Understanding Water Runoff, Infiltration, and Evapotranspiration

Posted on Feb 16, 2025 in Geology

Runoff and Infiltration

Water that falls as rain or snow can evaporate, drain, or infiltrate. This depends on:

Rate of precipitation (intensity and duration)
Soil water content (soil capacity)
Slope and vegetation

Runoff

Runoff is the flow of water from precipitation to rivers located at a lower elevation. This occurs when the capacity of soil infiltration is exceeded. It also refers to water leaving a drainage area.

Types of Runoff

Infiltration

Infiltration is the absorption and downward movement of water in the soil layers.

Soil Moisture

Groundwater

Groundwater (2 to 8 * 10⁶ km³) is water stored in soil and rock layers (aquifers). It will depend on the storage, porosity, and permeability of the aquifer.

Groundwater Contamination

Gasoline storage tanks
Poorly maintained septic tanks
Agriculture
Waste Landfill
Mining
Radioactive Waste

Average Residence Time for a Water Molecule in the Hydrological Cycle

Atmosphere: 9 – 10 days
Rivers: 12 – 20 days
Lakes: 1 – 100 years
Underground aquifers: 300 years
Ocean: 3000 years

Evaporation, Transpiration and Evapotranspiration

Evaporation: The process by which liquid water is converted to gas. It requires that the humidity of the atmosphere is less than the surface that is evaporating. This process requires energy (evaporation of 1 g of H₂O at 100 °C, 540 cal).

Transpiration: The process by which plants lose water through the stomata. It is a passive process controlled by atmospheric humidity and soil water content. Transpiration also transports nutrients from the ground to the roots and leads to various plant cells.

Evapotranspiration (ET): The current or real amount of water that returns to the atmosphere from the soil surface (evaporation) and through plant tissues (transpiration) in the course of a year. Potential ET is the ET that would occur in a surface vegetation cover if soil moisture was adequate to allow free transpiration of vegetation.

Estimating ET

Physical factors:

Solar radiation
Temperature
Wind Speed
Humidity

Biological factors:

Vegetation coverage
Stomatal conductance

Water vapor contributes to precipitation (snow and rain). 65% of the water that falls on Earth comes from evapotranspiration.

How could rainfall patterns affect the planet if there were vegetation?

How could rainfall patterns affect the planet if there were no vegetation?

a) With vegetation

b) Without vegetation

Interception and Anthropogenic Disturbance

Interception: The precipitation that falls on plants and evaporates without ever reaching the ground.

Anthropogenic Disturbance

Change in land use = Change in regional and global climate
Heating emission of other gases increases the amount of water vapor in the atmosphere

Acceleration of the hydrological cycle.