Water Management, Irrigation and Drainage
Water Management, Irrigation and Drainage
This domain tests crop water requirement, irrigation scheduling, irrigation methods and drainage. Numerical questions often use evapotranspiration, effective rainfall, application efficiency and the conversion between millimetres and cubic metres.
Core rules and concepts
- Crop water requirement is the amount of water lost through evapotranspiration plus water needed for plant tissue; irrigation supplies the deficit between crop demand and effective rainfall or stored soil moisture.
- Net irrigation requirement = crop evapotranspiration − effective rainfall − soil moisture contribution. Gross irrigation requirement = net requirement ÷ application efficiency.
- 1 mm of water over 1 ha = 10 m³ of water. This conversion is central to pump and canal calculations.
- Drainage removes excess water from waterlogged fields and helps control salinity. Surface drainage, subsurface drainage and controlled drainage differ in cost and purpose.
Worked example
Over 7 days, crop evapotranspiration is 5 mm/day, so total ETc = 35 mm. Effective rainfall is 12 mm. Net irrigation requirement = 35 − 12 = 23 mm. If application efficiency is 70%, gross requirement = 23 ÷ 0.70 = 32.9 mm. Volume per hectare = 32.9 × 10 = 329 m³/ha. The reasoning is to subtract effective rainfall first, then divide by efficiency.
Traps that cost marks
Do not multiply net requirement by efficiency; divide it. Do not use total rainfall as effective rainfall. Do not confuse 1 mm over 1 ha with 1 m³; it is 10 m³. Drainage is not only for removing flood water; it also manages salinity and waterlogging.