Question

The most suitable device for the measurement of flow in irrigation furrow is

• A. Portable Parshall flume
• B. Dethridge meter
• C. V-notch weir
• D. Submerged orifice

Hint:

To measure flow in small, sediment-laden channels:
- Use Flumes (like the Portable Parshall flume) because they are self-cleaning.
- Avoid Weirs (like the V-notch) because they cause sediment to deposit upstream.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Irrigation furrows are small, narrow channels constructed to guide water down a field slope between crop rows.
The flow rate in individual furrows is relatively small (typically ranging from $0.5$ to $3.0\text{ liters/second}$).
Measuring flow in these small channels requires a device that can be easily transported and installed, has low head loss, and is not easily clogged by suspended sediment.

Step 2: Detailed Explanation:
Let us evaluate each flow measurement device:
1. Portable Parshall flume (A):
This is a lightweight version of the standard Parshall flume, often with throat widths of $1$ or $2\text{ inches}$, made of sheet metal or fiberglass.
It is specifically designed for quick installation in small channels like furrows.
Its hydraulic advantages include:
- Extremely low head loss, making it suitable for flat slopes.
- Self-cleaning properties, as its high-velocity throat section sweeps away suspended sediment.
- It does not require a deep pond of water upstream, preserving the shape of the furrow.
Therefore, it is the most suitable device.
2. Dethridge meter (B):
This is a large, heavy paddle wheel device used to measure bulk water deliveries at canal outlets, and is too large for individual furrows.
3. V-notch weir (C):
While highly accurate for small flows, a weir requires a deep pooling basin upstream to stabilize the water.
This ponds water in the furrow, which can break the furrow walls and encourage sediment deposition.
4. Submerged orifice (D):
This device requires clean water and a stable head difference, and can easily clog with sediment and weeds in a furrow.

Step 3: Final Answer:
The most suitable device for measuring flow in an individual irrigation furrow is the portable Parshall flume.
Hence, the correct option is (A).