Design capacity of a canal

The design capacity (full supply discharge) of a canal is fixed from the water demand of the area it serves, plus the losses.

Steps

1. Gross commanded area (GCA) and culturable commanded area (CCA) of the canal (CCA = GCA minus unculturable land such as roads, villages, rocky areas).
2. Cropping pattern and intensity of irrigation for each crop (percentage of CCA).
3. Water requirement of each crop: field irrigation requirement from Kor depth $\Delta$ and Kor period $B$ (or duty). Discharge per hectare $= \dfrac{\Delta\ \text{(m)}\times 10^4}{B\ \text{(s)}}$ m³/s/ha.
4. Crop discharge $=$ area under the crop $\times$ discharge per ha. Crops of different seasons are not added; the critical period (peak demand) governs the canal capacity.
5. Add losses: seepage and evaporation from the canal (about 4-5% of discharge per km for unlined canals or by Punjab / Davis-Wilson formulas) and other operational losses (usually 10-20%).
6. Design discharge at the head of each reach = demand of the area below + conveyance losses below. A margin (about 10%) for future needs is generally added.