Methods of Estimating Evapotranspiration

Estimating Evapotranspiration (ET) is crucial for water resource management, and various methods exist based on data availability and desired accuracy. Methods can be broadly categorized into three groups:

1. Empirical Methods

Empirical methods primarily use observed climatological data and regression equations to estimate ET. Key examples include:
- Blaney-Criddle Method: Utilizes mean daily temperature and daylight hours, expressed as:

ET = p(0.46T + 8)

where p is the monthly percentage of annual daylight hours and T is mean monthly temperature (°C).

• Hargreaves Method: Simple and temperature-based, calculated as:

ET = 0.0023(T + 17.8)(T_max - T_min)^{0.5} Ra

where Ra represents extraterrestrial radiation.

• Pan Evaporation Method: Founded on the evaporation rate from a Class A pan, represented as:

ET = K_p * E_p

where K_p is the pan coefficient and E_p is the pan evaporation.

2. Energy Balance Methods

This category is based on the principle of energy conservation at land surface, summarized in the energy budget equation:

R_n - G - H = λE

where R_n is net radiation, G is soil heat flux, H is sensible heat flux, and λE corresponds to latent heat multiplied by ET.

3. Combination Methods

Combination methods integrate both empirical and energy balance approaches. Notable methods include:
- Penman Method: Blends energy balance with aerodynamic methods:

ET = rac{Δ(R_n - G) + γ rac{900}{T + 273}u(e_s - e_a)}{Δ + γ(1 + 0.34u)}

with variables defined as slopes, psychrometric constants, wind speed, and vapor pressure deficit.

• FAO Penman-Monteith Method: The standard method for estimating reference ET, using:

ET = rac{0.408Δ(R_n - G) + γ rac{900}{T + 273}u(e_s - e_a)}{Δ + γ(1 + 0.34u)}

This method employs a standard grass reference crop and is globally applicable, providing accuracy in ET estimations.