Rigid module (proportional outlet) in canal outlets: The discharge characteristic is best described as:

Introduction / Context:
Canal outlets (modules) control delivery from distributaries to watercourses. Their hydraulic behavior—how discharge varies with upstream and downstream levels—defines whether they are rigid, semi-rigid (proportional), or flexible modules.

Given Data / Assumptions:

• “Rigid module” is the term in question.
• Steady operating conditions intended.

Concept / Approach:
A rigid module (constant discharge module) aims to maintain nearly constant discharge irrespective of changes in head on either side, within a working range. In contrast, proportional (flexible) modules let discharge vary with upstream level, and orifices with submerged outlets may depend on both levels.

Step-by-Step Solution:
Recall definition: rigid module → discharge insensitive to level fluctuations.Match to statement: independence from both distributary and watercourse levels.Select option (a).

Verification / Alternative check:
Outlet classification tables clearly separate rigid (constant discharge), semi-rigid (proportional), and flexible (sensitive) behaviors.

Why Other Options Are Wrong:

• (b)/(c)/(d) imply sensitivity to one or both levels, contradicting “rigid”.
• (e) Not applicable.

Common Pitfalls:
Confusing “rigid module” with “proportional module.” The term “rigid” here means hydraulically insensitive (constant discharge).

Final Answer:
Independent of water levels in both the distributary and the watercourse