Irrigation is the artificial application of water to land to support crop growth. India irrigates over 90 million hectares — the largest in the world — fed by an extensive canal network, groundwater extraction, and an increasingly large micro-irrigation footprint. For civil engineers, irrigation design involves canal hydraulics, water requirement estimation, and water distribution systems.
Types of Irrigation Systems
| Type | Method | Efficiency | Best For |
|---|---|---|---|
| Surface / Flood | Water spread over field from ditches | 40–60% | Rice, sugarcane, flat fields |
| Border Strip | Long, narrow strips bounded by low ridges | 60–70% | Wheat, fodder crops |
| Furrow | Water flows in small channels between rows | 60–75% | Row crops — maize, cotton |
| Sprinkler | Pressurised water through nozzles | 75–90% | Undulating terrain, horticultural crops |
| Drip / Trickle | Water applied at root zone through emitters | 85–95% | Fruits, vegetables, water-scarce areas |
| Subsurface | Water delivered below soil surface via porous pipes | 90–95% | High-value crops |
Water Requirement Concepts
Crop Water Requirement (CWR)
CWR = Evapotranspiration (ET) of the crop under ideal conditions. Estimated by:
ET_crop = ET₀ × Kc
- ET₀ = Reference evapotranspiration (Penman-Monteith method, IS:1535)
- Kc = Crop coefficient (varies with crop growth stage)
Delta (Δ)
Total depth of water required by a crop during its entire growing season (mm or cm).
| Crop | Delta (cm) — Approximate India | Base Period (days) |
|---|---|---|
| Rice (kharif) | 120 | 120 |
| Wheat (rabi) | 40 | 120 |
| Sugarcane | 120 | 365 |
| Cotton | 60 | 180 |
| Gram (chana) | 30 | 90 |
Duty of Water (D)
Area of land (hectares) that can be irrigated by a continuous supply of 1 cumec (m³/s) of water throughout the base period.
D = 8.64 × B / Δ
where B = base period (days), Δ = delta (metres)
Example: Wheat, B = 120 days, Δ = 0.40 m → D = 8.64 × 120 / 0.40 = 2592 hectares/cumec
Relationship: Duty, Delta, and Base Period
Δ = 8.64 × B / D
This is the fundamental irrigation identity. Higher duty = more area per unit water = better irrigation efficiency.
Canal Irrigation System Hierarchy
- Head works / Diversion weir: Diverts river water into main canal
- Main canal: From head works, not used directly for irrigation
- Branch canal: Branches from main canal, supplies distributaries
- Distributary (minor): Supplies field channels (moghas)
- Field channel / Watercourse: From distributary outlet to field
Canal Design Theories
Kennedy's Theory (1895)
Based on observations on Upper Bari Doab Canal, Punjab. Proposes a regime velocity (Vo) that keeps silt in suspension without deposition or erosion:
Vo = C × y^0.64
- Vo = critical velocity (m/s)
- y = depth of flow (m)
- C = critical velocity ratio (CVR): 0.7–1.3 depending on silt grade (standard = 0.91–1.02 for Punjab silt)
Design: Assume depth y → calculate Vo → calculate area A = Q/Vo → width B = A/y − side slopes → check m = B/y ratio
Limitation: Does not consider side slope or bed slope explicitly.
Lacey's Regime Theory (1929)
More rigorous, based on 5 regime equations:
- V = 10.8 × R^(2/3) × S^(1/2) (Lacey's flow equation, like Manning's)
- V = 0.439 × √(f × R) (velocity-hydraulic radius relationship)
- f = 1.76 × √d₅₀ (silt factor, d₅₀ in mm)
- P = 4.75 × √Q (wetted perimeter)
- R = 0.47 × (Q/f)^(1/3) (hydraulic radius)
where f = Lacey's silt factor, R = hydraulic radius, Q = discharge.
Design steps: Given Q and f → find P, R → trapezoidal section → find V, S
Lacey's Silt Factor (f) Values
| Soil/Silt Type | d₅₀ (mm) | f |
|---|---|---|
| Very fine silt | 0.04 | 0.35 |
| Fine silt | 0.10 | 0.56 |
| Standard silt (Punjab) | 0.32 | 1.00 |
| Coarse silt | 0.50 | 1.25 |
| Coarse sand | 1.00 | 1.76 |
Worked Example — Canal Design by Lacey's Theory
Given
- Design discharge Q = 25 m³/s
- Silt: standard Punjab, d₅₀ = 0.32 mm → f = 1.0
Solution
Wetted perimeter: P = 4.75 × √25 = 4.75 × 5 = 23.75 m
Hydraulic radius: R = 0.47 × (25/1.0)^(1/3) = 0.47 × 2.924 = 1.374 m
Area A = P × R = 23.75 × 1.374 = 32.63 m²
Velocity V = Q/A = 25/32.63 = 0.766 m/s
Bed slope: from Lacey S = f^(5/3) / (3340 × Q^(1/6)) = 1 / (3340 × 1.71) = 1/5711 ≈ 1 in 5700
For trapezoidal section (SS = 1/2H:1V), B = (P − 2y√(1+ss²)) = solve → B ≈ 17.5 m, y ≈ 1.8 m
Waterlogging and Drainage
Waterlogging occurs when water table rises to within 1.5–2.0 m of ground surface, suffocating crop roots (lack of aeration).
Causes: Over-irrigation, seepage from canals, poor surface drainage, impermeable sub-strata.
Prevention: Lined canals (reduces seepage by 50–80%), proper drainage network, controlled irrigation scheduling, groundwater pumping.
Irrigation Efficiency Types
| Efficiency Type | Definition | Typical Value |
|---|---|---|
| Water conveyance efficiency (Ec) | Water delivered / Water diverted | 60–90% |
| Water application efficiency (Ea) | Water stored in root zone / Water delivered | 50–80% |
| Water use efficiency (Eu) | Water used by crop / Water applied | 60–90% |
| Overall project efficiency (Ep) | Ec × Ea × Eu | 30–50% |
Frequently Asked Questions
What is the difference between Kennedy's and Lacey's theory?
Kennedy's theory uses only hydraulic depth and an empirical velocity, ignoring bed slope design. Lacey's theory gives a complete set of regime equations to determine velocity, slope, wetted perimeter, and hydraulic radius — making it more versatile and widely used for Indian canal design. IS:7112 recommends Lacey's method.
What does a higher duty of water indicate?
Higher duty means more hectares irrigated per cumec — indicating more efficient use of water. Improvement in irrigation efficiency (lining canals, precise water scheduling, drip irrigation) increases duty. India's target is to improve overall irrigation efficiency from 35–40% to 60%+ under PMKSY scheme.