Question

The minimum land slope required for irrigation through surface methods is:

• A. 0.01%
• B. 0.05%
• C. 0.10%
• D. 0.15%

Hint:

Remember the standard design limits for surface irrigation:
- Minimum slope $= 0.05\%$ (to prevent water ponding and unequal distribution).
- Maximum safe slope $= 0.5\%$ to $2.0\%$ (depending on soil type, to prevent erosion).

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Surface irrigation methods (such as furrow, border, or basin irrigation) rely on gravity to distribute water over the field.
A minimal slope is necessary to allow water to flow smoothly and uniformly across the soil surface.
If the slope is too flat, water will pond near the inlet, leading to deep percolation losses and poor distribution uniformity.
Conversely, if the slope is too steep, it can cause high-velocity runoff and soil erosion.

Step 2: Detailed Explanation:
Let us analyze the slope limits required for surface irrigation:
- At a slope of $0.01\%$, the land is virtually flat (a drop of only $10\text{ cm}$ over $1000\text{ m}$).
This is insufficient to drive water forward by gravity unless extremely large stream flows are used, which typically results in waterlogging near the inlet.
- A minimum longitudinal slope of 0.05% (a drop of $5\text{ cm}$ per $100\text{ m}$) is required for graded borders and furrows.
This minimal slope provides enough gravitational force to overcome surface roughness and soil resistance, allowing water to advance down the field at a uniform rate while maintaining high application efficiency.
- Slopes of $0.10\%$ and $0.15\%$ are also used, but they represent steeper gradients rather than the absolute minimum required.
Therefore, $0.05\%$ is the minimum design slope recommended for surface irrigation methods.

Step 3: Final Answer:
The minimum land slope required for surface irrigation methods is $0.05\%$.
Hence, the correct option is (B).