Step 1: Understanding the Concept:
A watershed (also known as a catchment or drainage basin) is an area of land where all precipitation that falls on it drains to a common outlet, such as a river, lake, or ocean. Watersheds exist in a nested hierarchy of sizes. The question asks for the term for the smallest unit in this hierarchy.
Step 2: Detailed Explanation:
Let's define the terms related to watershed hierarchy:
(C) River basin: This is a very large-scale watershed, encompassing the entire area drained by a major river and all of its tributaries. For example, the Mississippi River Basin or the Amazon River Basin. It is the largest unit.
(A) Catchment area: This is a general term, often used interchangeably with watershed or drainage basin. It can refer to a watershed of any size.
Sub-watershed: A large river basin is made up of several smaller watersheds of its major tributaries. These are often called sub-watersheds or sub-basins.
(B) Micro-watershed: This is the term used to describe the smallest units of the watershed hierarchy. It refers to a very small area of land, typically from a few hectares to a few square kilometers, that drains to a single small stream or gully. These are the fundamental building blocks of the larger watershed systems.
(D) Flood plain: This is a geomorphological feature. It is the flat area of land adjacent to a river which is subject to flooding. It is a part of a watershed, not a unit of its hierarchical classification.
The hierarchy, from largest to smallest, is generally:
River Basin $\rightarrow$ Sub-basin / Sub-watershed $\rightarrow$ Watershed $\rightarrow$ Mill-watershed $\rightarrow$ Micro-watershed.
Step 3: Final Answer:
The smallest recognized unit in the watershed hierarchy is the micro-watershed. Therefore, option (B) is the correct answer.