Irrigation In Crop Production

Irrigation is the artificial application of water to the soil to supplement natural precipitation and ensure optimum crop growth. Water requirement (WR) of a crop refers to the total quantity of water needed for crop production from sowing to harvesting, including consumptive use (evapotranspiration, ET), metabolic needs, and water losses during application (such as deep percolation and run-off).

• Consumptive Use (CU): The quantity of water used by the vegetative growth of a crop through transpiration and evaporation from adjacent soil surfaces, expressed in depth units (mm or cm).
• Duty of Water: The area of land that can be irrigated by a continuous supply of one cusec (cubic foot per second) of water throughout the base period of the crop, expressed in hectares per cusec (ha/cusec).
• Delta of Water (\Delta): The total depth of water (in cm or meters) required by a crop from sowing to harvest, distributed across all irrigation events.
• Relationship between Duty (D), Delta (\Delta), and Base period (B):
  \Delta (meters) = 8.64 * B (days) / D (hectares/cusec)

Applying irrigation at moisture-sensitive physiological stages is critical for maximizing crop yield and water-use efficiency (WUE). Moisture stress during these periods causes irreversible yield reduction.

• Wheat (Triticum aestivum): Crown Root Initiation (CRI) at 20-25 days after sowing (DAS) is the most critical stage. Other stages include Tillering (40-45 DAS), Late Jointing (60-65 DAS), Flowering (80-85 DAS), Milk stage (105 DAS), and Dough stage (120 DAS).
• Maize (Zea mays): Tasseling and Silking are the most critical stages; moisture stress here drastically reduces grain setting.
• Rice (Oryza sativa): Panicle Initiation (PI), Flowering, and Milking stages are highly sensitive to water stress. Continuous submergence of 5 cm depth is generally maintained from transplanting to 10 days before harvest.
• Chickpea (Cicer arietinum): Pre-flowering and Pod-formation stages are critical.
• Sugarcane (Saccharum officinarum): Formative phase (tillering stage) is the most critical; drought during this phase halts cane elongation.

Selection of an appropriate irrigation method depends on topography, soil texture, water availability, and crop type.

• Surface Irrigation Methods: Includes flooding, check basin, border strip, and furrow methods. Best suited for medium- to heavy-textured soils with gentle slopes.
• Sprinkler Irrigation: Overhead application mimicking natural rainfall. Highly efficient for undulating topography, sandy soils, and closely spaced crops like wheat, pulses, and oilseeds. Saves 30-50% water compared to surface methods.
• Drip (Trickle) Irrigation: Application of water directly to the root zone drop by drop. Achieves the highest water-use efficiency (80-95%) and is ideal for wide-row horticultural and vegetable crops (e.g., tomato, Lycopersicon esculentum; banana, Musa paradisiaca).
• Sub-surface Drainage and Irrigation: Controlled water table management used in waterlogged or heavy clay soils.

Accurate measurement of irrigation water is essential for efficient farm water management. Common devices include Parshall flumes, V-notch weirs, and rectangular weirs.

• Soil Moisture Tension: Tensiometers are used to schedule irrigation in sandy to medium soils at low tension (0 to 0.8 bar), while resistance blocks are used for higher tensions.
• IW/CPE Ratio: Irrigation Water (IW) to Cumulative Pan Evaporation (CPE) ratio is widely used for scientific irrigation scheduling. An IW/CPE ratio of 0.75 or 1.0 means irrigation is applied when cumulative pan evaporation equals 1.33 or 1.0 times the depth of irrigation water applied (usually 50 mm).
• Depletion of Available Soil Moisture (ASM): Irrigation is typically scheduled when 50% of the Available Soil Moisture is depleted in the root zone.

Excess irrigation or poor natural drainage leads to waterlogging, soil salinization, and sodicity, severely impairing crop growth.

• Waterlogging Effects: Depletion of oxygen in the root zone, accumulation of toxic carbon dioxide, reduction in nutrient uptake (especially nitrogen and phosphorus), and shallow root development.
• Surface Drainage: Removal of excess surface water through field ditches, land grading, and bed-and-furrow systems.
• Sub-surface Drainage: Installation of perforated pipes or tile drains at predefined depths to lower the high water table and leach out excess soluble salts.
• Reclamation of Saline-Sodic Soils: Use of gypsum ( ext{CaSO}_4 \cdot 2 ext{H}_2 ext{O}) as an amendment to replace exchangeable sodium with calcium, followed by heavy leaching of salts through ponding.