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Rivers Flow into the Sea Without Overflowing: The Dynamic Balance of Earth's Water Cycle
GEOG1001C-PEP-CNLesson 3
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"Rivers flow into the sea without overflowing" reveals thedynamic equilibriumprinciple. At a global scale, surface water, atmospheric water, and groundwater remain constant in total volume over long periods. This balance is achieved through a closed-loop budget between oceans and land.

$$P_{ocean} + R = E_{ocean}$$ $$P_{land} = E_{land} + R$$

1. Global Water Balance Principle

For the ocean, its long-term average evaporation rate $E_o$ equals the sum of long-term average precipitation $P_o$ and river runoff into the sea $R$. For land, precipitation is divided into evaporation $E_l$ and runoff flowing to the ocean $R$. This "income-expenditure offset" mechanism ensures long-term sea level stability.

2. Atmospheric Medium and Pollution Intervention

The atmosphere is the most active component of the water cycle. However,atmospheric pollutionalters aerosol concentration (e.g., PM2.5 and PM10) as condensation nuclei, disrupting the frequency and intensity of precipitation. Major causes of current atmospheric pollution in China include:

  • A coal-based energy consumption structure.
  • Industrial emissions and a sharp increase in vehicle ownership.
  • Urbanization has altered infiltration conditions.
Key Pollutants in Air Quality Index (AQI)
Six primary indicators for evaluating air quality: PM2.5, PM10, SO2, NO2, O3, CO.