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Duty is the area a unit discharge can irrigate — 1800 hectares per cumec is a typical figure for a kharif crop. Net discharge is area divided by duty; the canal head must carry more than that because seepage and evaporation take a share along the way, so the gross figure is net divided by (1 - loss fraction).
Discharge from duty
Net discharge (cumec) = command area (ha) / duty (ha per cumec)
Gross at head
Gross = net / (1 - conveyance loss); volume = gross x 86,400 x rotation days
One cubic metre per second, equal to 1000 litres per second or about 35.31 cusecs. Volumes are often quoted in million cubic metres (MCM) for a full rotation or season.
Unlined earthen canals commonly lose 20-40% of head discharge to seepage and evaporation. Lined canals reduce this to 5-15%, and piped distribution networks lower again.