Outlet hydraulics: The ratio of the rate of change of discharge of an outlet to the rate of change of water level in the distributary (at normal depth) is called

Introduction / Context:
Equitable water distribution in irrigation depends on how outlets respond to fluctuations in parent channel levels. Key parameters are efficiency, sensitivity, flexibility, and modular limit.

Given Data / Assumptions:

• Outlet draws water from a distributary into a watercourse.
• Discharge varies as upstream and downstream levels change.
• Normal depth conditions in the distributary are referenced.

Concept / Approach:
Flexibility (F) = (rate of change of outlet discharge) / (rate of change of distributary water level at normal depth). It is a measure of how the outlet discharge tracks the parent channel level. Values near unity indicate proportional behavior; for modular outlets, dependence is minimized.

Step-by-Step Solution:
Identify the variables: discharge Q_out and water level H_d.Define F = dQ_out / dH_d (evaluated at normal depth).Hence, the named parameter is flexibility.

Verification / Alternative check:
Textbook definitions list: efficiency (ratio of water delivered to water measured), sensitivity (response to watercourse level), flexibility (response to distributary level), modular limit (head range for module performance).

Why Other Options Are Wrong:

• Efficiency: Involves conveyance/delivery, not level response.
• Sensitivity: Relates to watercourse/backwater effects.
• Modular limit: Range of levels preserving modularity.

Common Pitfalls:
Interchanging flexibility and sensitivity; ignoring the specific reference to distributary level at normal depth.

Final Answer:
flexibility