Hydrology - Evaporation

is the process in which a liquid changes to the gaseous state at the free surface, below the boiling point through the transfer of heat energy.

Evaporation

is the process by which water leaves the body of a living plant and reaches the atmosphere as water vapor.

Transpiration

is the sum of evaporation from the land surface plus transpiration from plants.

Evapotranspiration

all matter is made up of tiny particles called molecules that are in constant motion. In a liquid, the molecules are closely packed together and have strong intermolecular forces holding them together. However, some of the molecules at the surface of the liquid have enough kinetic energy to break free from the liquid's surface and enter the gas phase. These molecules are said to have "escaped" the liquid and become a gas or vapor.

kinetic theory

Factors Affecting Evaporation

❑ Vapor Pressure

❑ Temperature

❑ Wind

❑ Atmospheric Pressure

❑ Soluble Salts

❑ Heat Storage in Water Bodies

the pressure exerted by the vapor phase of a liquid that is in equilibrium with its liquid phase in a closed container

Vapor Pressure

The rate of evaporation increases with an increase in the water temperature.

Temperature

Aids in removing the evaporated water vapour from the zone of evaporation and consequently creates greater scope of evaporation.

Wind

A decrease in the barometric pressure, as in high altitudes, increase evaporation.

Atmospheric Pressure

When a solute is dissolved in water, the vapour pressure of the solution is less than that of pure water and hence causes reduction in the rate of evaporation.

Soluble Salts

Deep water bodies have more heat storage than shallow ones. A deep lake may store radiation energy received in summer and release it in winter causing less evaporation in summer and more evaporation in winter compared to shallow lake exposed to a similar situation.

Heat Storage in Water Bodies

Factors Affecting Transpiration

❑ Vegetation

❑ Stomatal Resistance

❑ Soil Characteristics

❑ Plant Maturity

Is a water-containing pan which is exposed to the atmosphere and the loss of water by evaporation measured in it at regular intervals.

Evaporimeter

HOW TO OBTAIN DATA

1. Install the evaporimeter in a suitable location: Choose a location that is representative of the area you want to monitor, and install the evaporimeter according to the manufacturer's instructions. Make sure it is level and stable.

2. Fill the evaporimeter with water: Fill the evaporimeter's reservoir with distilled water up to the recommended level.

3. Record the starting water level: Use a ruler or tape measure to measure and record the starting water level in the evaporimeter.

4. Place the evaporimeter cover: Place the cover on the evaporimeter to prevent wind, precipitation, or other factors from affecting the evaporation rate.

5. Monitor and record the water level: At regular intervals (e.g., daily), measure and record the water level in the evaporimeter.

6. Calculate the evaporation rate: To calculate the evaporation rate, subtract the current water level from the starting water level, and divide by the number of days elapsed.

is a special watertight tank containing a block of soil and set in a field of growing plants.

Lysimeter

The simplest of the three analytical methods and also the least reliable.

Water Budget Method

An application of the law of conservation of energy.

Energy Budget Method

This method is based on theories of turbulent mass transfer in boundary layer to calculate the mass water vapor transfer from the surface to the surrounding atmosphere.

With the use of quantities measured by sophisticated (and expensive) instrumentation, this method can give satisfactory results.

Mass Transfer Method