Vapor Pressure Deficit
Pressure Deficit, or VPD, is the difference (deficit) between the amount of moisture in the air and how much moisture the air can hold when it is saturated. Once air becomes saturated, water will condense out to form clouds, dew, or films of water over leaves.
Evapotranspiration (ET) is a term used to describe the combined loss of water due to evaporation from soil surfaces and transpiration from plants. This calculation is used to estimate the amount of irrigation required to exactly replace the water used by a crop. Unlike temperature, it is not possible to directly measure ET, but it is possible to estimate potential ET to within 10% by using equations that predict it from weather data.
If soil moisture is not limiting, ET increases in hot, windy, sunny, low humidity conditions. If the surface has short, healthy vegetation that completely covers the ground and is well supplied with water, the ET value will be determined by the weather conditions.
More than 20 equations have been developed to calculate reference ET from weather data. The most exact equation is based on biophysical principles and requires weather data that includes hourly averages of solar radiation, temperature, humidity, and wind. This is called the Penman-Monteith equation. Not all climate stations, however, record all the necessary measurements.
The Penman-Monteith equation was developed from energy transfer principles to calculate the potential, or reference ET. It is not the same as the actual ET. Rather, it is the largest value of ET that occurs under the weather conditions at the time.
Many studies have been conducted to find relationships between actual and reference ET for specific crops. The actual ET is determined by calculating the reference ET and multiplying it by a value called the crop coefficient. Note that there is usually considerably more error associated with the estimate of the crop coefficient than with the estimation of ET.
The reference ET from this station is calculated using the Penman-Monteith equation. The reference crop used is clipped grass, approximately 2.5 cm tall.
Additional, detailed information on evapotranspiration and methods used to calculate it are available at: