In irrigation hydraulics, how does pipe diameter affect friction loss at a given flow?

• A. Friction loss increases with larger diameter and decreases with higher flow.
• B. Friction loss increases with higher flow and decreases with larger diameter.
• C. Friction loss is independent of pipe diameter.
• D. Friction loss decreases with higher flow.

Answer: (B)

The key idea is that friction loss in pipes is driven by how fast the water is moving and the pipe size. For a fixed discharge, making the pipe bigger lowers the water velocity because the same amount of water has more cross-sectional area to pass through. Since the head loss from friction depends strongly on velocity (roughly with the square of velocity in common head-loss formulas), lowering velocity causes a big drop in friction loss. So larger diameter pipes reduce friction loss at the same flow.

Conversely, increasing the flow rate while keeping the same pipe diameter raises velocity, which increases friction losses because the velocity term drives the loss up (the higher the flow, the more resistance the water encounters along the pipe).

This explains why the right statement is that friction loss grows with higher flow and shrinks with larger diameter. The other ideas are not consistent with how velocity and pipe size control friction losses: larger diameter does not increase friction loss, friction loss is not independent of diameter, and friction loss does not decrease when flow increases.