Irrigation and Water Resources Engineering
Irrigation and water resources engineering covers crop water requirement, duty and delta, canal design, diversion weirs, dams, cross-drainage works and waterlogging. Questions often reduce to duty–delta conversions and regime channel formulas.
Core rules
- Duty D is the area irrigated per unit discharge, usually ha/cumec; delta Δ is the depth of water required over the base period.
- Δ (m) = 8.64 B/D, where B is base period in days and D in ha/cumec.
- Irrigation efficiency = water stored in root zone / water diverted; conveyance, application and distribution losses reduce it.
- Kennedy’s theory: critical velocity V = C m^0.64, with C about 0.55 for Indian canals.
- Lacey’s regime perimeter P = 4.75√Q; silt factor f = 1.76√dmm.
- Bligh’s creep and Khosla’s theory are used for weir floor design; dams are classified by material and hydraulic action.
Worked example
A crop has duty 1200 ha/cumec and base period 150 days. Delta = 8.64 × 150/1200 = 1.08 m. This means 1.08 m depth over the irrigated area is needed across the season. If duty improves to 1500 ha/cumec, delta falls to 0.864 m, showing the inverse relation.
Traps
Inverting duty and delta; using crop period instead of base period; forgetting the 8.64 conversion; treating Lacey’s P as area; applying Kennedy’s formula with m in centimetres.