Almost all irrigation waters contain some soluble salts in them. In some cases the amounts of salts are so much that the water is not suitable for irrigation. In some situations, even though the salt content of the water is low, continued irrigation may gradually build up the salt content in the rootzone.
Irrigation water quality is commonly assessed in terms of soluble salt content, percentage of sodium, boron and bicarbonates contents. The major dissolved constituents in natural waters are the salts of sodium, calcium and magnesium as their chloride, sulphate, carbonate and bicarbonates.
Potassium and nitrate ions are also present sometimes. Some other elements like boron, lithium, silicon, iron, manganese, fluorine, aluminium, lead, zinc, vanadium and chromium occur in minor quantities. Occurrence of these elements affects the quality of water for drinking purposes.
Assessing Water Quality:
The total salt content of the water is expressed using the Electrical Conductivity (EC) of the water.
EC of a water sample can be measured by using standard Wheatstone Bridges or Direct Indicating Bridges which are more common. The Direct Indicating Bridge is operated by alternating current and makes use of a cathode ray tube null indicator.
The temperature of the solution is set on the temperature compensating dial. The conductivity cell is dipped in the water sample and the null indicator is adjusted as per the directions given with the particular instrument. The EC is now read directly. The accuracy of calibration of the bridge is tested from time to time using standard solutions.
There are several attempts for classifying irrigation waters on the basis of their chemical characteristics for their suitability to irrigate different crops. However, no single classification holds true under all conditions.
This is mainly because of the large number of factors that affect the suitability of irrigation water. These include soil properties like texture, structure, pH, clay mineral etc., crops and their variety, agronomic practices, drainage and climatic factors. The criteria for the suitability of irrigation water are fixed for the particular location.
The U.S. Salinity Laboratory (USSL) classified the irrigation waters as given in Fig. 16.3. The diagram is based on electrical conductivity and the sodium adsorption ratio.
C1, C2, C3 and C4 refer to low salinity, medium salinity, high salinity and very high salinity of water. S1, S2, S3 and S4 refer to low, medium, high and very high sodium adsorption ratio of the water.
Low salinity water (C1) can be used for irrigation with most crops and most soils. Some leaching is required. Medium salinity water (C2) can be used with crops that can tolerate moderate salinity and leaching. High salinity water (C3) can be used with adequate drainage and salt tolerant crops.
Very high salinity water (C4) is not ordinarily suitable for irrigation but to be used under special conditions with good drainage and with salt tolerant crops. Low sodium water (S1), medium sodium water (S2), high sodium water (S3) and very high sodium water (S4) present the sodium problems in the same order.
It has been reported by several scientists that further classes must be added to the U.S. Salinity Laboratory Classification system as in several instances it has been found that even using high salinity waters successful crops have been grown. The limits fixed by US Salinity Laboratory are not acceptable under all conditions.
On the basis of studies carried out by different scientists in India, a water quality rating as given in Table 16.3 is proposed.
Table 16.4. indicates the general tolerance of different crops for salt tolerance.
Ayres and Westcot (1989), after reviewing the available information, provide guidelines for interpreting water quality for irrigation as in Table 16.5.
It is stated that these recommendations are for site conditions with soil texture ranging from sandy loam to clay loam with good internal drainage and located in arid and semi-arid regions of the world. It is also assumed that no uncontrolled shallow watertable is present within 2 m of the surface.
Crop Tolerance to Salts:
All plants are not affected by salinity in the same manner. Some plants are able to make needed osmotic adjustments enabling them to extract more water from a saline soil. Depending on their tolerance and effects on yields, crops could be grouped into three categories as indicated in Table 16.4. Fig. 16.4 shows the relative salt tolerance of agricultural crops in terms of relative crop yield.
Crops are affected by salts present in the soil as well as in the irrigation water. Tolerance during germination and early seeding stage may be different and these factors need to be understood in managing salinity.
The tolerance of crops for total salts, sodium, boron and other toxic constituents would vary not only with the crop variety but also with the amount of nitrate in the water, the application of fertilizers and manures, and agronomic practices like seedbed preparation, planting time, etc.
Residual Sodium Carbonate:
Presence of carbonate and bicarbonate in irrigation water results in the precipitation of calcium and magnesium present in the soil and thus increases the extent of sodium hazard. To assess the sodium hazard due to carbonate and bicarbonate of irrigation water, it is generally assumed that all the calcium and magnesium would precipitate as carbonates. The residual sodium carbonate (RSC) is expressed as follows – 
Where, the concentration of ions are expressed as meq/l. Based on field experience, the water with more than 2.5 meq/l RSC is not suitable for irrigation purposes. Waters containing 1.25 to 2.5 meq/l RSC are considered marginal, and those containing less than 1.25 meq/l RSC are considered safe.
Adjusted Sodium Adsorption Ratio:
Excessive sodium in irrigation water causes soil dispersion and structural breakdown when sodium exceeds calcium by more than a ratio of 3 : 1. This is due to lack of sufficient calcium to counter to dispersing effects of sodium. Excessive sodium also causes problems like soil crusting, poor seedling emergence and low infiltration.
The SAR values given in Table 16.5 gives an indication of the sodium hazard. In case of irrigation waters, in order to take into consideration the influence of calcium in the soil water system, a modified value of SAR is used to assess the quality of irrigation water. It is referred to as adj RNa (adjusted sodium adsorption ratio) and is given by the following equation –
To use the Cax table (Table 16.6), first determine the HCO3 to Ca ratio (HCO3/Ca) and ECW from the water analysis, using HCO3 and Ca in meq/l and the water salinity (ECw) in decisiemens per metre. An appropriate range of calculated HCO3/Ca ratios appears on the left side of the table and the range of ECw across the top.
Find the HCO3/Ca ratio that falls nearest to the calculated HCO3/Ca value for the subject water and read across to the ECw column that most closely approximates the ECW for the water being evaluated. The Cax value, shown represents the meq/l of Ca that is expected to remain in solution in the soil water at equilibrium and is to be used in equation (16.6).
The adj RNa values predict more correctly the potential infiltration problems due to sodium in irrigation water.
Boron Concentration:
Boron is needed for normal growth of all plants, but the quantity required is very small. When present in excess quantities, boron is toxic to plants. Table 16.7 presents the permissible limits of boron for several plant species.
Example:
An analysis of an irrigation water sample gives the following information –
Comment on the suitability of water for irrigation.
Solution:
From the classification chart given by U.S. Salinity Lab. Staff in Fig. 16.3, the irrigation water can be classified as high salinity water (C3) and low sodium water (S2) and accordingly this water cannot be used on soil with restricted drainage Even in soil with adequate drainage, special management for salinity control may be required and crops with good salt tolerance to salinity should be sown.
The RSC value of irrigation water is 3.3 meq/l, which is very high according to U.S. Salinity Lab. Staff and is unsuitable for irrigation. Practical experience in India has shown that this water can be used for irrigation successfully in soils with good drainage for semi-tolerant crops. However, gypsum will have to be applied to soil occasionally.