Water leaves a field in several ways. Some runs off the surface, some drains below the root zone, and a large share simply becomes vapor. That last pathway combines two processes: evaporation from soil and transpiration through plants. Together they are called evapotranspiration.
Why plants lose water on purpose
Transpiration is not waste. Plants open pores in their leaves to take in carbon dioxide for photosynthesis, and water vapor escapes through the same openings. Growth and water loss are physically coupled, which is why a healthy, actively growing crop uses more water than a stressed one.
What drives the rate
Temperature, humidity, wind, and sunlight all affect how fast water moves into the air. A hot, dry, windy day can pull several times more water from a field than a cool, humid, still day. Crop stage matters too, since a full canopy transpires far more than seedlings.
The practical consequence
Because evapotranspiration varies daily, a fixed watering schedule is guaranteed to be wrong most days. It over-waters during mild weather and under-waters during heat. Estimating evapotranspiration from weather data is the single most useful calculation in irrigation scheduling, and it is the backbone of nearly every serious water recommendation system.