Penman Equation Lake Evaporation Calculator

Penman Equation for Lake Evaporation:

\[ E = \frac{\Delta(R_n - G) + \gamma \frac{900}{T + 273} u_2 (e_s - e_a)}{\Delta + \gamma (1 + 0.34 u_2)} \]

Net Radiation (R_n):

MJ/m²/day

Soil Heat Flux (G):

MJ/m²/day

Mean Temperature:

°C

Wind Speed at 2m (u₂):

m/s

Saturation Vapor Pressure (e_s):

kPa

Actual Vapor Pressure (e_a):

kPa

Evaporation Rate:

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1. What is the Penman Equation?

The Penman equation is a widely used method for estimating evaporation from open water surfaces. It combines energy balance and aerodynamic principles to provide a comprehensive estimate of evaporation rates.

2. How Does the Calculator Work?

The calculator uses the Penman equation:

\[ E = \frac{\Delta(R_n - G) + \gamma \frac{900}{T + 273} u_2 (e_s - e_a)}{\Delta + \gamma (1 + 0.34 u_2)} \]

Where:

\( E \) — Evaporation rate (mm/day)
\( R_n \) — Net radiation at the surface (MJ/m²/day)
\( G \) — Soil heat flux (MJ/m²/day)
\( \Delta \) — Slope of saturation vapor pressure curve (kPa/°C)
\( \gamma \) — Psychrometric constant (kPa/°C)
\( T \) — Mean air temperature at 2m height (°C)
\( u_2 \) — Wind speed at 2m height (m/s)
\( e_s \) — Saturation vapor pressure (kPa)
\( e_a \) — Actual vapor pressure (kPa)

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

3. Importance of Evaporation Calculation

Details: Accurate evaporation estimation is crucial for water resource management, irrigation planning, and hydrological modeling.

4. Using the Calculator

Tips: Enter all required meteorological parameters. Typical values: R_n (5-25 MJ/m²/day), G (often small, ~0.1 MJ/m²/day), T (local temperature), u₂ (1-5 m/s), e_s and e_a from humidity data.

5. Frequently Asked Questions (FAQ)

Q1: Why use Penman equation instead of simpler methods?
A: Penman equation provides more accurate estimates by considering both energy balance and aerodynamic factors.

Q2: What are typical evaporation rates?
A: Varies by climate: 2-4 mm/day in temperate regions, up to 10 mm/day in hot arid regions.

Q3: How to estimate net radiation (R_n)?
A: Can be calculated from solar radiation, albedo, and thermal radiation using standard formulas.

Q4: What if I don't have all parameters?
A: Simplified versions exist (like Penman-Monteith), but accuracy decreases with fewer inputs.

Q5: Does this work for all water bodies?
A: Best for open water surfaces (lakes, reservoirs). Vegetated areas need modified approaches.