HOTS Science Primary 5 Cycles

A large forested region in Singapore is cleared to build a new town. Scientists predict that within ten years, the annual rainfall in the surrounding area will decrease significantly. Which of the following best explains the scientists' prediction, linking both plant processes and water cycle stages?

A The cleared land absorbs more solar energy, increasing the surface temperature and causing water to evaporate faster from drains and reservoirs, which increases overall rainfall in the region.
B The removal of trees reduces transpiration, lowering the amount of water vapour released into the atmosphere, which reduces condensation and cloud formation, leading to less precipitation over time.
C Without tree roots to absorb rainwater, more water flows directly into drains and the sea, reducing the amount of water available for evaporation and therefore lowering rainfall.
D The absence of trees means less oxygen is produced by photosynthesis, which changes the composition of the atmosphere and prevents water droplets from forming clouds.
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Worked Solution

Step 1: Identify the two processes to link Forests contribute to the water cycle primarily through transpiration — trees absorb water from soil and release it as water vapour through leaf stomata. This is separate from evaporation from land surfaces. Step 2: Trace the effect of removing transpiration When trees are removed, transpiration stops. This significantly reduces the amount of water vapour entering the atmosphere from the land area. Evaporation from the remaining hard surfaces (roads, rooftops) adds far less moisture than the original forest did. Step 3: Connect reduced water vapour to later water cycle stages Less water vapour in the atmosphere means less condensation occurs when air rises and cools. Less condensation means fewer clouds form, and precipitation (rainfall) decreases. Step 4: Evaluate each option Option A is partially true (more evaporation from surfaces) but the net effect is still reduced rainfall — the volume of transpiration lost far exceeds the small increase in surface evaporation. Option C focuses on runoff, not the atmosphere. Option D incorrectly links oxygen to cloud formation. Only Option B correctly chains transpiration → water vapour → condensation → precipitation.

Correct answer: The removal of trees reduces transpiration, lowering the amount of water vapour released into the atmosphere, which reduces condensation and cloud formation, leading to less precipitation over time.

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