Irrigation, Water Management and Drainage
What this domain tests
Water requirement, irrigation scheduling, methods of application, irrigation water quality, drainage and management of waterlogged or saline soils.
Core rules
- Water requirement = consumptive use (transpiration plus evaporation) plus conveyance and application losses. It is expressed in mm; 1 mm over one hectare equals 10 m³.
- Scheduling: the IW/CPE approach irrigates when cumulative pan evaporation reaches a chosen fraction of pan evaporation (commonly 0.8 for many field crops, 1.0–1.2 for hardy crops). Tensiometers and direct soil-moisture checks refine the decision.
- Critical stages that must not be missed: paddy at panicle initiation and flowering; wheat at crown-root initiation and grain filling; cotton at squaring and flowering; sugarcane at the formative stage (90–120 days); groundnut at pegging and pod development.
- Methods: surface (flood, furrow, basin, ring), sprinkler for light or undulating land, and drip for wide-row crops, orchards and vegetables. Drip saves 30–60% water and allows fertigation.
- Quality: check EC, sodium adsorption ratio and residual sodium carbonate; high RSC water needs gypsum, saline water needs leaching plus drainage.
- Drainage: surface drains remove standing water; subsurface pipe drains lower the water table. Waterlogging shows yellowing, shallow roots, iron toxicity in paddy and salt crusting at the surface.
Worked example
Pan evaporation over five days totals 25 mm and the crop's IW/CPE ratio is 0.8. Reasoning: irrigation depth = 0.8 × 25 = 20 mm. Per hectare that is 20 × 10 = 200 m³ (200,000 litres). A furrow stream of 30 litres per second needs 200,000 / 30 ≈ 6,667 seconds, about 1 hour 51 minutes per hectare.
Traps
Treating 1 mm of irrigation as 1 m³ per hectare; irrigating by calendar without checking evaporation or soil moisture; applying one IW/CPE ratio to every crop; and assuming a saline water source is safe because the soil looks black and fertile.