The Importance of Soil Moisture in Weather and Climate Prediction

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The Importance of Soil Moisture in
Weather and Climate Prediction

by John A. Leese and John Kermond
Office of Global Programs

Playing with mud -- we have all experienced the mixing of soil and water. We do it as children and as adult gardeners. But there is a lot of science, much known and some not yet fully understood, surrounding soil moisture.

Soil moisture is commonly defined as the amount of water contained in a unit volume of soil. The retention of moisture, or water, in the soil and the attendant runoff from naturally occurring rainfall, snow melt, or irrigation are fundamental processes upon which all civilization depends for food production, potable water, and navigable streams and waterways. But how is soil moisture linked to weather and climate prediction?

Put simply, soil moisture affects the transfer of moisture (remember that water vapor is a gas and one that can transport heat) into the atmosphere. Dry soil can contribute little to no moisture; saturated and super-saturated soils can contribute lots. So much, in fact, that large land surfaces that become flooded can create their own closed-loop as the evaporated moisture forms local clouds that continue to add to the system via continuing precipitation. This happened in the mid-west during the floods of 1993.

Soil containing a lack of moisture to sustain vegetation. Photo by: Gene Alexander

Soil containing a lack of moisture to sustain vegetation. Photo by Gene Alexander (USDA)

Flooded landscapes evaporate water vapor into the atmosphere in a manner similar to lakes or oceans.

Flooded landscapes evaporate water vapor into the atmosphere in a manner similar to lakes or oceans. Photo by Bob Nichols (USDA)

But how can scientists get an accurate handle on soil moisture given the enormous variations in land forms and the millions of square miles of land on this planet? They do this by using atmospheric general circulation models. These models are computer-driven mathematical representations of the major variables associated with the atmosphere. But remember that a model is just that, a model. Clouds and precipitation are not well represented in an equation! But the models do show us some valuable things.

Seeds need moisture in the soil to germinate and grow into mature plant.

Seeds need moisture in the soil to germinate and grow into mature plants. Photo by Tim McCabe (USDA)

For instance, in one atmospheric model study of the effects of soil moisture gradients on local scale precipitation patterns, it was found that evaporation from areas of high soil moisture could raise specific humidities and lower temperatures locally, leading to changes in thermal fields when warm or cool fronts are present in the area.

So, soil moisture is a key component in the land surface schemes in global climate models because it is linked to evaporation and thus to the distribution of heat fluxes from the land to the atmosphere. Just like the ocean where the surface temperature is a critical variable for model prediction, the first two meters of the soil and the moisture contained therein is also of paramount importance. Just ask any farmer!

The Office of Global Programs leads the NOAA Climate and Global Change Program. OGP assists NOAA by sponsoring focused scientific research aimed at understanding climate variability and its predictability. Through studies in these areas, researchers coordinate activities that jointly contribute to improved predictions and assessments of climate variability over a continuum of timescales from season to season, year to year, and over the course of a decade and beyond.

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