A Canal Turn, a Pump and a Wheat Crop
A 4 ha wheat field has sandy loam soil with field capacity 24% (w/w), permanent wilting point 10%, bulk density 1.45 g/cm³ and an effective root zone of 90 cm. The allowable depletion is 55% of total available water. Current crop evapotranspiration is 5.5 mm/day, and 8 mm of rain is forecast in the next 12 days. The canal turn gives 6 cm depth every 12 days, but the farmer's 5 hp pump can lift from a tube well at 22 m total head with an overall efficiency of 60%. He also has an old sprinkler system rated at 2.5 kg/cm²; the pump can deliver about 37 m³/h. Wheat is at crown-root initiation. The farmer wants to skip the canal turn, run the pump for 24 hours and then wait for rain.
1. Calculate total available water, allowable depletion and the net irrigation depth. Is the canal turn of 6 cm enough for the next 12 days? Show your assumptions.
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Calculations
Total available water (TAW) = (field capacity − wilting point) × bulk density × root depth.
In depth terms: (0.24 − 0.10) × 1.45 × 900 mm = 182.7 mm.
Allowable depletion (RAW) = 0.55 × 182.7 = 100.5 mm.
Crop water use over 12 days = 5.5 × 12 = 66 mm. Forecast rain = 8 mm, so net demand is about 58 mm after rain is credited.
Is 6 cm enough?
Six centimetres is 60 mm. If that is the gross depth delivered to the field and field application efficiency is about 70%, the net depth is only 42 mm. With 8 mm of rain, the net supply is about 50 mm against a 58 mm demand - a deficit of about 8 mm, and a larger deficit if the canal turn is late. Even at 100% efficiency, 60 + 8 = 68 mm barely covers 66 mm, leaving no margin for leaching or uneven application.
Assumption
The 6 cm is treated as gross depth at the field inlet; if the farmer measures 6 cm in the basin after standing water, the conclusion changes and a flow measurement is needed. On this evidence, the canal turn should not be skipped; it should be used and supplemented.
2. Will the existing pump and sprinkler system work? Calculate the pump discharge and the time needed to apply 60 mm over 4 ha, and advise on the sprinkler pressure.
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Pump discharge
Hydraulic power = ρgQH. For a 5 hp pump, shaft power is about 3.73 kW. At 60% overall efficiency:
Q = (3730 × 0.60) / (1000 × 9.81 × 22) = 0.0104 m³/s ≈ 37 m³/h.
That agrees with the stated 37 m³/h.
Time to apply 60 mm over 4 ha
Volume = 0.060 m × 40,000 m² = 2,400 m³.
Time = 2,400 / 37 = 64.9 hours, not 24 hours. In 24 hours the pump delivers about 888 m³, which is only 22 mm over 4 ha. The farmer's plan would apply roughly one-third of the intended depth and would still leave the crop short.
Sprinkler pressure
The sprinkler needs 2.5 kg/cm², which is about 25 m of head. The pump's total head is 22 m including lift and friction, so the pressure available at the sprinkler is below 22 m - well short of 25 m. The sprinkler will not perform at its rated pressure and nozzle flow. Options are a booster pump, a lower-pressure sprinkler set, or converting to furrow irrigation. Simply running the existing pump longer will not correct the pressure deficit.
3. Give a 12-day irrigation plan that uses the canal turn, the pump and the forecast rain. State what you would monitor and what you would change if the rain does not arrive.
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Plan
- Take the canal turn. Apply the 6 cm as soon as the turn starts, preferably in the morning, and divide the field so that the lower end is not over-irrigated. Do not wait for the forecast rain; wheat at crown-root initiation is sensitive to moisture stress.
- Supplement with the pump. The canal water at 70% efficiency supplies about 42 mm net. To reach the 58 mm net demand, add about 16–20 mm net, i.e. 640–800 m³ over 4 ha. At 37 m³/h this is 17–22 hours of pumping. Apply it after the canal water has soaked in, not at the same time, to avoid runoff.
- Use the rain if it comes. If 8 mm of effective rain falls, reduce the pump supplement to about 8–12 mm. Do not count the full forecast as effective; sandy loam may store most of 8 mm, but a short shower may evaporate.
- Do not attempt sprinkler operation at 2.5 kg/cm² with this pump. Use the pump for furrow or basin supplement until a booster or low-pressure sprinkler is available.
Monitoring
Check soil moisture at 30 cm and 60 cm by hand feel or a tensiometer before and after each application. Watch for runoff at the tail end and for leaf rolling. Record the canal turn date and pumping hours.
If rain fails
Apply the full 20 mm net supplement and bring the next pump run forward. If the next canal turn is delayed beyond 12 days, stretch the interval by applying a smaller depth more frequently with the pump rather than mining the soil to wilting point.