Evaporative demand is basically the pressure the atmosphere puts on plants and soil to draw water through evaporation.  Evaporative demand can tell us much about drying power we’re seeing over almost any time period we’re interested in tracking.  Often, we look at the Water Year, which runs from October 1 to September 30^th.  Usually the fall and winter are periods when you have very little plant growth and consequently very little water usage, which helps recharge the soil for the growing season starting in the spring.

This year, we have seen very little evaporative demand.  Some places, for example, like the Central to Northern Plains, that have experienced extreme flooding are seeing evaporative demand conditions that are way outside the norm. In some cases, they are seeing values that occur only about 5% to 2% of the time going back to 1978.