Evapotranspiration (ET) is the process by which water returns to the atmosphere due to evaporation and plant transpiration. Evaporation is the process by which water molecules go from the liquid state to the gas form, known as water vapor. Transpiration is the process in which plants release water molecules through the stomas of their leaves. The rate of evapotranspiration is determined by a complicated calculation based on temperature, solar radiation, humidity, and wind conditions. It is calculated for a reference plant crop, such as alfalfa. At Jackson Bottom Wetlands Preserve, there are several different plant communities. So the evapotranspiration rate should be used for general understanding and for comparing one time with another.
Relative Humidity is a measurement of the moisture in the air. It is expressed as a percentage. Relative humidity describes the percentage of water vapor in the air in relation to the total amount of water vapor that the air can hold. The amount of water vapor that the air can hold changes as the temperature changes.
Relative humidity affects how you feel on a warm day. If the relative humidity is low on a hot day, say 90-degrees, then it is easy for the body to cool itself by perspiring. However, when the relative humidity is high, the air does not want to take any additional water vapor, so it’s more difficult for your body to cool itself by perspiring.
Solar radiation is the energy that comes from the sun. Most of the radiation that reaches the Earth is short-wave radiation, including the visible light spectrum. Solar radiation is known as irradiance (with no directional component) and is measured in units of Watts per meter squared (W/m2). Radiation that reaches the Earth is either absorbed or reflected back into the atmosphere. The amount of absorption depends on the surface’s color, texture and other properties. Note how hot an asphalt surface gets during the summer when it absorbs the sun’s radiation.
The water year goes from October 1 through September 30 of the following year. It starts with the autumn months, goes through our rainy season and then through the relatively dry summer months.
Satellite Image from NOAA
Radar Image from National Weather Service