Moist Soil May Attract Rainfall

When we ponder about what Mother Nature might throw our way, we often look to the skies above for insight. But perhaps we should look below to “get the dirt!” Meteorologists know the amount of moisture in the soil can have a significant impact on our future weather. Currently, however, scientists have no direct way to observe soil moisture and rely on computer models and satellites to determine it.

Images/animations above: (left) Soil moisture plays an important role in summer rainfall, particularly in transition zones between very wet and very dry climates. The warm, moist air rises until it encounters colder air high above the Earth’s surface, leading to afternoon rainshowers. The water remains in the ground through the cool night, and the cycle repeats the next day. (right) Dry soil has the opposite effect on rainfall. As the temperature rises during the day, the air near the Earth’s surface heats up and rises, but does not contain enough moisture to form rainclouds. As each day passes more moisture is removed from the ground, enhancing the effect. Click on each still image to view its animation. Left animation is 1.2 MB, right animation is 697 Kb. Credit: Earth Observatory animations courtesy Susan Byrne, NASA GSFC )

New research that combined the findings from 12 different computer models may provide a way to learn more about how soil moisture interacts with the atmosphere. The study was led by Randal Koster, a researcher at NASA’s Goddard Space Flight Center in Greenbelt, Md. He and his colleagues used the computer models to detect several “hot spots” around the world, where soil moisture may strongly affect rainfall during the northern hemisphere summer. The study was conducted as part of the Global Land-Atmosphere Coupling Experiment (GLACE).

According to the models, the “hot spots” appear in the central plains of North America, the Sahel, equatorial Africa, and India. Less intense hot spots show up in South America, central Asia and China. In general, the hot spots have one thing in common: they occur in areas between wet and dry regions. This was expected. In wet climates, the Sun’s energy and cloudiness play a bigger role in determining evaporation rates than soil moisture. In dry climates, the limited water leads to reduced evaporation rates that are simply too small to have a large impact on the atmosphere.

Image/animation to left: This animation shows changes in soil moisture from July 1 through 15, 2002, derived from a land surface computer model. Soil moisture is important for knowing how much water is contained in the soil, which is vital for crop production, flood prediction, and evapotranspiration estimates. The blue colored areas represent wetter soils. Click on image to view animation (597 Kb) Credit: Images by Robert Simmon, Earth Observatory, NASA/GSFC and Univ. of Maryland-Baltimore County.

Soil moisture not only affects precipitation, but temperature as well. A very wet soil on a sunny day will give rise to more evaporation, while a very dry soil will allow more of the solar radiation to warm the surface, giving warmer daytime temperatures. Differences in soil moisture between neighboring regions can also influence regional weather, similar to a land-sea breeze.

Particularly in regional areas such as the northeastern U.S., scientists see trends in how moist soils are during different seasons. Accurate weather forecasts require detailed data on the rate of transfer of soil moisture to the atmosphere, whether by evaporation or plant transpiration. Evapotranspiration is the combination of evaporation – the transfer of water from the liquid to the gaseous state, and transpiration – the process by which plants remove liquid water from the soil and release it to the air as gaseous water.

Dry soils in the spring and summer can lessen the threat of flooding but harm agriculture. The duration and intensity of dry spells largely determine irrigation needs, which are in those parts of the country where water resources and river systems are most depleted during hot dry summers.

Image to right: The red areas are “Hot Spots” where soil moisture changes can affect rainfall, according to the GLACE multi-model study. The bars in the insets show the individual results for each of the 12 climate models, called Atmospheric General Circulation Models (AGCMs), averaged over the indicated regions. According to the insets, the models clearly do not show perfect agreement in the “strength” of the hot spots. Still, many independent models place the hot spots in the same place. The results pertain to Northern Hemisphere summer months, June, July and August. Red areas show the highest connection between soil moisture and rainfall. The units for the insets are the same as those for the color bar..Credit: Koster et. al, 2004, Copyright Science

By identifying how much moisture stays in soils, hydrologists will be able to determine how much more water they can absorb and better estimate the potential for flooding, drought, and fire danger. Also, by knowing which factors create large or small rainfall, hydrologists can provide better forecasts and know how much water will be available to people.

NASA helps in the design of satellites and instruments to measure soil moisture. Currently, the Advanced Microwave Scanning Radiometer for EOS on NASA’s Aqua satellite measures the moisture in surface soil down to a depth of a few centimeters (almost 1 inch). In 2009, NASA plans to launch the Hydrosphere State (HYDROS) mission that will provide the first global view of the Earth’s changing soil moisture down to five centimeters (1.97 inches).