Past IEM Features tagged: pet
The past few days have seen near perfect drying conditions with bright sunshine, strong winds, warm temperatures and low humidity. The featured chart displays potential evapotranspiration estimates from the ISU Soil Moisture network since 1 May along with daily averages computed from the longer term data from the legacy ISU AgClimate station. The past two days are shown at 0.20+ inches which implies that depth of water leaving the ground surface and moving into the atmosphere. The number is an estimate and a potential maximum value, so actual values may be less. Much of that water looks to reverse path today with thunderstorms present already this morning.
Having water available in the soil for plant use is obviously important during the growing season. Different plants require different amounts of water to thrive. Potential evapo-transpiration (PET) is an estimate of the amount of water that a plant could transpire given always available soil water and observed air temperature, humidity, and solar radiation conditions. This is sometimes expressed in a unit like how we measure precipitation (depth of liquid water) and represents an upper bound on the amount of water a plant could utilize. The featured chart presents the daily accumulation of PET for this year and past years since 1987 for the Ames ISUAG station. The two recent big drought years of 2012 and 1988 appear with the larger values. This may seem counter-intuitive that drought years have the highest potential water use. Drought conditions often include less humid conditions and hot temperatures, both of which promote water usage by plants when soil water is available. The availability of soil water is the key to how much PET is actually materialized into evaporation and transpiration. Unfortunately ET is difficult and expensive to measure, so we are stuck with PET estimates to provide insight into water usage by plants.