Which soil conservation method uses stones, grass and soil to build barriers along contour lines and digs trenches in front of them to collect runoff water?

Introduction / Context:
Soil conservation techniques are essential in agriculture and environmental management to prevent erosion and maintain fertility. Different methods are used depending on slope, rainfall and soil type. This question describes a particular method in which stones, grass and soil are used to build small barriers along contour lines, and trenches are dug in front of these barriers to collect water. You are asked to identify the name of this method from the options given.

Given Data / Assumptions:

- Stones, grass and soil are used to construct barriers along contour lines.
- Trenches are made in front of these barriers to collect runoff water.
- The question is about a soil and water conservation practice on sloping land.
- Options are Mulching, Contour barriers, Rock dam and Terrace farming.

Concept / Approach:
Contour barriers are structures placed along contour lines on a slope to slow down surface runoff and encourage water infiltration into the soil. They can be made using stones, soil, grasses or a combination of these. Sometimes trenches or small channels are dug upslope of the barriers to capture water and sediment. This practice reduces erosion, improves moisture retention and supports crop growth. Mulching involves covering the soil with straw or other materials, rock dams are built across gullies rather than along contour lines, and terrace farming creates step like flat surfaces on slopes.

Step-by-Step Solution:
Step 1: Note that the method described uses material such as stones and grass to make low barriers.Step 2: These barriers are aligned along contour lines, which means they follow lines of equal elevation across a slope.Step 3: Trenches are placed in front of the barriers to collect water, indicating a design meant to slow runoff and increase infiltration.Step 4: This description matches the technique called contour barriers, used widely in soil conservation programs.Step 5: Therefore the correct answer is contour barriers.

Verification / Alternative check:
You can verify by comparing each option with the description. Mulching does not involve constructing physical barriers along contours. A rock dam is typically a structure built across a small stream or gully, not along contour lines across a field. Terrace farming involves cutting broad level steps into the hillside, which is different from placing narrow barriers at intervals. Only contour barriers use small stone and grass lines along contour paths combined with trenches to intercept runoff, matching the scenario in the question.

Why Other Options Are Wrong:
Mulching: This method protects soil by covering the surface with straw, leaves or plastic, reducing evaporation and weed growth, but it does not involve contour aligned stone and soil barriers with trenches.
Rock dam: A rock dam is constructed mainly across a channel or gully to slow water flow and capture sediment. It is not usually repeated along contour lines across farmland.
Terrace farming: Terracing involves creating large flat steps on a hillside by cutting and filling soil. While it is an effective erosion control method, it does not match the description of small contour barriers and trenches.

Common Pitfalls:
Students may mix up various soil conservation terms because many involve controlling runoff. The key is to focus on the pattern of structures. Contour barriers follow contour lines and are relatively low, while terraces are larger level platforms and rock dams cross channels. Remembering typical diagrams from textbooks that show contour barriers as lines of small bunds helps keep these concepts distinct.

Final Answer:
The described soil conservation method is called Contour barriers.