Question

Overland flow during irrigation in a border strip is an example of

• A. Steady flow
• B. Unsteady flow
• C. Steady flow with decreasing discharge
• D. Unsteady flow with decreasing discharge

Hint:

Any irrigation flow over a soil bed (borders, furrows) is always unsteady and spatially varied because infiltration continuously reduces the flow rate as it advances.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Overland flow in border strip irrigation involves water advancing over a porous soil bed, where infiltration occurs dynamically.
To classify this flow, we must analyze how flow rate and water depth vary with time and distance.
Detailed Explanation:
- As water is turned into a border strip, the water front advances downslope.
- At any fixed point, the depth and velocity of the water change as the front passes and continues to advance.
- Because the hydraulic parameters vary with time, the flow is classified as unsteady flow.
- Concurrently, as water moves over the soil surface, a portion of the flow continuously infiltrates into the soil profile.
- Consequently, the volume of surface water flowing downslope decreases progressively along the length of the border strip.
- This results in a decreasing discharge along the channel length.
- Therefore, the flow is hydrologically classified as an unsteady, spatially varied flow with decreasing discharge.

Step 2: Final Answer:
The overland flow during border irrigation is an unsteady flow with decreasing discharge, corresponding to option (D).