New USDA Model Predicts Drought
May 7, 2012 Growers and water managers may soon have an online tool to help them assess drought and irrigation impacts on water use and crop development, thanks to the work of two USDA researchers. Agricultural Research Service (ARS) scientists Martha Anderson and Bill Kustas have developed an evapotranspiration (ET) and drought modeling system at the ARS Hydrology and Remote Sensing Laboratory in Beltsville, MD. The modeling system also will help forecasters monitor ET and drought conditions across the U.S. and overseas.
The model, known as ALEXI (Atmosphere-Land Exchange Inverse), uses thermal infrared imagery from satellites and calculates soil and plant temperatures that can be used to create maps of ET rates of plants growing in cultivated areas around the world.
ET consists of the water evaporated from soil and plant surfaces, and the water vapor that escapes, or transpires, through plant leaf pores (stomata) as the plants absorb carbon dioxide through photosynthesis. Generally, evaporation cools surfaces, so a cooler land surface is an indicator of higher ET rates and wetter soils.
Water stress elevates soil and leaf temperatures, which can be detected by satellites. Anderson and Kustas can use satellite temperature data to create ET maps. The maps are capable of detecting areas under water stress.
The work is funded by the National Oceanic and Atmospheric Administration (NOAA) and NASA. NOAA plans to use the system to generate ET estimates over the entire U.S. The system is expected to become particularly relevant as climate change presents challenges for growers across the U.S.
Getting routine ET estimates for individual fields is laborious, but the researchers are streamlining the process. With help from satellite imagery, they hope to be able to move toward routine mapping at the “field” level.
ALEXI has been estimating evapotranspiration (ET) rates since 2000, but researchers continue to refine the system and plan to make the maps available online soon on the U.S. Drought Portal at www.drought.gov.
More And More Uses
Besides drought monitoring, water management, and irrigation scheduling, uses of the ALEXI/satellite package include crop yield prediction. “If crops suddenly show stress, we can ask whether that will affect yield, which will depend on the crop and whether it’s in a critical growth stage when drought occurs,” Anderson says.
Another use is in weather forecasting. Differences between land and air temperatures have major effects on weather. “With five- to 15-minute readings from the geostationary satellites, we can monitor the changes in land and air temperatures as the sun rises. Since heat transfer from the land surface is largest around noon, late morning to early afternoon is when there is the greatest potential for turbulence caused by the temperature difference between land and air,” Anderson says.
In addition, remotely sensed ET and soil moisture maps can be assimilated into meteorological models, improving short-range weather forecasts.
A full-length version of this article appears in the February 2012 issue of Agricultural Research, published by USDA’s Agricultural Research Service. To find the article, go to www.ars.usda.gov/is/AR/archive/feb12/drought0212.htm.