CHAPTER 7: Water quality for irrigation

1 CHAPTER 7: Water quality for irrigationINTRODUCTIONI rrigation waters whether derived from springs, diverted from streams, orpumped from wells, contain appreciable quantities of chemical substancesin solution that may reduce crop yield and deteriorate soil fertility. Inaddition to the dissolve salts, which has been the major problem forcenturies, irrigation Water always carry substances derived from its naturalenvironment or from the waste products of man s activities (domestic andindustrial effluents). These substances may vary in a wide range, butmainly consist of dirt and suspended solids (SS) resulting into the emitters blockages in micro- irrigation systems and bacteria populations andcoliforms harmful to the humans and the OF Water quality FOR IRRIGATIONIn several studies carried out in the eighties on the various causes forthe clogging of the emitters, the engineers, based on the three majorinterrelated factors contributing to that specific problem, classified thewater according to the chemical quality , the physical and the classification although primary and simple seemed convenient for amore or less broad evaluation to cover the whole spectrum of irrigationwaters quality for crop production.

2 With the inclusion of reuse of treatedmunicipal wastewater in agriculture during that period, the Water qualityconsiderations were broadened in order to cover all the physicochemical,biological and microbiological properties of Water that may cause anyimpact on soil, plants, environment and the consumers, human orlivestock. The Water quality evaluation method in this CHAPTER , although inbrief, draws on the important parameters and criteria for a more or lesspractical evaluation of the chemical, the physical and the biologicalquality of the Water for irrigation with pressurized techniques as follows:a) Chemical, ( salinity /toxicity hazards for the soil, the plants and theirrigation system such as it is pipe corrosion and emitter chemicalclogging),b) Physical (emitters blockages problems from suspended solid particlesand other impurities content),c) Biological (problems from bacteria and other contents harmful forhuman and animal health as well as for the soil the plants and theirrigation systems).

3 Pressurized irrigation TechniquesPressurized irrigation information on the physical and biological evaluation is given inbrief as compared to the chemical evaluation. Yet an in-depth andcomplete examination should include the soil physical properties and theclimatic conditions as well as many other factors with direct or indirectinfluence on Water use in agriculture and CHEMICAL quality OF irrigation WATERC omposition and concentration of soluble saltsSalinity is a common problem facing farmers who irrigate in aridclimates. This is because all irrigation waters contain soluble salts. Whetherderived from springs, diverted from streams, or pumped from wells, thewaters contain appreciable quantities of chemical substances in solution,dissolved from the geological strata through and over which the watershave flowed. Waters with a high salt content may have moved from asaline Water table. In areas with intensive agriculture, fertilization is amajor cause of aquifer composition of salts in Water varies according to the source andproperties of the constituent chemical compounds.

4 These salts includesubstances such as gypsum (calcium sulphate, ), table salt(sodium chloride NaCl) and baking soda (sodium bicarbonate NaHCO3).When dissolved in Water , salts separate into ions; sodium chloridebreaks down into sodium and chloride ions. Thus, it is customary to referto ions rather than salts. The principle ions in irrigation Water and theircharacteristics are listed in Table ions are expressed in the form of milligrams per litre (mg/litre orppm) and milliequivalents per litre (meq/litre). The latter unit is preferablebecause Water quality criteria involve milliequivalents per irrigation TechniquesChapter 7 Water quality for - Principle ions present in irrigation waterIonsChemical symbolEquivalent weightAnions (acidic ions)ChlorideCl 48 CarbonateCO3 30 BicarbonateHCO3 61 NitrateNO3 62 Cations (basic ions)SodiumNa+23 PotassiumK+ ++20 MagnesiumMg++ conversion formula is:Boron is also present in irrigation waters as un-ionized boric acidexpressed as boron element (B) in milligrams per litre.

5 The saltconcentration in most irrigation waters ranges from 200 to 4 000 mg/litretotal dissolved solids (TDS). The pH of the Water is also an indicator of itsquality and it normally ranges from to common method for evaluating the total salts content in Water is bymeasuring the electrical conductivity of Water (ECw) at 25 C. Electricalconductivity is expressed in deciSiemens per metre. There is a relationbetween the electrical conductivity and the concentration of salts inmilliequivalents per litre and in milligrams per litre when the ECw is in therange of 1 5 dS/m. Thus, every 10 meq/litre of salts (cation concentration)create 1 dS/m ECw. The relationship between electrical conductivity andtotal dissolved salts (TDS) is:ECw (dS/m) x 640 = TDS (mg/litre)The sum of cations should equal the sum of anions. The accuracy of thechemical Water analyses should be checked on the basis of the of soluble salts on plantsThe application of irrigation Water to the soil introduces salts into theroot zone.

6 Plant roots take in Water but absorb very little salt from the soilsolution. Similarly, Water evaporates from the soil surface but salts remainbehind. Both processes result in the gradual accumulation of salts in theroot zone, even with low salinity Water . This situation may affect the plantsin two ways: a) by creating salinity hazards and Water deficiency; and b)by causing toxicity and other hazards and Water deficiencyThe build-up of salinity in the root zone increases the osmotic pressureof the soil solution and causes a reduction in both the rate of waterabsorption by the plants and the soil Water availability. Thus, a continuouswater deficiency may exist even though the field is heavily irrigated. Plantwilting symptoms may not become apparent, but growth and yield aredepressed. Under such circumstances it is not possible to maintain goodmeq/litremg/litre=equivalentweightPr essurized irrigation TechniquesPressurized irrigation irrigation TechniquesChapter 7 Water quality for development conditions and obtain high yields.

7 Instead, plant growthis delayed and there is a considerable reduction in yield. Seed germinationis also affected by the presence of salts. It is usually delayed and in somecases does not level of salinity build-up depends on both the concentration andthe composition of salts in the Water . Chloride is highly soluble andremains in the soil solution, while sulphate and bicarbonate combine withcalcium and magnesium, where present, to form calcium sulphate andcalcium carbonate, which are sparingly soluble hazards Many fruit trees and other cultivations are susceptible to injury from salttoxicity. Chloride, sodium and boron are absorbed by the roots andtransported to the leaves where they accumulate. In harmful amounts, theyresult in leaf burn and leaf necrosis. Moreover, direct contact duringsprinkling of Water drops with a high chloride content may cause leaf burnin high evaporation conditions. To some extent, bicarbonate is also symptoms of toxicity include premature leaf drop, reduced growthand reduced yield.

8 In most cases, plants do not show clear toxicityproblems until it is too late to remedy the and sodium ions are both present in the solution. Thus, it isdifficult to determine whether the damage caused is due to the one or tothe other. Chloride ions in high concentrations are known to be harmful tocitrus and many woody and leafy field crops. A chloride content exceeding10 meq/litre may cause severe problems to crops. The effect of sodiumtoxicity is not very clear. However, it has been found that it may causesome direct or indirect damage to many is an essential element to the plants. However, where present inexcessive amounts, it is extremely toxic, even at relatively very lowconcentrations of mg/litre. Toxicity occurs with the uptake of boronfrom the soil solution. The boron tends to accumulate in the leaves until itbecomes toxic to the leaf tissue and results in the death of the plant. Inarid regions, boron is considered the most harmful element in problems In addition to the moisture availability effect and the toxicity problemsto which the soluble salts contribute, certain salt constituents may interferewith the normal nutrition of various crops.

9 Bicarbonate ions in highconcentrations may affect the uptake of mineral nutrients and theirmetabolism in the plant. Chlorotic symptoms in sensitive plants may bedue to the direct or indirect effects of bicarbonate, an increase in nitrate contents, higher than 100 mg/litre, may affecttransplants and sensitive crops at the initial growth stage. However, nonegative effects have been reported in the last three decades fromfertigation with pure nitrogen concentrations in irrigation Water of about200 ppm. Although there is no doubt about the problem s existence, itseems that the main concern should be the nitrate content in the irrigationwater, when calculating the total nitrogen application, NO3equals (pure nitrogen).Effects of soluble salts on soil Sodium hazardA soil permeability problem occurs with a high sodium content in theirrigation Water . Sodium has a larger concentration than any other cationin saline Water , its salts being very soluble. Positively charged, it isattracted by negatively charged soil particles, replacing the dominantcalcium and magnesium cations.

10 The replacement of the calcium ions withsodium ions causes the dispersion of the soil aggregates and thedeterioration of its structure, thus rendering the soil impermeable to waterand air. The increase in the concentration of exchangeable sodium maycause an increase in the soil pH to above and reduce the availability ofsome micronutrients, iron and degree of absorption to the clay particles of the sodium depends onits concentration in the Water and the concentration of the calcium andmagnesium ions. This reaction is called cation exchange and it is areversible process. The capacity of soil to adsorb and exchange cations islimited. The percentage of the capacity that sodium takes up is known asthe exchangeable sodium percentage (ESP). Soils with ESP > 15 areseriously affected by adsorbed sodium problem is reduced if the amount of calcium plusmagnesium is high compared with the amount of sodium. This relation iscalled the sodium adsorption ratio (SAR) and it is a calculated value fromthe formula:(ions units meq/litre)2 MgCaNaSAR+=Pressurized irrigation TechniquesPressurized irrigation irrigation TechniquesChapter 7 Water quality for use of Water with a high SAR value and low to moderate salinitymay be hazardous and reduce the soil infiltration rate.