Irrigation
Sources, scheduling, crop water requirements, methods and water quality.
What irrigation does
Irrigation is the artificial application of water to soil to meet crop water needs when rainfall is insufficient or uneven.
Sources
Wells and tube wells (groundwater) are the largest source in India, followed by canals, tanks and other sources such as rivers and ponds.
Crop water requirement
Water requirement is the total water needed to grow a crop from sowing to harvest. It covers evapotranspiration (ET) plus unavoidable losses and special needs such as puddling in rice. Water requirement is often expressed as a depth (cm or mm). Useful conversion: 1 cm of water over 1 hectare = 100 m³ (because 10,000 m² × 0.01 m).
Scheduling
Scheduling means deciding when and how much to irrigate. Approaches:
- Soil moisture depletion (irrigate when a set fraction of available water is used).
- Critical growth stages: stages when water stress causes the biggest yield loss, e.g. crown root initiation (CRI) in wheat (about 21 days after sowing), flowering and grain filling in most crops.
- Climatological approach (IW/CPE ratio: irrigation water depth ÷ cumulative pan evaporation).
- Plant indicators such as wilting or leaf colour.
Methods
- Surface: flooding, check basin, border strip, furrow, ring basin (for orchards). Cheap but less efficient.
- Sprinkler: water sprayed like rain; suits uneven land and sandy soils.
- Drip (trickle): water delivered drop by drop at the root zone; highest efficiency (about 90%), saves water, suits orchards, vegetables and sugarcane; fertigation possible.
Water quality
Poor-quality water has high salts, high sodium or toxic ions like boron. Quality is judged by electrical conductivity (salinity), sodium adsorption ratio (SAR) and residual sodium carbonate (RSC). Continued use of salty water creates saline or sodic soils.
Key points
- Groundwater is India's main irrigation source.
- CRI is the most critical stage for irrigating wheat.
- Drip has the highest application efficiency.
- 1 cm depth on 1 ha = 100 m³ of water.
- SAR and EC judge irrigation water quality.
Exam tips
- Practise depth-to-volume conversions.
- Remember which methods suit which crops: furrows for row crops, basins for orchards, flooding for rice.