15 Jan Main parameters used to assess water quality for irrigation of citrus trees
by Pieter Raath (CRI) and Tariena Nel (Labserve)
In the light of decreasing availability and quality of water used for irrigation of citrus in many regions of South Africa, producers are compelled to investigate the possibility of using new water sources, i.e., groundwater or wastewater. However, the quality of these sources often raises concern, making citrus producers unsure whether they can use them to supplement limited irrigation water supplies or for new developments. In this article, the most common factors that determine whether water is suitable for irrigation and possible mitigation options are discussed.
Water salinity
The first quality parameter that must be considered is total dissolved solids (TDS), which measures the total quantity of various inorganic salts dissolved in water. The TDS is assessed by measuring the electrical conductivity (EC) of the water. The principle of EC measurement is based on the fact that salts in water increase the rate at which electricity is conducted in the water – the higher the salt content, the higher the conductance. Therefore, the TDS is typically calculated mathematically by laboratories from the EC by using a factor that will differ according to the unit in which the EC is expressed. The relevant factors are provided in Table 1 below, with the TDS being expressed as milligrams per litre (mg/L), which is equal to parts per million (ppm).
Interpretation of the total salt content of water is made using a scale of EC values – the higher the EC, the less suitable the water becomes for citrus production. Different EC ranges are used to classify the water quality – the different classes that are internationally used as an indication of the suitability of the water for citrus production are provided in Table 2. The EC, however, does not indicate the type of salts that are present in the water.
The sodium content of the water
Even though the water’s total salinity places it in salinity Class C1, C2 or C3, it can still be unsuitable for irrigation due to disproportionately high sodium (Na) in the water. In addition to being a salt that can damage plant tissues in extremely high concentrations, Na is harmful to soils since it displaces Ca and Mg on clay particles resulting in clay dispersion and breakdown of soil structure. A decreased water infiltration and/or drainage, with an exponential increase in the soil salinification rate, is the end result.
To evaluate the risk that Na poses to the soil, the ratio of Na in relation to calcium (Ca) and magnesium (Mg) in the water is used, which is expressed as a factor called the sodium adsorption ratio (SAR). The SAR is a value that is mathematically calculated by the laboratory from the water Na, Ca, and Mg analysis results. An interpretation of the SAR values is provided in Table 3.
Irrigation water analysis reports will therefore often contain a so-called irrigation class, which is expressed with C and S symbols denoting the salinity (C = conductivity) and sodium status (S = sodium). For example, highly saline water (EC > 2.25) with a low SAR (<5) will be classified as C4S1 water. From the tables above, these classifications can therefore be interpreted.
Crust formation that typically occurs on the surface of soil that is irrigated with water that has a high residual sodium carbonate index or an excessive SAR – the result is poor water infiltration, i.e., excessive run-off
Residual sodium carbonate index
When the pH of the irrigation water exceeds 8.0, a useful alternative measure of the risk that sodium poses is the residual sodium carbonate (RSC) index. In this index, the bicarbonate (HCO3-) and carbonate (CO32-) concentrations in the water are brought into consideration. The logic is that precipitation of soil Ca and Mg with HCO32- and CO32- in the irrigation water subsequently leads to an increase in the relative proportions of sodium to the other cations in the soil solution. This situation, in turn, will increase the sodium hazard of the soil-water to a level greater than indicated by the SAR value of the irrigation water. Therefore, a high RSC of irrigation water results in an increase in the soil’s exchangeable sodium percentage (ESP), with the consequent dispersion of clay, as referred to above.
On their own, high HCO3- and/or CO32- levels in irrigation water also increase the water pH, cause clogging of irrigation systems due to calcite or lime deposition, and a whitish deposition forms on leaves and fruit from the droplets of irrigation water.
In arid and semi-arid regions, underground water often has a high pH and RSC. The higher the pH of the water, the higher the HCO32- and CO32- concentration will be, with a progressive shift towards CO32- as the pH increases. The clogging of irrigation systems increases as the HCO32- and CO32- concentration and water pH increase. The guidelines in Table 4 can be used to assess the risk of drip irrigation systems becoming blocked due to calcite/lime deposition.
To assess the risk of sodification of soil, a calculation of the RSC should be done if the HCO32- and CO32- concentration of the water exceeds 120 mg/L and 15 mg/L respectively, and the SAR > 3. The RSC can be calculated from normal irrigation water analysis results, using the following formula:
RSC index = {(HCO3/61) + (CO3/30)} – {(Ca/20) + (Mg/12)},
where the concentrations are in mg/L, and the resulting RSC index value is expressed in milliequivalents per liter (meq/L).
The following indicates the suitability of water for irrigation on account of the RSC index (meq/L):
Management and mitigation possibilities
From the above tables, it is implicit that while it is possible to use a wide variety of water quality types for irrigation, management procedures become more demanding as the water quality decreases – the suitable range of soils and citrus also become more restrictive. The possible management options to prevent the build-up of salt in the soil, and ensure long-term tree performance, are:
Soil drainage: To avoid the build-up of salts, the soil must be permeable – permeable soil can be leached to wash out the salts. In this regard, sand is much more forgiving than heavy soil.
Leaching: Salt build-up in the soil is avoided through leaching. Through regular monitoring of salt levels in the soil (measuring the soil’s EC or resistance), the need for leaching (or its effectiveness when practiced) can be determined. Generally, the higher the salt level of the water, the more arid the climate, the greater the leaching requirement will be.
Irrigation frequency and duration: Irrigation scheduling has a significant impact on salt accumulation or reducing levels in the soil. Short, regular cycles will more likely result in the salt build-up in the topsoil because of evaporation. On the other hand, long cycles leading to deep water penetration mitigate the build-up of salts in the root zone.
Soil and water amelioration: Irrigation water can be modified with gypsum to improve its SAR. The quantity of gypsum needed for adding to irrigation water depends upon the quality of water (the SAR and RSC) and the annual quantity of water required for irrigation of the trees. The addition of gypsum increases water salinity, so this is only an option when the water’s salinity is not too high, and when the soil is well-drained. Soils can also be modified using gypsum to ensure proper drainage (it helps to maintain and improve the soil’s structure), but this should only be used when the soil’s exchangeable sodium percentage exceeds 10% – otherwise, the soil is actually further salinified.
In this article, the three most important water quality parameters were discussed, i.e., total salinity, SAR, and RSC. These are the most common restricting factors that must be considered. Water quality problems can also be associated with the presence of other constituents, like excessive levels of iron (Fe) or manganese (Mn), that can result in drip irrigation being blocked.
Drip irrigation lines become blocked when irrigation water is used with high bicarbonate and carbonate concentrations
Literature cited
Prasad, A., Kumar, D., Singh, D.V., 2001. Effect of residual sodium carbonate in irrigation water on the soil sodication and yield of palmarosa (Cymbopogon martinni) and lemongrass (Cymbopogon flexuosus). Agricultural Water Management 50, 161-172.
Wilcox, L.V., 1955. Classification and the use of irrigation waters. Circular No. 969, USDA, Washington.
Zaman, M., Shahid, S.A. & Heng, L., 2018. Guideline for salinity assessment, mitigation, and adaptation using nuclear and related techniques. IAEA, Vienna. https://doi.org/10.1007/978-3-319-96190-3.
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