Step 1: Understanding the Concept:
Terraces are earth embankments constructed across a slope to control runoff and erosion. The design of a terrace depends on the climate (rainfall) and soil properties. The question asks for the type of terrace suitable for conditions where conserving moisture is the primary goal.
Step 2: Detailed Explanation:
Let's analyze the conditions given:
- Low rainfall: This means water is a scarce resource and needs to be conserved.
- Very permeable soil: This means the soil can absorb water quickly without becoming waterlogged.
- Moisture conservation is important: This is the main objective.
Now let's look at the different types of terraces:
(A) Level terrace (or Ridge type terrace): These are constructed perfectly level along the contour. Their purpose is to trap all the rainfall that falls between them, preventing runoff from leaving the field. The trapped water then has time to infiltrate into the soil, thus maximizing moisture conservation. This design is ideal for the conditions described: low rainfall (so the terrace won't be overtopped) and permeable soil (to absorb the stored water). "Ridge type terrace" (Option D) is another name for this, as it consists of a ridge built on the contour.
(B) Graded terrace (or Channel type terrace): These are constructed with a gentle slope or grade along their length. Their primary purpose is not to store water, but to intercept runoff and convey it at a non-erosive velocity to a safe outlet. This design is used for runoff disposal in areas with higher rainfall and/or less permeable soils, where moisture conservation is less important than preventing erosion from excess runoff. "Channel type terrace" (Option C) is another name for this.
Step 3: Final Answer:
For low rainfall areas where moisture conservation is the main goal, level terraces are constructed to impound water and promote infiltration. Therefore, option (A) is the correct answer.