Penman's Equation Calculator

Penman's Equation:

\[ PET = \frac{A \times Hn + Ea \times \gamma}{A + \gamma} \]

Slope of Saturation Vapour Pressure (A):

mm Hg/°C

Net Radiation of Evaporable Water (Hn):

mm/day

Parameter of Wind Velocity and Saturation Deficit (Ea):

mm/day

Psychrometric Constant (γ):

kPa/°C

Daily Potential Evapotranspiration (PET):

Unit Converter ▲

Unit Converter ▼

From:	To:

1. What is Penman's Equation?

Penman's Equation is a comprehensive formula that combines energy balance and mass transfer methods to estimate potential evapotranspiration (PET). It provides a more accurate assessment of water loss from surfaces than simpler methods, accounting for both radiative and aerodynamic components.

2. How Does the Calculator Work?

The calculator uses Penman's equation:

\[ PET = \frac{A \times Hn + Ea \times \gamma}{A + \gamma} \]

Where:

\( A \) — Slope of saturation vapour pressure vs temperature curve (mm Hg/°C)
\( Hn \) — Net radiation of evaporable water (mm/day)
\( Ea \) — Parameter of wind velocity and saturation deficit (mm/day)
\( \gamma \) — Psychrometric constant (kPa/°C)

Explanation: The equation combines the energy available for evaporation (radiation term) with the atmospheric demand for moisture (aerodynamic term) to estimate potential evapotranspiration.

3. Importance of PET Calculation

Details: Accurate PET estimation is crucial for irrigation scheduling, water resource management, hydrological modeling, and climate studies. It helps determine water requirements for crops and natural vegetation.

4. Using the Calculator

Tips: Enter all required parameters with appropriate units. Ensure values are positive and within reasonable ranges for accurate results.

5. Frequently Asked Questions (FAQ)

Q1: What is the difference between PET and actual evapotranspiration?
A: PET represents the maximum possible water loss under ideal conditions, while actual evapotranspiration is the real water loss limited by available moisture.

Q2: How accurate is Penman's equation?
A: Penman's equation is considered one of the most accurate methods for PET estimation, particularly in temperate climates with adequate data.

Q3: What are typical PET values?
A: PET values vary widely by climate, ranging from 1-3 mm/day in cool climates to 10-15 mm/day in hot, arid regions.

Q4: What data is required for Penman's equation?
A: The equation requires meteorological data including temperature, humidity, solar radiation, and wind speed.

Q5: Are there simplified versions of Penman's equation?
A: Yes, several modified Penman equations exist, such as Penman-Monteith, which is widely used in agricultural applications.