First Irrigation of Wheat on a Light Soil with a Weak Canal Turn
A farmer in Etawah has 1.0 ha of sandy loam - field capacity 24% by volume, permanent wilting point 10%, bulk density 1.45 g/cm³ - under wheat var. HD-2967 sown on 22 November after rice. The canal outlet runs on a 21-day rotation. The 6 December turn delivered only a thin flow for two hours, enough to wet perhaps one-third of the field. His own tube well discharges 25 litres per second, but three-phase power is available for only about six hours a day. The root zone at crown-root initiation is 60 cm. Rainfall since sowing is 4 mm; daily evaporation is around 3 mm. On 12 December the crop is at 4–5 leaf and the farmer is unsure whether to irrigate from the tube well now or wait for the canal turn on 27 December, because he has been told that December irrigation causes lodging and that the first irrigation can be delayed to save water.
1. Calculate the plant-available water, the safe depletion limit, and the depth and running hours needed for one irrigation. Use these figures to decide whether to irrigate on 12–13 December.
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Step 1 - available water capacity. AWC = (field capacity − wilting point) × root depth = (0.24 − 0.10) × 600 mm = 0.14 × 600 = 84 mm in the 60 cm root zone.
Step 2 - safe depletion. Wheat is usually irrigated at about 50% depletion of AWC: 0.5 × 84 ≈ 42 mm. Allowing for the 4 mm of rain and three days of evaporation at 3 mm/day, the deficit by 12–13 December is close to that limit, and the crop is exactly at crown-root initiation - the most sensitive irrigation stage.
Step 3 - water and time required. A net 45 mm over 1 ha = 450,000 litres = 450 m³. At 70% application efficiency the gross requirement is about 640 m³. At 25 L/s (90 m³/hour) that is roughly 7 hours of pumping - more than the six-hour power window.
Decision. Irrigate now, using the tube well, but plan the application in two consecutive six-hour blocks, or finish the field in alternate furrows on the second day. Waiting until 27 December would push the crop past CRI with the profile near wilting point on a soil that holds only 84 mm; that is a yield loss of 20–30% for the sake of one saved watering. The 6 December canal flow was too uneven to count as an irrigation.
2. Plan the full irrigation schedule to harvest, showing which water source serves which stage and which irrigation you would drop first if water runs short.
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Stage priority, not a fixed calendar. For wheat the sensitive stages are crown-root initiation (20–25 days), flowering (about 85–95 days) and milk (about 105–115 days); tillering and dough are the first to be sacrificed.
Schedule:
- CRI, 12–13 December - tube well, about 45 mm net, split over two power windows.
- Tillering, early January - canal turn if it arrives; if not, skip and monitor.
- Jointing, late January - tube well or canal, 45 mm.
- Flowering, mid-February - canal water reserved for this stage; do not risk it on a late turn.
- Milk, early March - 40 mm, canal or tube well according to the turn.
- Dough, late March - optional on this light soil, only if the profile is dry.
Reasoning behind the sequencing. The canal is unreliable, so it should be spent on flowering, where a shortfall is least recoverable. The tube well is limited by power, so it should cover CRI and jointing, which fall between canal turns.
If water runs short, drop the dough irrigation first, then tillering. Never drop flowering. Record the actual canal arrival each cycle - a 21-day rotation in practice drifts by several days, and planning to the nominal date is how farmers miss CRI.
3. Evaluate the farmer's belief that December irrigation causes lodging, and state what else he should watch after irrigating.
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The belief confuses cause and correlation. Lodging in wheat comes from excessive nitrogen, tall varieties, dense stands, wind and over-irrigation late in the season - not from a correctly timed CRI irrigation. A 45 mm application at crown-root initiation improves tillering, root development and ultimately grain number; withholding it on a soil with only 84 mm of available water is a false economy.
Where the caution is valid: heavy applications of 80–100 mm in one go, standing water for more than a day, and irrigation combined with high nitrogen and strong wind. On this sandy loam, 45 mm will infiltrate within a few hours, so the lodging risk is low.
After irrigating, check:
- Evenness - the two-hour canal flow left one-third of the field wet; the tube well application must cover the rest or the field stays patchy.
- Surface crusting and emergence of Phalaris and other weeds in the moist strips; a light hoeing at 30–35 days helps.
- Nitrogen top dressing: split it, and apply the second dose just before the jointing irrigation rather than on wet soil where volatilisation and leaching losses are high.
- Yellowing in low patches, which signals waterlogging rather than drought - do not irrigate those spots again.