Water Management and Irrigation
What you must understand
Know the soil water terms used in scheduling: field capacity, permanent wilting point, available water, infiltration, percolation and evapotranspiration (ET). Understand irrigation scheduling - when to irrigate and how much - using feel-and-appearance tests, tensiometers, evaporation pans and crop water-use tables. Know the methods: surface (furrow, basin), sprinkler, drip (trickle) and sub-irrigation, with their efficiency and suitability. Drip is the most efficient; flood furrow is the least.
Understand critical stages for water: flowering and grain filling in cereals, flowering and pod fill in legumes. Know the effects of waterlogging (root asphyxiation, denitrification, yellowing) and drought (wilting, stomatal closure, reduced photosynthesis, abortion of flowers). Know drainage: surface drains, cut-off drains, and subsurface drains. Understand water harvesting and conservation structures such as tied ridges and contour bunds.
Traps in questions
- Irrigating too often with small amounts wets only the surface and encourages shallow roots.
- Waterlogging causes nitrogen loss by denitrification, not leaching.
- Drip saves water but needs filtration and clean water.
- ET includes evaporation from soil plus transpiration from the crop.
- Scheduling by calendar alone ignores rainfall and crop stage.
Rule of thumb
Irrigate when about half the available water has been used, and apply enough to refill the root zone to field capacity without deep percolation.
Worked example
Available water is 18% by mass in a 0.3 m root zone with bulk density 1.3 g/cm³, giving about 70 mm. Irrigate at 50% depletion, so apply about 35 mm. With drip at 90% efficiency, gross requirement = 35 ÷ 0.90 ≈ 39 mm; with furrow at 55% efficiency, gross ≈ 64 mm.