D = duty in hectares/cumec

∆ = total depth of water supplied in metres

B = base period in days

If we take a field of area D hectares, water supplied to the field corresponding to the water depth ∆ metres will be = ∆ x D hectares-metres.

$= D \times ∆ \times 10^4$ cubic-metres. .... (1)

Again for the same field of D hectares, one cumec of water is required to flow during the entire base period. Hence, water supplied to this field.

$ = (1) \times (B \times 24 \times 60 \times 60) m^3$ .... (2)

Equating Equations (1) and (2), we get

$D \times ∆ \times 10^4 = B \times 24 \times 60 \times 60$

$∆ = (B \times 24 \times 60 \times 60)/(D \times 10 4) = 8.64 B/D metres.$