Water-resources planning context: For planning a water-supply project, which method is considered the most important and widely used for calculating the discharge of a natural stream or river?

Introduction / Context:
Estimating river discharge accurately is fundamental to designing intakes, transmission mains, and treatment capacities in water-supply projects. Among several techniques, hydrometric gauging in the field provides a representative measure of streamflow during different seasons and stage conditions.

Given Data / Assumptions:

• The stream has a defined cross-section where velocities can be measured.
• No permanent hydraulic control (like a calibrated weir) is available along most reaches.
• Standard hydrometry equipment (current meter, Acoustic Doppler if available) can be used.

Concept / Approach:
The velocity–area method divides the cross-section into verticals, measures point velocities, and integrates velocity over area to obtain discharge. It is versatile for a wide range of stages and is the workhorse method for establishing discharge ratings. Weir/spillway methods require a stable, calibrated control structure; Venturi-meters suit closed conduits, not open channels; power-plant usage is an indirect proxy and not reliable for basin design.

Step-by-Step Solution:

Verification / Alternative check:
Cross-check with float (surface) methods corrected by a coefficient during reconnaissance; compare to any nearby benchmark gauges or historical ratings for consistency.

Why Other Options Are Wrong:

• Weir/spillway method: accurate only where a calibrated structure exists.
• Venturi-meter: intended for pressurized pipes, not rivers.
• Power plant consumption: reflects local operational patterns, not full river discharge.
• None of these: incorrect because velocity–area gauging is standard practice.

Common Pitfalls:
Using too few verticals, ignoring secondary currents, or gauging during rapidly changing unsteady flow can bias results.

Final Answer:
Velocity–area method (current-meter gauging)