Evaporation Loss Per Day Given Change in Vapour Pressure Calculator

Rohwer's Formula:

\[ E = C' \times (1.465 - (0.00732 \times P_a)) \times (0.44 + (0.0732 \times u)) \times \delta V \]

Rohwer's Formula Constant (C'):

Atmospheric Pressure (Pₐ):

cmHg

Mean Wind Velocity (u):

m/s

Change in Vapour Pressure (δV):

cmHg

Evaporation Loss per Day (E):

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1. What is Rohwer's Formula?

Rohwer's Formula is an empirical equation used to estimate daily evaporation loss from water surfaces. It takes into account atmospheric pressure, wind velocity, and vapor pressure changes to provide a more accurate estimation of evaporation rates.

2. How Does the Calculator Work?

The calculator uses Rohwer's formula:

\[ E = C' \times (1.465 - (0.00732 \times P_a)) \times (0.44 + (0.0732 \times u)) \times \delta V \]

Where:

\( E \) — Evaporation loss per day (cm)
\( C' \) — Rohwer's formula constant
\( P_a \) — Atmospheric pressure (cmHg)
\( u \) — Mean wind velocity (m/s)
\( \delta V \) — Change in vapour pressure (cmHg)

Explanation: The formula accounts for the combined effects of atmospheric pressure, wind speed, and vapor pressure difference on evaporation rates.

3. Importance of Evaporation Loss Calculation

Details: Accurate evaporation loss estimation is crucial for water resource management, irrigation planning, reservoir operations, and climate studies. It helps in predicting water availability and managing water storage systems efficiently.

4. Using the Calculator

Tips: Enter Rohwer's constant, atmospheric pressure in cmHg, mean wind velocity in m/s, and change in vapor pressure in cmHg. All values must be positive numbers.

5. Frequently Asked Questions (FAQ)

Q1: What is a typical value for Rohwer's constant?
A: Rohwer's constant typically ranges from 0.4 to 0.8, with 0.5 being commonly used for general calculations.

Q2: How does wind velocity affect evaporation?
A: Higher wind velocities increase evaporation rates by removing saturated air from the water surface and replacing it with drier air.

Q3: Why is atmospheric pressure important in evaporation calculation?
A: Lower atmospheric pressure reduces the boiling point of water and generally increases evaporation rates.

Q4: What factors influence vapor pressure change?
A: Temperature, humidity, and solar radiation are the main factors that affect vapor pressure differential.

Q5: When is this formula most accurate?
A: Rohwer's formula provides good estimates for open water surfaces under normal climatic conditions, but may require calibration for specific locations.