Soil moisture has an obvious, visible effect on the landscape. The high-profile examples are droughts and floods. But the water in the soil has a more subtle, yet equally important role in day-to-day weather.
Soil moisture forms a vast, thin, and mostly out-of-sight reservoir of water that accumulates in the root zone of plants. The water is released to the atmosphere through evaporation and plant transpiration. Averaged globally, this evapotranspiration contributes to more than 60 percent of the precipitation that falls over land each year.
“The first time we were struck by the importance of soil moisture for weather forecasts was in July 1993,” said Patricia de Rosnay, a researcher at the European Centre for Medium-Range Weather Forecasts (ECMWF). During the first six months of that year, extreme amounts of rain and snow fell on the central United States. Yet the existing weather models were not accounting for the storage and evaporation of all of that water. They could not see how the water on the land was feeding back into the weather patterns to make the deluge more extreme. By July 1993, the Upper Mississippi River faced its worst flooding on record.
July 1993 also happened to be the same month that ECMWF scientists began testing a new weather forecast model. Their model accounted for soil moisture in the root zone, allowing researchers to see how the soil sustained a high level of evaporation and fed the extreme rainfall event. The new model had produced a closer representation of reality.
The strength of the connection between soil moisture and the weather is not the same everywhere. According to NASA scientist Randy Koster, there are hot spots—about 10 percent of Earth’s surface where the amount of soil moisture plays a more critical role in the weather.
Water that evaporates from Earth’s surface is linked to the formation of clouds and rainfall. In dry areas, variations in the amount of evaporation are too small to have much of an effect on the atmosphere. In humid regions, particularly the tropics, changes in soil moisture do not matter much for evaporation because it is limited by the amount of water that the atmosphere can hold.
The soil moisture hot spots are areas that are neither too dry nor too wet. They are located in the transition zones between dry and wet areas—places that have suitably high evaporation that is more dependent on moisture on the ground than in the atmosphere. The Midwestern United States is one of those hot spots. So, too, are northern India and the African Sahel.
Better estimates of soil moisture in weather models will not necessarily make for perfect long-range forecasts. Randomness in the variables that cause weather will always hinder the accuracy beyond a few days. But with better information on the thin reservoir in the soil, forecasters can tip the scales further in favor of getting weather prediction right.