**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**

Evapotranspiration (ET) refers to the combined loss of water due to evaporation from soil surfaces (E) and transpiration from plants (T). It is not possible to directly measure ET, but we can calculate it from weather data. This calculated value is used to determine the irrigation required to replace the water used by a crop.

More than 20 equations have been developed to calculate the potential ET from weather data. The most detailed approach is the Penman-Monteith equation. This is based on biophysical principles and requires hourly averages of solar radiation, temperature, humidity, and wind. Not all weather stations, however, record the measurements to calculate ET from the PM equation.

The potential, or reference ET is not the same as the actual ET. Rather, it is the largest value of ET that would occur if soil water is not limiting.

Many studies have been conducted to find relationships between actual and potential ET for specific crops. The actual ET is determined by multiplying the reference ET by a value called the crop coefficient, which varies from zero to one. There is considerably more error associated with the estimate of the crop coefficient than with the estimation of ET.

The potential ET from this station is calculated using the Penman-Monteith equation. The reference crop is 2.5 cm tall grass.

Additional, detailed information on evapotranspiration and methods used to calculate it are available at:

http://www.fao.org/docrep/X0490E/x0490e04.htm