Irrigation and Water Management
What the exam asks
Irrigation and Water Management tests water requirement, evapotranspiration, irrigation scheduling, critical stages, methods and efficiency. Numerical items often convert depth of water to volume and account for application losses.
Core rules
- Water requirement includes crop transpiration, evaporation and unavoidable losses; irrigation requirement is the part supplied by irrigation after rain and soil storage.
- Critical stages include crown-root initiation in wheat, panicle initiation in rice, flowering and pod formation in pulses, and formative growth in sugarcane.
- Surface methods include flooding, furrow, basin and border; sprinkler and drip are pressurized methods. Drip saves water but needs filters and maintenance.
- Application efficiency = water stored in the root zone ÷ water delivered. Distribution uniformity matters as much as total volume.
- Drainage prevents waterlogging and salinity; over-irrigation can leach nutrients and reduce aeration.
Worked example
A 0.5 ha field needs 60 mm net irrigation depth. Application efficiency is 75%. Gross depth = 60 ÷ 0.75 = 80 mm. Volume = 0.5 ha × 10,000 m2 ha−1 × 0.080 m = 400 m3. Reasoning: first correct for efficiency, then convert millimetres to metres and multiply by area. If the farmer applies 400 m3 but distribution is uneven, parts of the field may still be under-irrigated.
Traps
Do not equate 1 mm of water with 1 m3 ha−1: 1 mm over 1 ha is 10 m3. Net irrigation is not gross irrigation. A critical stage is not every growth stage. More water is not always better: waterlogging can reduce yield.