What is evapotranspiration?Evapotranspiration is a calculated estimate of the water that evaporates from soil and plant surfaces and the water plants lose through their leaves. The term used for water loss from plants is transpiration. “Evapo-” from evaporation and “transpiration” are combined to create the term, evapotranspiration. Evapotranspiration rates are the inches of water loss for each day. Daily evapotranspiration rates are added together to estimate a cumulative water loss over a given number of days. This water loss are the inches of water needed to maintain healthy plant growth.
Each plant type has its own unique evapotranspiration rate. For example, tall fescue uses more water per day than bermudagrass, so the daily evapotranspiration rate for tall fescue will be higher than bermudagrass under the same weather conditions.
How is evapotranspiration calculated?Daily evapotranspiration rates are calculated from air temperature, relative humidity, wind speed, sunlight and air pressure. For many years, different formulas were used to calculate evapotranspiration rates. In 2002, the American Society of Civil Engineers proposed a standardized reference evapotranspiration formula. This formula provided reference evapotranspiration rates for a short crop canopy, similar to a 4-inch clipped grass and a tall crop canopy, similar to alfalfa.
The reference evapotranspiration rate is adjusted by a unique coefficient for each crop or plant species. In calculating evapotranspiration rates for the Simple Irrigation Plan unique grass coefficients for bermudagrass, zoysiagrass and tall fescue are used. To better estimate water use these coefficients are adjusted as the air temperature changes.
Evapotranspiration models from the Oklahoma Mesonet operate year-round and are updated once each day using the 24-hour period from midnight to midnight Central Standard Time. Model output is updated daily, shortly after midnight.
How is evapotranspiration used?For the Simple Irrigation Plan, Oklahoma Mesonet weather data are used to calculate evapotranspiration rates for each Mesonet site. By selecting the nearest Mesonet tower and entering customized watering information, watering times are calculated for different grass and sprinkler types.
Using evapotranspiration to schedule turfgrass watering can improve water use efficiency and plant health. It is a tool that makes water management more efficient by calculating how much water is needed since the last rainfall or irrigation.