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)

2 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.

3 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.

4 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).Pressurized irrigation TechniquesPressurized irrigation information on the physical and biological evaluation is given inbrief as compared to the chemical evaluation.

5 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.

6 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. 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.

7 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.

8 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.

9 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. Plant roots take in Water but absorb very little salt from the soilsolution. Similarly, Water evaporates from the soil surface but salts remainbehind.

10 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.