Duty & Delta & The Canal's Promise to the Crop
Sizes canals
Enter the duty of water and the base period to get the delta — the depth of water the crop needs — in metres and millimetres.
Set your canal & command area
Next: plan for a duty of 1,000 ha/cumec and a delta of 1,037 mm; if crop demand exceeds the delta, raise discharge or shrink the command area.
Duty (8.64 × B ÷ Δ) and delta vary with soil, crop and conveyance losses; field duty is lower than head duty after canal seepage.
Duty & delta — key facts
- Delta
- Δ = 8.64 × B ÷ D (metres)
- Duty
- Area per cumec (ha/cumec)
- Delta
- Depth crop needs per season
- Base period (B)
- Days first to last watering
- 1 cumec for 1 day
- = 8.64 m over 1 ha
- Higher duty
- Smaller delta (fixed B)
- Use
- Sizes canals & command areas
- Privacy
- Runs in your browser; nothing uploaded
Two numbers that size a whole canal system
In canal irrigation, two figures connect the discharge at the head to the land it can serve. The duty is the area, in hectares, that one cumec of water can irrigate over a crop's whole base period; the delta is the total depth of water that crop needs over the same period. They are tied together by Δ = 8.64 × base period ÷ duty, where the 8.64 comes from spreading one cumec flowing for a day over a hectare. Get one and you have the other.
This tool computes the delta in metres and millimetres from the duty and base period, and reports the duty so you can design from either end. Use it to size canals and command areas, to compare crops on how thirsty they are, and to check that a given discharge can reliably command the area you intend. Pair it with the Tank Command Area and Channel Flow tools to plan the full conveyance system.
Size the command area
Duty fixes how much land a cumec can serve.
Know the crop's thirst
Delta is the depth of water a season demands.
One clean relation
Δ = 8.64 × base period ÷ duty, both ways.
Design from either end
Work from duty to delta or delta to duty.
Frequently Asked Questions
What is the duty of water in irrigation?+
Duty is the area of land a unit discharge can irrigate over the whole base period of a crop — expressed in hectares per cumec (ha/cumec). A higher duty means one cumec of canal water serves more land, so the same headworks can command a larger area. It is one of the central design figures in canal irrigation.
What is the delta of a crop?+
Delta is the total depth of water a crop needs over its base period, usually given in metres or millimetres. If you imagine all the water supplied to a field spread evenly over its surface, the depth of that layer is the delta. Each crop has a typical delta — rice is high, while wheat and pulses are much lower.
How are duty and delta related?+
They are linked through the base period: Δ = 8.64 × B ÷ D, where Δ is delta in metres, B is the base period in days and D is the duty in ha/cumec. The constant 8.64 comes from converting one cumec flowing for one day into a depth over one hectare. So as duty rises, delta falls for the same base period.
What is the base period?+
The base period is the number of days between the first and last watering of a crop in a season — essentially how long the crop is drawing irrigation. It appears directly in the duty–delta relation, so using the correct base period for your crop is essential to getting a sensible delta or duty.
Why does the duty–delta relation matter?+
It lets engineers size canals and command areas. Knowing the duty fixes how much area a given discharge can serve; knowing the delta fixes how much depth of water each hectare needs. Together they connect the canal capacity at the head to the total area that can be reliably irrigated downstream.
Where does the constant 8.64 come from?+
One cumec is one cubic metre per second. Over one day that is 86,400 m³. Spread over one hectare (10,000 m²) that depth is 8.64 metres, and over the base period of B days it becomes 8.64 × B metres per hectare. Dividing by the duty (ha served per cumec) gives the delta per hectare.
Does a higher duty always mean less water per hectare?+
For a fixed base period, yes — a higher duty spreads the same cumec over more land, so each hectare receives a smaller depth, i.e. a smaller delta. That is why low-water crops like wheat and pulses have high duties, while heavy-water crops like rice have low duties and large deltas.
Can I work backwards from delta to duty?+
Yes. The relation rearranges to D = 8.64 × B ÷ Δ. If you know the crop's delta and base period you can find the duty the canal must achieve, which then tells you the command area a given discharge will cover. This tool reports both so you can design from either end.
Are these values reliable for design?+
The duty–delta relation is a clean engineering identity, so the maths is exact. The judgement lies in choosing realistic delta, duty and base-period values for your crop, soil and climate, and in allowing for conveyance losses between the head and the field — outlet duty is lower than field duty because of these losses.