Question

The overland flow in an irrigation border strip is a case of

• A. unsteady flow only
• B. steady flow only
• C. steady flow with decreasing discharge
• D. unsteady flow with decreasing discharge

Hint:

Any irrigation flow over a permeable soil surface (such as in furrow, basin, or border irrigation) is always a case of decreasing discharge because infiltration continuously removes water from the surface.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Open-channel hydraulics classifies fluid flow based on how velocity and discharge change over time and space.
- Steady flow: Flow parameters (velocity, depth) do not change with time at a given location.
- Unsteady flow: Flow parameters change with time at a given location.
- Spatially varied flow: Discharge changes along the length of the flow channel due to water entering or leaving the channel.

Step 2: Detailed Explanation:
Let us analyze the hydraulic behavior of water during border strip irrigation:
When water is introduced at the upstream head of a border strip, it advances down the slope as a thin sheet of overland flow.
As the water front advances:
1. The depth and velocity of flow at any fixed point along the border change continuously over time as the wetting front passes and stabilizes.
This makes the flow unsteady over time.
2. As the water flows over the soil surface, some of it continuously infiltrates into the ground.
This infiltration reduces the volume of water remaining on the surface as it moves downstream.
This means the discharge ($Q$) decreases along the direction of flow.
Therefore, the flow is characterized as a spatially varied, unsteady flow with decreasing discharge.

Step 3: Final Answer:
The overland flow in an irrigation border strip is classified as an unsteady flow with decreasing discharge.
Hence, the correct option is (D).