The relationship between duty (D), delta (Δ), and base period (B) is:

D = 8.64B/Δ where D = duty in hectares/cumec (land irrigated per unit discharge), B = base period in days (duration of crop from sowing to harvest), Δ = delta in metres (total depth of water needed for crop). 8.64 is the conversion factor (86400 s/day × 10⁻⁴ ha/m²). Higher Δ → lower D (more water per unit area = less area served per cumec).

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