H2. Evapotranspiration
The hydrologic cycle is completed by evapotranspiration, a process that restores water to the atmosphere. Evaporation and transpiration make up the two components of evapotranspiration. The loss of water molecules from soil and water surfaces is known as evaporation. Water is lost from plants through transpiration in the form of vapor.
Evapotranspiration, a process of evaporation from soil and water bodies and transpiration from plant leaves, is crucial for irrigation, water quality management, urban development, and flood control. It depends on atmospheric conditions and water availability. The resulting evapotranspiration is known as prospective evapotranspiration (PET) if there is always enough moisture available to fully satisfy the needs of vegetation covering the area. Actual evapotranspiration (AET) is the evapotranspiration that actually occurs in a given scenario.
1. Potential Evapotranspiration (PET)
Pan evaporation
Pans measure evaporation rates from water surfaces, indicating the impact of radiation, wind, temperature, and humidity on water depth. However, factors like solar radiation reflection, heat storage, and air turbulence can affect water loss. Pans can be used to predict ETo for periods of 10 days or longer, as they provide a measurement of the combined effect of these factors on water surface evaporation.
ETo = Kp x Epan
where:
ETo reference evapotranspiration [mm/day],
Kp pan coefficient [-],
Epan pan evaporation [mm/day].
Evapotranspiration gauges
The modified Bellani plate atmometer offers a simpler and more efficient method for estimating evapotranspiration (ET) rate from green grass surfaces.
2. Actual Evapotranspiration (AET)
Simple methods
Soil water depletion method
Evapotranspiration can be measured using the soil water depletion method, which involves comparing soil moisture levels at different time intervals.
Water balance method
The method estimates water balance by calculating soil water deficit, which is the volume of water needed to saturate the soil.
Ea = P – Gr + ΔS – Ro
where:
Gr = recharge;
P = precipitation;
Ea = actual evapotranspiration;
ΔS = change in soil water storage; and
Ro = run-off.
Complex methods
Lysimeters
A lysimeter is a watertight tank used to measure evapotranspiration in a field of plants. It accurately reproduces soil conditions, moisture content, and vegetation type. However, studies are time-consuming and expensive, and must be done quickly to maintain consistent soil levels.
Energy balance method
The energy balance in evapotranspiration involves four main components: net radiation input, soil energy exchange, air heat exchange, and latent energy exchange for water evaporation:
Net Radiation – Ground Heat Flux = Sensible Heat + Latent Energy
Evapotranspiration can be determined using energy balance, eddy diffusion equations, or combinations of these methods, depending on available data, accuracy, and computational capability.
Mass transfer method
The analytical method for determining lake evaporation uses turbulent mass transfer theories to calculate mass water vapor transfer from the surface to the surrounding atmosphere.
Estimation of Evapotranspiration
Due to insufficient field data, various methods have been developed to predict Potential Evapotranspiration (PET) using climatological data, including commonly used methods:
• Blaney-Criddle method
• Modified Penman Method
• Jansen-Haise method
• Hargreaves method
• Thornwaite method
Modi, P N (2000) provides detailed discussions on various methods for estimating irrigation demand, which can be found in “Estimating irrigation demand.”
