Evaporation Rate Calculator

Calculate Water Evaporation Rate

Estimate the amount of water evaporating from a surface based on environmental conditions. Our evaporation rate calculator helps you understand water loss.

Evaporation Rate at Different Wind Speeds

Wind Speed (m/s)	Evaporation (mm/day)	Evaporation (liters/day)
0	0.00	0.00
1	0.00	0.00
2	0.00	0.00
3	0.00	0.00
4	0.00	0.00
5	0.00	0.00

Table showing estimated evaporation rate at various wind speeds, assuming current air/water temp, humidity, and surface area.

What is Evaporation Rate?

The evaporation rate is a measure of the amount of a substance, typically water, that changes from a liquid to a gaseous or vapor state from a given surface area over a specific period. It is a crucial parameter in hydrology, agriculture, meteorology, and engineering, particularly in managing water resources, designing reservoirs, and understanding climate patterns. A higher evaporation rate means more water is lost to the atmosphere.

This evaporation rate calculator helps estimate water loss from surfaces like pools, lakes, reservoirs, or even moist soil. It’s useful for homeowners with pools, farmers managing irrigation, and engineers designing water storage facilities.

Common misconceptions include thinking evaporation only happens when water boils (it happens at all temperatures) or that it’s solely dependent on temperature (wind and humidity are also very important).

Evaporation Rate Formula and Mathematical Explanation

The evaporation rate can be estimated using various formulas, from simple empirical ones to complex energy balance methods like the Penman-Monteith equation. This calculator uses a simplified Dalton-type equation, which relates the evaporation rate to the vapor pressure deficit and wind speed:

E = f(U) * (e_ws – e_a)

Where:

• E is the evaporation rate (e.g., in mm/day).
• f(U) is a wind function, often linear like (a + bU), where U is wind speed. In our calculator, we use f(U) = (0.2 + 0.1 * U) with U in m/s.
• e_ws is the saturation vapor pressure at the water surface temperature (in kPa).
• e_a is the actual vapor pressure of the air (in kPa).

The saturation vapor pressure (e_s) at a given temperature (T in °C) can be estimated using the Tetens equation:

e_s(T) = 0.6108 * exp((17.27 * T) / (T + 237.3)) (in kPa)

So, e_ws = e_s(T_water) and e_a = (RH / 100) * e_s(T_air), where RH is relative humidity (%).

The final evaporation in mm/day is then: E (mm/day) = (0.2 + 0.1 * U) * (e_ws – e_a).

To get the total volume lost in liters per day: Evaporation (liters/day) = E (mm/day) * Surface Area (m²) (since 1 mm over 1 m² is 1 liter).

Variables Used in the Evaporation Rate Calculation

Variable	Meaning	Unit	Typical Range
A	Water Surface Area	m²	0.1 – 1,000,000+
Tair	Air Temperature	°C	-20 – 50
Twater	Water Surface Temperature	°C	0 – 40
RH	Relative Humidity	%	10 – 100
U	Wind Speed at 2m	m/s	0 – 20
ews	Saturation Vapor Pressure at Twater	kPa	0.6 – 7.4
ea	Actual Vapor Pressure	kPa	0.1 – 7.4
E	Evaporation Rate	mm/day or liters/day	0 – 20+

Practical Examples (Real-World Use Cases)

Example 1: Swimming Pool Evaporation

A homeowner has a swimming pool with a surface area of 30 m². On a summer day, the air temperature is 30°C, the water temperature is 28°C, relative humidity is 50%, and the wind speed is 1.5 m/s.

• Surface Area (A) = 30 m²
• Air Temperature (T_air) = 30°C
• Water Temperature (T_water) = 28°C
• Relative Humidity (RH) = 50%
• Wind Speed (U) = 1.5 m/s

Using the calculator, we find an estimated evaporation rate of around 6 mm/day, leading to a water loss of about 180 liters per day from the pool.

Example 2: Small Farm Pond

A farmer has a small pond with a surface area of 500 m². On a dry, windy day, air temperature is 25°C, water temperature is 22°C, relative humidity is 30%, and wind speed is 4 m/s.

• Surface Area (A) = 500 m²
• Air Temperature (T_air) = 25°C
• Water Temperature (T_water) = 22°C
• Relative Humidity (RH) = 30%
• Wind Speed (U) = 4 m/s

The calculated evaporation rate might be around 8-9 mm/day, resulting in a significant water loss of 4000-4500 liters per day from the pond, impacting water availability for irrigation.

How to Use This Evaporation Rate Calculator

Using this evaporation rate calculator is straightforward:

1. Enter Surface Area: Input the area of the water surface in square meters (m²).
2. Enter Air Temperature: Input the average air temperature in degrees Celsius (°C).
3. Enter Water Temperature: Input the average water surface temperature in °C. Often, this is close to air temperature but can differ.
4. Enter Relative Humidity: Input the average relative humidity as a percentage (%).
5. Enter Wind Speed: Input the average wind speed at about 2 meters above the surface in meters per second (m/s). You might need a wind speed converter if you have data in other units.
6. View Results: The calculator automatically updates the primary result (Evaporation Rate in liters/day) and intermediate values like vapor pressures and mm/day evaporation. The chart and table also update.
7. Interpret Results: The primary result shows the estimated total volume of water lost per day. Intermediate values help understand the driving forces. The chart and table show how the evaporation rate varies with wind and humidity.

This tool helps in estimating water loss, planning make-up water for pools or ponds, and understanding the impact of local weather conditions on the evaporation rate.

Key Factors That Affect Evaporation Rate Results

The evaporation rate is influenced by several interconnected environmental factors:

1. Temperature (Air and Water): Higher temperatures increase the energy available for water molecules to escape into the air and also increase the saturation vapor pressure, thus increasing the potential for evaporation.
2. Relative Humidity: Lower relative humidity means the air is drier and can hold more water vapor, leading to a higher vapor pressure deficit and a higher evaporation rate. High humidity reduces it.
3. Wind Speed: Higher wind speeds remove the moist air layer just above the water surface and replace it with drier air, maintaining a higher vapor pressure gradient and increasing the evaporation rate.
4. Water Surface Area: A larger surface area naturally leads to a larger total volume of water evaporated, even if the rate per unit area (mm/day) remains the same. Check our surface area calculator for help.
5. Solar Radiation: Although not a direct input in this simplified model, solar radiation is the primary energy source that heats the water and air, thus indirectly influencing temperatures and evaporation. More complex models include net radiation.
6. Water Body Characteristics: The depth, shape, and surroundings (shelter) of the water body can influence water temperature and wind exposure, affecting the evaporation rate. Salinity also reduces the evaporation rate slightly compared to fresh water.

Frequently Asked Questions (FAQ)

Q1: Why is my pool losing more water than the calculator estimates?

A1: Besides evaporation, pools can lose water through leaks, splash-out, and filter backwashing. The calculator only estimates the evaporation rate component.

Q2: Can I use this calculator for other liquids?

A2: No, this calculator and the formulas are specifically for water evaporation. Other liquids have different vapor pressures and properties.

Q3: How accurate is this evaporation rate calculator?

A3: It provides a reasonable estimate based on a simplified model. For highly accurate scientific or engineering purposes, more complex models like Penman-Monteith, which require more data (like solar radiation), are used. The accuracy of our weather data explained page might be useful.

Q4: Does water salinity affect the evaporation rate?

A4: Yes, dissolved salts reduce the vapor pressure of water slightly, so saline water evaporates at a slightly lower rate than fresh water under the same conditions. This calculator assumes fresh water.

Q5: What is the difference between evaporation and evapotranspiration?

A5: Evaporation is the water loss from open water or bare soil. Evapotranspiration includes evaporation plus transpiration (water released by plants).

Q6: How can I reduce evaporation from my pool or pond?

A6: Using a pool cover is very effective. Windbreaks can also reduce wind speed over the surface. For ponds, floating covers or shade structures can help reduce the evaporation rate. See our pool maintenance guide.

Q7: Why does the water temperature input matter?

A7: The saturation vapor pressure at the water surface (e_ws) is determined by the water surface temperature, which is a key driver of the evaporation rate.

Q8: How do I get accurate temperature, humidity, and wind speed data?

A8: You can use a local weather station, online weather services for your area, or portable weather meters. For best results, use data measured close to the water body.