Evaluation of Irrigation Water: 10 Main Criterions

This article throws light upon the ten main criterions for the evaluation of irrigation water. The criterions are: 1. Salinity Hazard or Total Soluble Salt Concentration 2. Sodium Hazard 3. Salt Index 4. Bicarbonate Hazard 5. Boron Concentration 6. Chloride Concentration 7. Soluble Sodium Percentage 8. Magnesium Hazard 9. Nitrate Concentration 10. Lithium.

Criteria # 1. Salinity Hazard or Total Soluble Salt Concentration:

The concentration of soluble salts in irrigation water can be classified in terms of electrical conductivity (EC) and expressed as + mSm^-1 (milliSiemens per meter) formally as micromhos + cm^-1 at 25°C (micromhos× 0.10 = + mSm^-1).

Criteria # 2. Sodium Hazard:

High concentrations of sodium are undesirable in water because sodium adsorbs onto the soil cation exchange sites, causing soil aggregates to break-down (de-flocculation), sealing the pores of the soil, and making it impermeable to water flow.

The tendency for sodium to increase its proportion on the cation exchange sites at the expense of other types of cations is estimated by the ratio of sodium content to the content of calcium plus magnesium in the irrigation water. This is called the sodium adsorption ratio (SAR).

The adjusted SAR is a value corrected to account for the removal of calcium and magnesium by their precipitation with bicarbonate and carbonate ions in the irrigation water added, giving higher values for “adjusted SAR” than for SAR. The sodium hazard of irrigation water as expressed through SAR does not take into account the effect of anionic composition.

Criteria # 3. Salt Index:

It is also used for predicting sodium hazard. It is the relation between Na⁺, Ca²⁺ and CaCO₃ present in irrigation water.

Salt index = (Total Na – 24.5) – [(Total Ca – Cain + CaCO₃) × 4.85]

where all quantities being expressed in ppm and all values of magnesium being reckoned as calcium.

The salt index is negative (-24.5 to 0) for irrigation water of high quality and any positive value of the salt index is harmful for irrigation purposes. The relative degree on both sides (negative and positive sides) depends on the magnitude of the “salt index” factor.

Criteria # 4. Bicarbonate Hazard:

The bicarbonate (HCO₃^–) anion is an important in irrigation water as regards calcium and to a lesser degree also of magnesium as their carbonates in the soil. This brings about a change in the soluble sodium percentage (SSP) in the irrigation water and therefore, an increase of the sodium hazard.

The residual sodium carbonate (RSC) is used to evaluate the quality of irrigation water and is expressed in + mel^-1,

RSC (mel^-1) = (CO₃^2- + HCOD₃^– – (Ca²⁺ + Mg²⁺)

RSC Value (mel^-1) – Water quality

<1.25 – Water can be used safely.

1.25-2.5 – Water can be used with certain management

> 2.5 – Unsuitable for irrigation purposes

Criteria # 5. Boron Concentration:

It is evident that boron is essential for the normal growth of the plant, but the amount required is very small. The occurrence of boron in toxic concentration in certain irrigation waters makes it necessary to consider this element in assessing the water quality. The permissible limits of boron in irrigation water are given in Table 16.1.

Criteria # 6. Chloride Concentration:

Since the chloride ion has no effect on the physical properties of a soil and is not adsorbed on the soil complex and so it has generally not been included in modern classification system. However, it can be used as a factor in some regional water classification.

Chloride (Cl^–) concentration (mel^-1) = Cl^–/CO₃^2- + HCO₃^– + HCO₄^2- + NO₃^–

Criteria # 7. Soluble Sodium Percentage (SSP):

Soluble Sodium Percentage (SSP) = Na × 100/Ca + Mg + Na

where all soluble cations are expressed in mel^-1

Irrigation waters having SSP value of 60 and above are considered as harmful.

Criteria # 8. Magnesium Hazard:

It is believed that one of the important qualitative criteria in judging the irrigation water is its magnesium content in relation to total divalent cations, since high magnesium adsorption by soils affects their physical properties. A harmful effect on soils appears when Ca: Mg ratio declines below 50.

Mg-adsorption ratio = Mg²⁺/Ca²⁺ + Mg²⁺

Magnesium hazard in irrigation water is expected having Mg: Ca ratio more than 1.

Criteria # 9. Nitrate Concentration:

Very frequently ground waters contain high amount of nitrate. When such type of irrigation water is applied on soils continuously various physical properties will be affected very badly which causes poor growth of the plants.

Criteria # 10. Lithium:

Lithium is an important trace element which may be found in most of saline ground-waters and irrigated soils. It has been found that small concentrations (0.05- 0.1 ppm) of lithium in irrigation water produced toxic effects on the growth of citrus crops. It has also been reported that saline soils of varying degrees found in India contain lithium up to 2.5 ppm.

Fortunately the germination of majority of crops including rice, wheat, barley etc. are not affected with this level of lithium content in soils. However, guidelines for the quality of irrigation water considering various criteria are presented in Table 16.2.

Quality of irrigation water can also be assessed by calculating the D value. The D value includes both salinity and alkalinity classes of irrigation water and also the textual variation of soils. D value can be calculated based on salinity, alkalinity and variation of soil textural classes. So, D value is more appropriate to assess the quality of irrigation water.

Total I can be calculated taking salinity (c) and alkalinity (s) classes from the figure 16.1 of quality of irrigation water after determining the value of SAR (alkalinity) and EC (salinity) of the irrigation water. Total II can be calculated by adding Total I and weightage value of soil textural variation as give.

Total II = Total I + Weightage value of soil textural class

Therefore, D value = 9 – Value of Total II

Rating of irrigation water based on D value:

Problem 1:

Adjusted SAR. 

where all cationic concentrations were expressed in mel^-1.

The SAR indicates the tendency for the soil to become higher in exchangeable sodium; higher SAR values mean higher exchangeable sodium percentages and lower soil permeability. If an irrigation water contains bicarbonate (HCO₃^–) and carbonate (CO₃^2-) ions, these will precipitate calcium and magnesium, which increases the SAR.

The formula, taking into account these changes, is called the adjusted SAR. It is defined as:

and pH_c = (pk₂‘-pk_c‘)+p[HCO₃^–] + p[ Ca²⁺ + Mg²⁺]

where (pk₂‘ – pk_c‘) = Concentration of Ca²⁺, Mg²⁺ and Na⁺ ions

p( HCO₃^–) = Concentration of carbonate and bicarbonate

p(Ca²⁺ + Mg²⁺) = Concentration of Ca²⁺ and Mg²⁺ ions

Calculate the adj SAR for an irrigation water with these ions contents: 7 mel^-1 of Ca²⁺, 2 mel^-1 of Mg²⁺, 5 me^-1 of Na⁺, 4 mel^-1 of HCO₃^–, and a total ionic concentration of 14 mel^-1.

Solution:

Interpolation to get values between those listed in the above table, (pk{ – pk_c‘) at 14 mel^-1 total ions = about 2.30.

The p( Ca²⁺ + Mg²⁺) = (7 + 2) + mel^-1 = 9 mel^-1 = 2.35,

and p (HCO₃^–) = 4 mel^-1 = 2.40.

Therefore,

pHc = (pk₂‘–pk_c‘) + p[HCO₃^–] + p[Ca²⁺ + Mg²⁺]

= 2.30 + 2.40 + 2.35 (by putting the values derived from interpolation) = 7.05

= 2.36 [1 + 1.35]

= 2.36 × 2.35 = 5.55.

So adj. SAR of the irrigation water is 5.55 rather than the uncorrected SAR of 2.36.

Problem 2:

An irrigation water contains carbonate (CO₃^2-), bicarbonate (HCO₃^–), calcium (Ca²⁺) and magnesium (Mg²⁺) ions 1, 4, 2.5 and 1.5 mel^-1 respectively. Calculate the residual sodium carbonate (RSC) content of the irrigation water and give comments for the use of irrigation purposes.

Solution:

RSC = (CO₃^2- + HCO₃^–) – (Ca²⁺ + Mg²⁺)

= (1+ 4)-(2.5+ 1.5) = 5-4= 1.0

So the residual sodium carbonate content of the irrigation water is 1.0 and by fitting with the RSC scale of irrigation water classes (mentioned in the text) it is found suitable for irrigation purposes.

Problem 3:

An irrigation water having EC value of 450 mSm^-1(milliSiemens per metre) contains calcium (Ca²⁺) and magnesium (Mg²⁺), 2.0 and 1.0 mel^-1 respectively. Calculate (i) the concentration of sodium (Na⁺) in met¹ and soluble sodium percentage (SSP) of the irrigation water (ii) SAR of the irrigation water. Give comments on the irrigation water.

Solution:

From the equation it is found,

Na⁺ = EC/100 – (Ca²⁺ + Mg²⁺)

where

EC = mSm^-1

Na⁺ = Ca²⁺ and Mg²⁺ concentration = mel^-1

Now we get,

The irrigation water has no sodium hazard and it can be safely used for the irrigation purposes.

Problem 4:

An irrigation water contains 414, 120 and 24 mgl^-1 of sodium (Na⁺), calcium (Ca²⁺) and magnesium (Mg²⁺) respectively. Calculate (a) total cation concentration in mel^-1, (b) SAR value of the irrigation water, (c) approximate EC value in mSm^-1, (d) osmotic pressure of the irrigation water containing such soluble cations and (e) total dissolved salts in + mg l^-1 of the irrigation water.

Solution:

We know,

(i) Total dissolved or soluble salts (mgl^-1) = EC (dSm^-1) × 640

(ii)

where all concentrations are expressed in mel^-1

(iii) Osmotic Pressure (OP) = EC (dSm^-1) × 0.36 (in bars or atmospheres)

(iv) Total cation concentrations (mel^-1) = EC (dSm^-1) × 10

(a) 414 mgl^-1 Na⁺ = 414/23 mel^-1Na⁺ = 18 mel^-1

120 mgl^-1 Ca²⁺ = 120/20 mel^-1 Ca²⁺ = 6 mel^-1

24 mgl^-1 Mg²⁺ = 24/12 mel^-1 Mg²⁺ = 2 mel^-1

Therefore, the irrigation water contains total cations

= (18 + 6 + 2) mel^-1 = 26 mel^-1

= 18/2 = 9

Therefore, the irrigation water has a SAR value of 9.

(c) EC + (dSm^-1) =Total Cation (mel^-1)/10 = 26/10 = 2.6

We know,

1 dSm^-1 at 25°C = 100 mSm^-1 at 25°C

1 µmhos cm^-1 = 0.1 mSm^-1

Therefore, the irrigation water has an EC value of (2.6 × 100) mSm^-1

= 260 + mSm^-1 or 2600 µmhos cm^-1.

(d) Osmotic Pressure (OP) in bars or atmosphere

= EC (dSm^-1) × 0.36

or

EC (mSm^-1) × 10-2 × 0.36

= 260 (mSm^-1) ×1/100 x 0.36

= 260/100 × 0.36 = 0.936

Therefore, the irrigation water has an osmotic pressure of 0.936 bars or atmospheres.

(e) Total dissolved or soluble salts (mgl^-1)

= EC (dSm^-1) × 640

= EC (mSm^-1) × 10^-2 x 640

= 260 ×1/100× 640= 1664

Therefore, the irrigation water contains total dissolved or soluble salts of 1664 mgl^-1.

Problem 5:

An irrigation water contains 1920 mgl^-1 soluble salts and drainage water contains 2560 mgl^-1 soluble salts. Calculate the electrical conductivities of both irrigation and drainage water in mSm^-1 and leaching requirement (LR) of the irrigation water in percentage.

Solution:

Total soluble salts (mgl^-1) in irrigation water

= EC_iw (mSm^-1) × 10^-2× 640

1920 = EC_iw × 1/100 × 640

.^.. EC_iW (mSm^-1) = 1920 ×1920 × 100/640 = 300

Again, EC_dw (mSm^-1) = 2560 × 100/640 = 400

Therefore, leaching requirement (LR) of the irrigation water (in percentage)

= EC_iw/EC_dw × 100 = 300/400 × 100 = 75