Water Quality Parameters and Irrigation Suitability

Water quality for agricultural irrigation is evaluated based on the total concentration of dissolved salts, the relative proportion of sodium to other cations, the concentration of potentially toxic elements, and the presence of bicarbonate ions. Poor quality irrigation water directly deteriorates soil physical and chemical properties, impairs crop water uptake through osmotic pressure effects, and causes specific ion toxicities that reduce crop yields.

• Total Dissolved Solids (TDS): Measured either directly by evaporation or indirectly through electrical conductivity (EC).
• Electrical Conductivity (EC): Expressed in deciSiemens per meter (dS/m) or micromhos per centimeter (μmhos/cm) at 25°C.
• pH Range: Ideal irrigation water pH ranges between 6.5 and 8.4; extreme values cause nutrient fixation and pipe corrosion.
• Suspended Solids: High silt load can clog micro-irrigation emitters and accumulate undesirable sediments on agricultural land.

Salinity hazard is primarily evaluated by the electrical conductivity of water (EC_w). High salinity increases the osmotic pressure of the soil solution, making it difficult for plant roots to extract water, leading to physiological drought even when soil moisture appears adequate.

• Low Salinity (EC_w < 0.25 dS/m): Class C1; suitable for almost all crops and soils with little likelihood of salinity development.
• Medium Salinity (0.25 - 0.75 dS/m): Class C2; can be used if moderate leaching occurs; plants with moderate salt tolerance can be grown.
• High Salinity (0.75 - 2.25 dS/m): Class C3; cannot be used on soils with restricted drainage; management for salinity control is required.
• Very High Salinity (> 2.25 dS/m): Class C4; generally unsuitable for ordinary irrigation except for highly salt-tolerant crops under special leaching management.

Sodicity hazards relate to the relative proportion of sodium ions (Na^+) compared to calcium (Ca²⁺) and magnesium (Mg²⁺) ions. When irrigation water with high sodium is applied continuously, it displaces calcium and magnesium on the soil exchange complex, causing clay dispersion, surface crusting, and drastic reduction in infiltration rate and hydraulic conductivity.

• Sodium Adsorption Ratio (SAR): Calculated mathematically as SAR = Na^+√Ca²⁺ + Mg²⁺2} where ionic concentrations are expressed in milliequivalents per liter (meq/L).
• Residual Sodium Carbonate (RSC): Calculated as RSC = (CO_3^2- + HCO_3^-) - (Ca²⁺ + Mg²⁺) in meq/L. High RSC causes precipitation of calcium and magnesium carbonates, leaving residual sodium behind.
• Classification of RSC: Safe (< 1.25 meq/L), Marginal (1.25 - 2.5 meq/L), and Unsuitable (> 2.5 meq/L).

Certain ions present in irrigation water are toxic to sensitive crops even at low concentrations, causing leaf burn, defoliation, and reduced photosynthetic activity. These specific toxicities must be monitored rigorously in intensive agricultural command areas.

• Boron Toxicity: Essential micronutrient but toxic in small excesses. Sensitive crops show toxicity at < 0.5 mg/L, while tolerant crops can withstand up to 2.0 - 4.0 mg/L.
• Chloride Toxicity: Common in saline groundwaters; accumulates in woody perennial crop leaves causing marginal necrosis.
• Sodium Toxicity: Particularly damaging in sprinkler irrigation where foliage absorbs sodium and chloride directly through wet leaves, causing leaf scorch.
• Heavy Metals and Trace Elements: Elements such as cadmium, lead, arsenic, and nickel present in industrial effluents render water unfit for crop production and pose severe food safety risks.

When superior quality surface water is unavailable, saline or sodic groundwaters can be utilized effectively through specific agronomic and engineering interventions to sustain crop productivity without permanent land degradation.

• Gypsum Application: Used to neutralize high RSC and supply calcium to lower the effective SAR of irrigation water or soil.
• Leaching Requirement (LR): Defined as the fraction of applied irrigation water that must pass through the root zone to control soil salinity at a specified level (LR = EC_wEC_e).
• Amending Strategies: Conjunctive use of canal water and saline groundwater in cyclic or blended modes to keep salinity thresholds below critical crop limits.
• Cultural Practices: Growing salt-tolerant crops such as barley (Hordeum vulgare), cotton (Gossypium hirsutum), and sugarbeet (Beta vulgaris); adopting drip irrigation to maintain high matric potential in the root zone.