Last reviewed 16 Sept 2026 · 8 min read
Irrigation
Irrigation is the artificial application of water to land to meet the water needs of crops when rainfall is inadequate or badly distributed.
Necessity: insufficient or uneven rainfall; growing more than one crop a year; commercial and high-water-requirement crops; controlled water supply for higher yields.
Benefits: increased and stable food production; protection against famine; cash crops and higher farm incomes; hydropower, navigation and water supply from multipurpose projects; groundwater recharge; general prosperity.
Ill-effects (if mismanaged): waterlogging and salinity/alkalinity; breeding of mosquitoes (malaria); damp climate; loss of soil fertility by leaching; wastage of water.
Soil–water relations
| Term | Meaning |
|---|---|
| Saturation capacity | All pores filled; excess drains by gravity (gravitational water) |
| Field capacity (FC) | Moisture held after free drainage stops (usually 1–3 days after irrigation) — upper limit of useful water |
| Permanent wilting point (PWP) | Moisture at which plants can no longer extract water and wilt permanently |
| Available moisture | — water plants can use |
| Readily available moisture | The easily extracted part of available moisture; irrigation is normally given before this is used up |
| Hygroscopic water | Thin film held tightly by particles; unavailable to plants |
Depth of water stored in the root zone when soil moisture is raised from to field capacity (moisture contents by weight):
= depth of root zone; = dry unit weight of soil.
Frequency of irrigation (interval): , where = daily consumptive use rate.
Crop seasons in India
| Season | Sowing – harvesting (approx.) | Typical crops |
|---|---|---|
| Kharif | June/July – October | Rice, maize, jowar, bajra, cotton, groundnut, soybean |
| Rabi | October/November – March/April | Wheat, barley, gram, mustard, peas |
| Zaid | March – June (between rabi and kharif) | Watermelon, cucumber, vegetables, fodder |
Perennial or long-duration crops such as sugarcane span seasons.
Duty, delta and base period
- Base period (B) — total time between the first watering for preparing the land and the last watering before harvest (days).
- Crop period — time from sowing to harvesting (slightly longer than B).
- Delta (Δ) — total depth of water required by a crop during the base period (m or cm).
- Duty (D) — area (hectares) irrigated by a continuous flow of 1 cumec (m³/s) running throughout the base period (ha/cumec).
Approximate textbook deltas (actual values vary with climate, soil and method): rice about 120 cm, sugarcane about 90 cm, cotton about 50 cm, maize about 45 cm, wheat about 40 cm.
Factors affecting duty
Type of crop; climate and season (evaporation); rainfall; soil type and topography; method and system of irrigation (flow vs lift, surface vs sprinkler); canal conditions (lined canals — higher duty); quality of water; methods of cultivation; skill of cultivators and charging method (volumetric pricing encourages economy).
Improving duty
Proper ploughing and levelling; suitable irrigation methods; lining canals and watercourses; volumetric assessment of water rates; crop rotation; using good-quality water; training farmers.
- Duty is smallest at the head of the canal system (all losses still ahead) and largest at the field — i.e. duty increases as we move towards the field.
- Outlet discharge factor — duty at the head of a watercourse.
Consumptive use and irrigation requirements
- Consumptive use (evapotranspiration) — water used by plants in transpiration and building tissue plus evaporation from the soil and plant surfaces.
- Effective rainfall — the portion of rainfall usefully stored in the root zone and used by the crop.
- Consumptive irrigation requirement:
- Net irrigation requirement:
- Field irrigation requirement: ( = water application efficiency)
- Gross irrigation requirement: ( = conveyance efficiency)
Irrigation efficiencies
| Efficiency | Definition |
|---|---|
| Conveyance | Water delivered to the field ÷ water diverted into the canal at the headworks |
| Application | Water stored in the root zone ÷ water delivered to the field |
| Storage | Water stored in the root zone ÷ water needed to bring the root zone to field capacity |
| Distribution uniformity | ( = average absolute deviation of depths stored; = mean depth) |
| Water use efficiency | Crop yield ÷ water used (or water stored/delivered for the crop) |
| Overall (project) | (× distribution efficiency where used) |