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Ecology and Earth Science - The Water Cycle

Grade 7IB

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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The Water Cycle (Hydrologic Cycle) is the continuous movement of water within the Earth and atmosphere, involving the hydrosphere, lithosphere, and biosphere.

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Energy from the Sun drives the cycle by providing thermal energy for phase changes, such as the transition from H2O(l)H_2O_{(l)} to H2O(g)H_2O_{(g)}.

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Evapotranspiration is the sum of evaporation from land/water surfaces and transpiration, where H2OH_2O is released from the stomata of plants.

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Condensation occurs when water vapor loses heat energy and changes back into liquid droplets, forming clouds around microscopic particles called condensation nuclei.

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Precipitation occurs when water droplets or ice crystals in clouds become too heavy to be suspended and fall to Earth as rain, snow, sleet, or hail.

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Infiltration and Percolation: Infiltration is the entry of water into the soil surface, while percolation is the downward movement of water through soil and rock layers to replenish groundwater in aquifers.

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Surface Runoff happens when the soil is saturated or the surface is impermeable, causing water to flow over the land into rivers, lakes, and oceans.

📐Formulae

P=Q+ET+ΔSP = Q + ET + \Delta S

H2O(l)+Heat Energy→H2O(g)H_2O_{(l)} + \text{Heat Energy} \rightarrow H_2O_{(g)}

Relative Humidity=(Actual Water VaporSaturation Water Vapor)×100%\text{Relative Humidity} = \left( \frac{\text{Actual Water Vapor}}{\text{Saturation Water Vapor}} \right) \times 100\%

💡Examples

Problem 1:

A local ecosystem receives 1200 mm1200\text{ mm} of precipitation (PP) annually. If the surface runoff (QQ) is 300 mm300\text{ mm} and the change in storage (ΔS\Delta S) is 00, calculate the total amount of water lost through evapotranspiration (ETET).

Solution:

Using the water balance equation P=Q+ET+ΔSP = Q + ET + \Delta S, we rearrange for ETET: ET=P−Q−ΔSET = P - Q - \Delta S Substituting the values: ET=1200 mm−300 mm−0=900 mmET = 1200\text{ mm} - 300\text{ mm} - 0 = 900\text{ mm}

Explanation:

The water balance equation represents the law of conservation of mass, ensuring that all water entering the system as precipitation is accounted for by water leaving as runoff, water lost to the atmosphere, or water stored in the ground.

Problem 2:

Explain the role of latent heat when H2OH_2O changes from a liquid to a gas during evaporation.

Solution:

During evaporation, water molecules absorb energy to overcome intermolecular forces. This energy is known as the Latent Heat of Vaporization (Lv≈2260 kJ/kgL_v \approx 2260\text{ kJ/kg} at 100∘C100^\circ\text{C}).

Explanation:

Even though the temperature might not change during the phase transition, the energy is used to break the hydrogen bonds between H2OH_2O molecules, allowing them to escape into the atmosphere as water vapor.