Rate Of Evaporation Calculator

Penman Evaporation Equation:

\[ ER = \Delta \times Rn + \gamma \times 6.43 \times (1 + 0.536 U2) \times (es - ea) / (\Delta + \gamma) \]

Delta (Δ):

kPa/°C

Net Radiation (Rn):

MJ/m²/day

Gamma (γ):

kPa/°C

Wind Speed (U2):

m/s

Saturation Vapor Pressure (es):

kPa

Actual Vapor Pressure (ea):

kPa

Evaporation Rate (ER):

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1. What is the Penman Evaporation Equation?

The Penman equation is a comprehensive physical-based model that estimates evaporation from open water surfaces. It combines energy balance and aerodynamic principles to provide accurate evaporation rates under various meteorological conditions.

2. How Does the Calculator Work?

The calculator uses the Penman evaporation equation:

\[ ER = \Delta \times Rn + \gamma \times 6.43 \times (1 + 0.536 U2) \times (es - ea) / (\Delta + \gamma) \]

Where:

\( ER \) — Evaporation rate (mm per day)
\( \Delta \) — Slope of vapor pressure curve (kPa/°C)
\( Rn \) — Net radiation (MJ/m²/day)
\( \gamma \) — Psychrometric constant (kPa/°C)
\( U2 \) — Wind speed at 2m height (m/s)
\( es \) — Saturation vapor pressure (kPa)
\( ea \) — Actual vapor pressure (kPa)

Explanation: The equation combines energy available for evaporation (first term) with the drying power of air (second term) to estimate total evaporation.

3. Importance of Evaporation Rate Calculation

Details: Accurate evaporation estimation is crucial for water resource management, irrigation scheduling, hydrological modeling, and climate studies. It helps in predicting water losses from reservoirs and planning agricultural water requirements.

4. Using the Calculator

Tips: Enter all parameters in correct units. Delta and gamma are temperature-dependent parameters. Ensure es ≥ ea for physically meaningful results. Typical values: Δ (0.1-0.4 kPa/°C), γ (0.066 kPa/°C), Rn (5-25 MJ/m²/day), U2 (1-10 m/s).

5. Frequently Asked Questions (FAQ)

Q1: What surfaces does Penman equation apply to?
A: Primarily designed for open water surfaces, but modified versions exist for reference crop evapotranspiration (Penman-Monteith equation).

Q2: How accurate is the Penman equation?
A: Very accurate when proper meteorological data are available. It's considered one of the most reliable physical-based evaporation models.

Q3: What's the difference between evaporation and evapotranspiration?
A: Evaporation is from water surfaces, while evapotranspiration includes both evaporation from soil and transpiration from plants.

Q4: When should measurements be taken?
A: Ideal measurements are daily averages, preferably from standard meteorological stations at 2m height.

Q5: Are there limitations to this equation?
A: Requires comprehensive meteorological data. Less accurate in extreme conditions or when data quality is poor. Not suitable for vegetated surfaces without modification.