In every method of irrigation, many solvent salts are mixed in the supply of water.
Production of these salts like sodium chloride, gypsum, sodium bicarbonate and sodium nitrate takes place during weathering of minerals from nature. Quantity of salts in groundwater depends on the nature of the source of water.
When surface water infiltrates, minerals keep dissolving with it. Chemical deposition in water depends on source and pH value of water, nature of solvent gases, rate of weathering of rocks, soil geology, nature of soil profile, existence of quantity of salts, solution, water planning, hydrolysis, evaporation and rainfall etc.
In areas where productivity is continuously falling down, use of saline water is resorted to in irrigation. Sodium, magnesium, calcium, chloride, sulphate and bicarbonate are the main ingredients of irrigation water. Apart from these, boron, lithium, silicon, iron, manganese and fluorine are also found in small quantities at some places. They are harmful for growth of boron plants.
Characteristics Determining Water Quality:
The characteristics of irrigation water that appear to be most important in determining its quality, which however depend on climatic conditions, irrigation practices, soil-water receptivity characteristics, crop tolerance, depth to water table and agronomic practices, etc., are the following:
1. Salinity hazard (total concentration of soluble salts); Electrical Conductivity (EC).
2. Specific ion toxicity hazard (Ionic composition):
(a) Major constituents (Sodium, Magnesium, Chloride, Bicarbonate, Carbonate, Silica, Nitrate)
(b) Minor constituents (Boron, Lithium, Fluorine and other micro toxicants)
3. Sodicity hazard (Relative proportion of sodium to other actions); Sodium Adsorption Ratio (SAR).
4. Alkalinity hazard (Bicarbonate concentration as related to the concentration of calcium plus magnesium); Residual Sodium Carbonate (RSC).
In addition to the above individual parameters, combined evaluation of the two parameters given below are also of practical importance:
5. EC and SAR
6. SAR and RSC
1. Salinity Hazard:
The total concentration of soluble salts is the single most important criterion which has been used conventionally for determining the quality of irrigation water. It is measured quantitatively in terms of ‘Electrical Conductivity (EC), because this is very closely related with the sum of major actions (or anions) determined by chemical analysis, and because this correlates well with the value of total dissolved solids, as well as osmotic potential.
The EC of a water sample is determined by measuring the electrical resistance between two parallel electrodes immersed in the solution. The EC is also called ‘specific conductivity’ or the conductance per unit cross-sectional area and across unit distance.
2. Specification Toxicity Hazard:
Sodium is the most abundant chemical in all natural waters whereas, its presence is least desirable. Excess of sodium ions characterises that the water is saline or alkaline depending upon its occurrence in association with chloride/sulphate or carbonate/bicarbonate ions, and accordingly the irrigated soil may develop salinity or sodicity problem. For some time in the past, the quality of irrigation water used to be evaluated with respect to sodium on the basis of Soluble Sodium Percentage (SSP).
Soils and plants are adversely affected by high sodium in irrigation water. Sodium soils are relatively impermeable to air and water. They are hard when dry, difficult to tilt, and plastic and sticky when wet. These adverse physical conditions retard or prevent germination and are generally unfavourable for plant growth.
Magnesium is the second most abundant anion usually found in high salinity waters. However, in low salinity waters, calcium dominates over magnesium ions. Obviously, it can be stated that with an increase in EC of water, Mg Ca ratio tends to increase. Magnesium is known to affect plant growth mainly by reducing calcium uptake and causing calcium deficiency. However, when high concentrations of magnesium occur together with high concentrations of calcium, there is no specific ion toxicity effect due to magnesium (Meiri and Shalhavet, 1973).
The occurrence of chloride ions in natural irrigation water increases with an increase in EC and sodium ions. Therefore, these ions are most dominant in very high salinity waters. Unlike sodium ions, neither do the chloride ions have any effect on the physical properties of the soil nor are they absorbed by the soil.
Bicarbonates and Carbonates:
Bicarbonates occur in low salinity waters and their concentration usually decreases with an increase in EC. The proportion of bicarbonate ions is higher than calcium ions and have been considered to be undesirable because after evaporation of irrigation water, bicarbonate ions tend to precipitate calcium ions.
Silicon is the more abundant element in igneous rocks than some other types of deposits. Most silica in water is probably derived from the decomposition or metamorphism of silicate minerals rather than from a solution of quartz, as quartz is one of the rock minerals most resistant to attack by water.
Many waters contain less than 10 mgL-1of silica; those that drain deposits high in silicate minerals, particularly feldspars, often contain up to 60 mgL-1and are not commonly found. It is believed that most silica is present in a non-ionsed form, but ionized silicate(s) is undoubtedly present in some waters.
Although nitrates are highly soluble, natural waters contain only small quantities, usually less than 10 mgL-1. In Rajasthan and Uttar Pradesh, groundwater in some districts of Rajasthan has been shown in Table 7.2. Legumes take nitrogen from the atmosphere and fix it in the soil as nitrate. Nitrates in water may result from leaching of soils and rocks, from fertilizers, normal decomposition of plants and animals, sewage, industrial effluents, and living organisms.
The nitrates occurring in saline waters from 2 to 5 meqL-1have been observed to cause beneficial effect on the plant growth and partly help in the concentration of boron ranges usually from traces to 5 mgL-1in natural groundwater’s. However, in very high salinity waters sometimes it can occur up to 10 mgL-1 (Gajbhiye et al., 1973).
It increases with an increase in EC and SAR of the waters. In sea water, it occurs up to the extent of 4.8 mgL-1In salt affected soils, boron may occur up to 10 mgL-1 (Bhargava et al., 1974). It differs from the other constituents of water in that it occurs as unionized boric acid. It is essential to plant growth but may prove exceedingly toxic at concentrations slightly above optimum.
Lithium is present in some minerals but is not abundant in nature. The more common minerals which contain from 2 to 10 per cent lithium oxide are petalite, spodumene, lepidolite and amblygonite. It is generally found in soils developed from mica minerals and is more frequent in arid regions. Although surface waters generally do not contain lithium more than 0.3 mgL-1 lake brines have been reported to contain Li up to 16 mgL-1
Unlike chlorides, fluorides are only sparingly soluble and are present in most natural waters in only small amounts. Calcium fluoride (fluorite) is the principal source of fluoride, but there are some other complex fluoride rich areas that are underlain by crystalline rocks at places known to contain fluorspar and apatite, the main sources of fluoride in groundwater.
The element is often characteristic of waters from deep strata and is frequently found in salt water from oil wells and in water from areas that have been subjected to recent volcanism. The concentration of fluoride ranges from traces to more than 10 mgL-1in natural waters, although surface waters generally do not exceed 0.3 mgL-1 unless they are polluted from other sources.
Apart from boron, lithium and fluorine, there are some more trace elements, which if present in irrigation waters may also cause a toxic effect. Maximum concentrations of all trace elements in irrigation waters including B, Li and F, as recommended by National Academy of Science and National Academy of Engineering (1972), are shown in Table 7.2. These concentrations are based on the protection of soils for plant production under long continued use of water. Criteria for short-term is also suggested for soils that have high capacities to inactivate these trace elements.
3. Sodicity Hazard:
Sodium Adsorption Ratio (SAR):
Apart from the total salinity, the next important consideration is whether the use of irrigation water of a given quality will cause sodicity in the soil. Water which might be considered suitable for irrigation on the basis of EC, may not be suitable if sodium predominates. In the past, sodic (alkali) soil was defined as one in which the exchangeable sodium percentage was greater than 15 and pHs (pH of the saturated soil paste) values usually ranged between 8.5 and 10.
It implies that the quality of irrigation water should be such that the irrigated soil does not exceed ESP value of 15 and pHs value of 8.5 However, now the sodic soil is defined as one in which SAR is greater than 15. Obviously, the irrigation water which does not increase SAR of the soil beyond 15, is considered to be of good quality.
Sodium to Calcium Activity Ratio (SCAR):
The application of SAR to the group of waters which have EC higher than 5 dSm-1and Mg/Ca ratio higher than 1, is obviously questionable. About 35 per cent of the groundwater occurring in 11 arid districts of Rajasthan have EC greater than 5 dSm-1and nearly all waters are dominated by magnesium over calcium. Although some waters have Mg/Ca ratio as high as 16, the majority falls between 2 and 4.
Gupta and Abichandani (1970) examined field condition, the effect of irrigation with high salinity water having Mg/Ca ratio as 5.4 and SAR 8, on a deep sandy to sandy loamy soil, containing CaC02 from 1 to 2 per cent. Although SAR of the irrigated soil was low and in line with SAR of the irrigation water, ESP values were much higher than expected, than the SAR of irrigation water or irrigated soil.
4. Alkalinity Hazard:
Residual Sodium Carbonate:
Eaton (1950) gave the concept of residual sodium carbonate (RSC) and pointed out that development of alkali soils (saline or non-saline) may be expected when irrigation water containing COs + HCOs higher than Ca + Mg is used for irrigation, provided that it is used so sparingly that little leaching occurs.
In water containing high concentrations of biocarbonate ion, there is tendency for calcium and to some extent for magnesium to precipitate as carbonates as the soil solution becomes more concentrated. On the other hand, if sodium is not too greatly in excess of calcium-l-magnesium and enough of such water is used that the drainage water continues to contain a substantial proportion of Ca and Mg, there may be no ill effects.
Reddy and Raddy suggested new classification of irrigation waters.
In the classification of irrigation waters, it is presumed that the water will be used under average conditions with respect to several factors which influence its quality in relation to its use. Whether the salts dissolved in irrigation water will accumulate in the soil in injurious amounts and affect plant growth, depends on one or more of the several parameters discussed.
Large deviations from the average for one or more of the variable may make it unsafe to use what under average conditions would be a good water; or may make it safe to use what under average conditions would be a water of doubtful quality. This relationship to average conditions has therefore, to be kept in consideration with the use of any general method for the classification of irrigation waters.
5. Salinity (EC):
The classification of irrigation waters with respect to salinity hazard on the basis of EC is based primarily on the development of salinity in the soil to the extent that the yield is affected adversely.
On the basis of electrical conductivity the irrigation water may be classified into six salinity classes:
1. CO, Non-saline waters (EC<0.2 dSm-1) may create a severe permeability problem in the soil because infiltration rate into the soil is adversely affected due to lack of salts in the water to such an extent that the crop is not adequately supplied with water, and yield is reduced.
2. C-1, Normal water (EC 0.2-1.5 dSm-1) can be used for irrigation of most crops and of most soils with little likelihood that soil salinity will develop. Large areas of citrus trees (very sensitive plants) have developed. They have been irrigated for many years with waters having EC 1.5 dSm-1. These waters do not have any leaching requirement. These waters are generally relatively high in Ca and low in chlorides. There is seldom any problem of high SAR/SCAR or of RSC/RSBC and toxic elements for water falling under this group.
3. C-2, Low salinity waters (EC 1.5-3.0 dSm-1) can be used if a moderate amount of leaching occurs due to the current irrigation practices. Most of the crops except sensitive ones comprising some horticultural and leguminous plants can be grown on all soils except very heavy textured ones, with impeded drainage. These waters do not have any significant problem of SAR/SCAR and toxic elements but may have some problem due to RSC/RSBC.
4. C-3, Medium salinity waters (EC 3.0-5.0 dSm-1) can be used of soils provided with good drainage such that leaching fraction is not less than 0.3. Most semi-tolerant and tolerant crops can be grown under good management. These waters may have a moderate problem of high SAR/SCAR and RSC/RSBC but seldom a significant problem of toxic elements. Interestingly, there is evidence to show that optimum yields of many tolerant crops are obtained under irrigation with waters belonging to this EC group.
5. C-4, High salinity waters (EC 5.0-10.0 dSm-1) are suitable for irrigation of soils provided with very good drainage so that leaching fraction is always greater than 0.3 and ECe does not exceed ECiw. Only salt tolerant crops can be grown under good management. Higher the total and seasonal rainfall during the crop growth period, better the usability. These waters generally may have a significant problem of medium to high SAR/SCAR but no problem due to RSC/RSBC. However, high SAR (>30) values disqualify these waters form satisfactory use for prolonged periods. Summer fallowing is sometimes necessary for effecting desalinisation, and amendments may also have to be used to take care of hazards due to sodium and/or toxic elements.
6. C-5, Very high salinity waters (EC> 10 dSm-1). These waters are directly not suitable for irrigation but may be used in cycle or in conjunction with low salinity waters, for example, canal waters.
6. Sodicity (SAR/SCAR):
Among the soluble salts or irrigation water, sodium is considered to be the most hazardous. The water which might be considered as suitable according to salinity may not be suitable if sodium predominates. The classification of irrigation waters with respect to sodic hazard on the basis of SAR/SCAR is established primarily on the increase of exchangeable sodium and its effect on the physical condition of the soil.
Since, ESP of the soil is equal to SAR of the soil in equilibrium in turn with SAR of the water when ECiw and EQ are less than 5 dSm-1, Mg to Ca ratio is less than one, SAR of the irrigation water directly reveals whether it is excessive for any crop and if it will deteriorate the physical properties of the soil. When ECiw is higher (5-10 dSm-1) and Mg/Ca ratio is also higher (2-4) then SCAR will give a better idea than SAR if ESP is to be depended on for soil appraisal and/or crop performance.
On the basis of SAR/SCAR, the irrigation waters may be classified in six classes:
1. S-0, Non-sodic waters (SAR/SCAR < 5) can be used for irrigation of almost all soils, for all crops even those sensitive to sodium such as stone-fruit trees or wood trees.
2. S-1, Normal waters (SAR/SCAR 5-10) can be used for irrigation on almost all soils with little danger of development of harmful levels of exchangeable sodium for growing all crops except some stone-fruit trees or wood trees which are specially sensitive to sodium. These waters do not have any requirement for leaching and/or amendments.
3. S-2, Low sodicity waters (SAR/SCAR 10-20) can be used for crops which are semi-toleranf’ or tolerant to sodium on almost all soils so that LF is around 0.3. If soils contain gypsum or calcium carbonate, these waters can be used more successfully. EC should not be less than 1 to 2 dSm-1lest permeability problem develops.
4. 5-3, Medium sodicity waters (SAR/SCAR 20-30) can be used only for crops which are tolerant to sodium on soils provided with good drainage such that LF is always greater than 0.3. If soils contain gypsum or calcium/carbonate and if rainfall is appreciable and effective, these waters can be used very successfully. EC should not be less than 2 to 3 dSm-1lest permeability problem develops.
5. S-4, High sodicity waters (SAR/SCAR 30-40). These waters are directly unsuitable for irrigation without drastic treatment.
The classification of irrigation waters with respect to bicarbonate or carbonate alkalinity hazard on the basis of RSC or RSBC is based primarily on the precipitation of CA/or Mg and pairing of residual CO3 or HCO3 with sodium and formation of Na2C03 in the soil and increasing SAR/ESP, characterizing it as alkali soil. Since, appreciable amount of carbonates occurs only in the waters that have high pH (>8.5), RSC should be calculated for high pH waters and RSBC for low pH (<8.5) waters. These water, also cause a permeability problem in the soil.
On the basis of RSC/RSBC, the irrigation water may be classified in six classes:
1. A-0, Non-alkaline waters (RSC/RsBC-ye) can be used for irrigation on almost all soils, for all crops for indefinitely long periods without any problem.
2. A-1, Normal water (RSC/RSBC 0 meqL-1) can be used for irrigation on almost all soils for all crops even those very sensitive to carbonates or bicarbonates. These waters may not create any permeability problem.
3. A-2, Low-alkalinity water (RSC/RSBC tr-2.5 meqL-1) can be used for irrigation on almost all soils for all crops. These waters may not create any permeability problem unless drainage is impeded, rainfall is unduly low and evaporation is very high.
4. A-3, Medium-alkalinity waters (RSC/RSBC2.5-5.0 meq L-1) can be used for irrigation on almost all soils with little danger of the development of harmful levels of alkali for growing all crops except those which are specifically sensitive to carbonate or bicarbonates. Optimum yields of several alkali tolerant crops are obtained when RSBC is in this range.
5. A-4, High-alkalinity waters (RSC/RSBC 5.0-10.0 meqL-1) can be used for irrigation on soils provided with good drainage such that LF is not less than 0.3, for growing semi-tolerant and tolerant crops to sodium. EC should be <3.0 dSm-1 and SAR <10. Rainfall should be appreciable and effective (>400 mm) and evaporation must be low (<2000 mm) for the prolonged successful utilization of such waters. If SAR is > 10, use of gypsum may be required.
6. A-5, Very high-alkalinity waters (RSC/RSBO 10.0 meqL-1) These waters are directly not suitable for irrigation but may be used in cycle or in conjunction with low alkalinity waters or with the use of amendments such as gypsum. High alkalinity with preponderance of carbonates associated with high EC and SAR/SCAR, prohibits the use of these waters for irrigation.