Atmospheric Moisture, Humidity, and Evapotranspiration Dynamics
Atmospheric Moisture
Source of Atmospheric Moisture
Atmospheric moisture originates with the gasification of water from oceans, lakes, rivers, soils, vegetation, and wildlife, as well as humanity (breathing). Moisture also results from human activities such as waste combustion and the advent of steam engines.
Humidity and Water Vapor Concepts
Moist Air and Water Vapor
Water vapor is a component of the mixture of atmospheric air. Moist air is defined as a mixture of dry air and water vapor.
Vapor Pressure (e)
It is the pressure exerted by water vapor. The units are the same as atmospheric pressure (e.g., mb, mm Hg).
Saturation Vapor Pressure (s)
This is the maximum vapor pressure that a moist air sample can contain at its current temperature.
Absolute Humidity
The amount of water vapor contained in the atmosphere at a specific time and location. It is the number of grams of water in 1 m³ of air, commonly known as the density of water vapor in the air.
Relative Humidity (RH)
The relationship between the actual amount of water vapor (absolute humidity) and the maximum amount of water vapor the air could hold at that place and temperature.
Dew Point Temperature
The temperature at which water vapor in the air begins to condense, producing dew, fog, or, if the temperature is sufficiently low (below zero), frost.
For a given mass of air containing a specific amount of water vapor (absolute humidity), saturation (relative humidity equals 100%) leads to the dew point. The device that measures the amount of dew is called a Rociógrafo.
Evaporation
The Evaporation Process
Evaporation is the physical process where water turns into vapor from free surfaces such as reservoirs, lakes, dams, and raceways. The change from liquid to vapor state is the result of increased kinetic energy of the water particles or molecules. This increase in kinetic energy requires the consumption of energy per unit mass, which is called the latent heat of evaporation.
Factors Affecting the Evaporation Rate
- Source of Energy: The primary energy source is solar radiation. If the net radiation balance is positive, there is energy available to be used in evaporation.
- Driving Force: This is determined by the vertical gradient of atmospheric moisture, which is affected by the following factors:
- Wind speed
- Air temperature (T)
- Vertical gradient of air temperature
- Evaporating surface temperature (T)
Evaporation Intensity
Evaporation intensity is the speed with which the phenomenon occurs and is expressed in mm/hour or mm/day. There are many mathematical relationships that express the intensity of evaporation, including:
- Dalton’s Law
- Mass Balance Method or Aerodynamic Method
- Meyer Ratio
- Ven Te Chow Ratio
Evapotranspiration
Definition of Evapotranspiration
Evapotranspiration is the loss of water to the atmosphere through evaporation from any evaporative surface (wet soils, water surfaces, etc.) and transpiration from the vegetation in that area or medium.
Classification of Evapotranspiration
Evapotranspiration is classified as follows:
Potential Evapotranspiration (ETP)
Occurs when evapotranspiration is the maximum possible, given favorable conditions: soil moisture within its field capacity, fitted with a dense, uniform, low cover (8 to 15 cm, like grasses). It is also known as reference crop evapotranspiration (ET).
Actual Evapotranspiration (ETR)
Occurs in real environmental conditions, taking into account variations in soil moisture and incomplete cover, as normally occurs in most crops (potato, maize, etc.).
Relationship Between ETP and ETR
The ETP and ETR are related as follows: [Relationship implied here]
Where:
Kc is a coefficient that depends on each plant species according to its specific development conditions, physiological demands, vegetative pace, sowing and harvest times, and variety. This value is weighted when several crops are set simultaneously within a given project.