Duty of canal water — select the correct set of statements about definition, location for calculation, and outlet factor in irrigation practice.

Introduction / Context:
“Duty” links discharge to the command area irrigated during a crop’s base period. Recognizing where it is evaluated and how losses change its value along the system is crucial for canal planning and water allocation.

Given Data / Assumptions:

• Open canal system with conveyance losses along its length.
• Base period defined for the crop under consideration.
• Outlet discharge factor refers to duty at the distributary outlet.

Concept / Approach:

Duty = Area irrigated per unit discharge during the base period. Because seepage and evaporation reduce flow downstream, the duty calculated at the head differs from that at outlets. The outlet duty is often referred to as the outlet discharge factor. For planning, head reach duty is commonly used as a reference.

Step-by-Step Solution:

Verification / Alternative check (if short method exists):

Convert duty to Delta (depth) using Δ = 8.64 * B / D (with consistent units) to cross-check planning numbers; head vs outlet differences reflect loss allowances.

Why Other Options Are Wrong:

Options B–D either misdefine duty, confine it to an outlet only, assert constancy, or limit it to wells—all incorrect.

Common Pitfalls (misconceptions, mistakes):

Forgetting that duty varies spatially due to losses; mixing duty (area/discharge) with delta (depth of water on the field).

Final Answer:

Duty is the total area irrigated by one unit discharge during the base period; it is usually calculated at the channel head; it differs along the system due to losses; the outlet value is known as outlet discharge factor.