When temperature is high, evaporation and runoff is......

Question

Examoogle · Accepted Answer

Option B: evaporation is high and runoff is low

Explanation & Concepts:
High temperature increases the rate of evaporation from water bodies and soil surfaces due to greater energy availability for phase change. Concurrently, since evaporation is an abstraction from the total precipitation, the net water available to form surface runoff is reduced, resulting in low runoff.

Working Principle: The principle is governed by the energy balance in the hydrologic cycle. When temperature rises, the vapor pressure deficit increases, accelerating the latent heat of vaporization process. Because evaporation acts as a depletion mechanism, high rates of vapor loss from a catchment leave less water to fill surface depressions and exceed infiltration capacities, thereby reducing the volume of surface runoff.

Key Formulas: $E = K_e \cdot (e_s - e_a)$ — where $E$ is evaporation, $e_s$ is saturation vapor pressure (highly temperature-dependent), and $e_a$ is actual vapor pressure.; $Q = P - E - \Delta S$ — where $Q$ is runoff, $P$ is precipitation, and $\Delta S$ is storage change.

Key Points: Evaporation is directly proportional to temperature due to the Clausius-Clapeyron relation increasing saturation vapor pressure.; Runoff is the residual of precipitation after accounting for evaporation and infiltration.; Higher temperatures typically promote increased infiltration rates if the soil remains moist, further reducing surface runoff.

Advantages: High temperature aids in faster drying of agricultural fields.; Reduces water logging risks in poorly drained areas.

Disadvantages: Reduces storage capacity of reservoirs through high evaporative loss.; Decreases water availability for downstream irrigation and municipal use.

Applications: Hydrological modeling for catchment yield analysis.; Reservoir management and climate change impact assessments.

Additional Info: In arid regions, extreme temperatures often lead to high evaporation rates, rendering runoff almost zero during summer months.. Option A is incorrect because high evaporation consumes the potential runoff. Option C and D are incorrect as they defy the fundamental thermodynamic relationship between temperature and evaporation.

Answer: B is correct — High temperatures enhance evaporative loss, leaving significantly less water available as surface runoff.

Exam Tip: Always remember that in the water balance equation $P = R + E + I$, $E$ (evaporation) and $R$ (runoff) compete for the same input $P$ (precipitation).

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Option A: evaporation is high and runoff is high

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Option C: evaporation is low and runoff is low

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Option D: evaporation is low and runoff is high

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