How to predict evaporation rate?

How to Predict Evaporation Rate from a Lake

Estimating how quickly water evaporates from a lake isn’t just guesswork—it’s science. One of the most practical tools for this is a simplified version of the Penman formula, widely used in hydrology and agriculture. This formula helps predict daily evaporation rates based on a few key environmental factors.

The equation looks like this: E₀ = 700 Tₘ / (100 − A) + 15 (T − T_d) (80 − T), with the result in millimeters per day. Here, Tₘ stands for the temperature adjusted for elevation—calculated as T + 0.006h, where T is the mean air temperature (°C) and h is the site’s elevation in meters. The higher you go, the thinner the air, and this adjustment accounts for that.

A represents latitude in degrees, influencing how much solar energy the location receives. Closer to the equator? Expect higher potential evaporation. Meanwhile, T_d, the mean dew-point temperature, reflects humidity levels—lower dew points mean drier air, which pulls more moisture from the lake surface.

The formula combines energy availability (via temperature and location) with aerodynamic factors (like humidity and wind, indirectly captured through temperature spread). While it’s simplified, it’s surprisingly effective for regions without complex instrumentation.

For example, a warm, dry day at a low elevation near 40° latitude will yield a much higher evaporation rate than a cool, humid day at high latitudes. This makes the formula especially useful for water resource management, irrigation planning, and climate studies.

Of course, real-world conditions vary—wind speed, cloud cover, and water quality also play roles—but this model offers a solid starting point. With just a few measurements, you can get a reliable estimate of how fast a lake might lose water to the atmosphere.