Irrigation and Water Management
What the domain tests
Irrigation and Water Management tests crop water requirement, irrigation scheduling, methods, efficiency and drainage. Numerical questions often convert irrigation depth to volume or compare application methods.
Core rules
- Crop water requirement is the water needed for transpiration and tissue building; it is met by effective rainfall, soil moisture and irrigation.
- Evapotranspiration (ET) combines evaporation from soil and transpiration from plants. Irrigation scheduling can use cumulative pan evaporation (IW/CPE ratio) or soil-moisture depletion.
- Critical stages are the growth stages most sensitive to water stress, such as flowering and grain filling.
- Methods: surface (flood, furrow, basin), sprinkler and drip. Drip saves water and places it near roots; sprinkler suits closely sown crops and light soils.
- Irrigation efficiency is the proportion of applied water used beneficially; losses include evaporation, deep percolation, runoff and conveyance.
- Drainage removes excess water; waterlogging reduces oxygen in the root zone.
Worked example
A 1 ha field needs 50 mm of irrigation. Volume = area x depth = 10,000 m2 x 0.05 m = 500 m3. Reasoning: 1 mm of water over 1 ha equals 10 m3, so 50 mm equals 500 m3. If application efficiency is 80%, water to apply = 500/0.80 = 625 m3.
Traps
Do not equate rainfall with effective rainfall; some is lost as runoff or evaporation. Do not apply the same schedule to all crops: stage, soil and weather matter. More irrigation is not always better; over-irrigation leaches nutrients and causes waterlogging. Drip is not suitable for every crop and layout without design.