Warabandi Turn with a Saline Tubewell
A 6 ha wheat field in a canal command receives a warabandi turn of 6 hours every 7 days at an outlet discharge of 80 L/s. The farmer also owns a tube well rated at 30 L/s, but its water has an EC of 1.6 dS/m; canal water EC is 0.25 dS/m. The field soil is sandy loam with a root-zone ECe of 3.8 dS/m and a field application efficiency of about 70%. Local wheat ET averages 4.5 mm/day, and the farmer irrigates when about 50% of available water is depleted. Last week the canal turn arrived 30 hours late, and the crop is now at crown-root initiation. The farmer proposes extending the canal turn by blocking neighbours’ outlets and skipping the tube well to save fuel. The watercourse is shared, and the canal department has warned against tampering. Rainfall forecast is 5 mm in the next 10 days.
1. Calculate the weekly crop water demand, the canal supply, and the deficit. How many hours should the tube well run to close the deficit?
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Crop demand: 4.5 mm/day × 7 = 31.5 mm/week. Over 6 ha: 31.5 mm × 60,000 m²/ha × 6 ha / 1000 = 1,890 m³/week net.
At 70% application efficiency, gross requirement = 1,890 / 0.70 = 2,700 m³/week.
Canal supply: 80 L/s × 6 h × 3,600 s/h = 1,728,000 L = 1,728 m³.
Deficit = 2,700 − 1,728 = 972 m³.
Tube-well runtime at 30 L/s: 972 m³ / 0.030 m³/s = 32,400 s = 9.0 h.
So the canal turn covers only about 64% of the gross weekly requirement. The tube well must run about 9 hours, preferably split into two or three applications, to avoid a yield-limiting deficit at crown-root initiation.
2. Assess the salinity risk of using the tube well at 1.6 dS/m. Is skipping the tube well and extending the canal turn technically justified?
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Wheat has a salinity threshold of about 6 dS/m ECe, and the soil is at 3.8 dS/m, so there is some margin. The leaching requirement for the tube well is approximately LR = ECw / (5ECe − ECw) = 1.6 / (5×6 − 1.6) = 1.6 / 28.4 ≈ 0.056.
That means about 5.6% extra water should be applied when using this tube well to prevent salt accumulation. The canal water, at 0.25 dS/m, provides a strong leaching fraction and will help flush salts.
Skipping the tube well is not technically justified. Extending the canal turn by blocking neighbours is a watercourse offence and creates equity and conflict problems; it also does not reliably deliver the missing 972 m³. The salinity risk from 9 hours of tube-well water is modest if the field is monitored and the canal turn is used for leaching. The main precaution is to avoid using the tube well alone during germination or early seedling stages, and to check root-zone ECe after the season.
3. Recommend a practical irrigation and monitoring plan for the next 10 days, considering the warabandi rules and the 5 mm rainfall forecast.
Read the worked answer
The plan should respect the warabandi and use the tube well only for the calculated deficit:
- Take the full 6-hour canal turn at the scheduled time and irrigate the most stressed or highest part of the field first.
- Run the tube well for about 9 hours, split into two 4.5-hour blocks, and apply the water to the same areas rather than spreading it thinly.
- Treat the 5 mm rainfall as only about 300 m³ over 6 ha at best; it does not remove the deficit, so do not rely on it.
- Monitor soil moisture at 0–30 cm and 30–60 cm before the next turn. If the next canal turn is again delayed by more than 24 hours, add a short tube-well run rather than tampering with the outlet.
- Sample soil ECe at the start of the next season. If ECe rises above about 5 dS/m, increase the canal-water leaching fraction and avoid tube-well use during sensitive stages.
- Report the late turn to the canal operator through the watercourse association; do not block other outlets.