigation management are critical to successful crop production.

The quality of the irrigation water may affect both crop yields and soil physical conditions, even if all other conditions and cultural practices are favorable/optimal. In addition, different crops require different irrigation water qualities.

Therefore, testing the irrigation water prior to selecting the site and the crops to be grown is critical. The quality of some water sources  may change significantly with time or during certain periods (such as in dry/rainy seasons), so it is recommended to have more than one sample taken, in different time periods.

The parameters which determine the irrigation water quality are divided to three categories: chemical, physical and biological. In this review, the chemical properties of the irrigation water are 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 Origin of Salts in Irrigation Water

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

Irrigation Water Quality-Related Problems

Irrigation water salinity

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

High level of salts in the irrigation water reduces water availability to the crop (because of osmotic pressure) and causes 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 the irrigation water quality, in relation with its salinity, are EC and TDS.

In case the irrigation water salinity exceeds the threshold for the crop, yield reduction occurs. Equations were developed to estimate the yield potential, based on the irrigation water salinity.

% Yield (of maximum) = 100 - b (ECe - a)

Where (b), is the percent loss in relative yield per unit increase in salinity, (a) the EC threshold the crop can tolerate and ECe is the electrical conductivity of the saturated soil paste, which is measured in the laboratory.

ECe is proportional to the electrical conductivity of the irrigation water, depending on the percentage of irrigation water leached below the root zone.

Sodium hazard and irrigation water infiltration

The parameter used to determine the sodium hazard is SAR - Sodium Adsorption Ratio. This parameter 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 to become hard and compact when dry. 

Ironically, higher salinity reduces the negative effect of sodium on soil structure. So, when sodium levels in the soil are high in relation with calcium and magnesium, i.e SAR is high, flushing the soil with good irrigation water quality will only worsen the problem.

Toxicity of specific ions

The irrigation water quality can be also determined by 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 accumulation of a specific ion in the leaves.

The most common ions which might cause a toxicity problem are chloride, sodium and boron. The same as with salinity, crops differ in their sensitivity to these ions.

Special attention should be given to boron because toxicity occurs in very low concentrations, even though it is an essential plant nutrient. 

Toxic levels of even a single ion in the irrigation water might make the water unsuitable for irrigation.

Nevertheless, there are some management practices that can help in reducing the damage.

These practices include proper leaching, increasing the frequency of irrigations, avoiding overhead irrigation, avoiding the use of fertilizers containing chloride or boron, selecting the right crops, etc.

Alkalinity and pH

Alkalinity is the sum of the amounts of bicarbonates (HCO_3^-), 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.