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ET maps reveal drought stress

11/02/2012 @ 2:47pm

The U.S. Drought Monitor provides abundant information on dry conditions on a national basis or by state. But its use for scheduling irrigation is limited.

Martha Anderson and Bill Kustas are working to change that.

The USDA Agricultural Research Service (ARS) scientists have developed an evapotranspiration (ET) and drought modeling system at ARS's Hydrology and Remote Sensing Laboratory. In addition to assisting farmers in scheduling irrigation in a local setting, this modeling system will also 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. This data can be used to create maps of ET rates of plants growing in cultivated areas.

What ET reveals

Evapotranspiration consists of the water evaporated from soil and plant surfaces, and the water vapor that escapes – or transpires – through plant leaf pores as the plants absorb carbon dioxide through photosynthesis. Generally, evaporation cools surfaces. As such, 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 tap into satellite temperature data to create ET maps. These maps are capable of detecting irrigated cropland under water stress. As Anderson and Kustas' research advances and is made publicly available, it can be accessed by farmers in the future to determine if individual fields or parts of fields need irrigation, particularly when ET readings are combined with local information from soil-moisture sensors.

Streamlined ET Readings

Getting routine ET estimates for individual fields is laborious. But Anderson and Kustas are streamlining the process. With help from new satellite imagery, they are able to move toward routine mapping at the field-scale level.

Alexi has been estimating ET rates since 2000. But the researchers continue to refine the system, and they plan to make the maps available online soon on the U.S. Drought Portal at www.drought.gov.

In the long run, Anderson and Kustas hope to provide local ET data for use in irrigation scheduling, just as is currently done from field weather stations. But the data from satellites would be for individual farm fields (rather than from the nearest field station), so it would more accurately reflect local conditions. This would be particularly helpful in places where there are no extensive networks of field weather stations.

The ARS work is being funded by the National Oceanic and Atmospheric Administration (NOAA) and the National Aeronautics and Space Administration (NASA). NOAA plans to use the system to generate ET estimates all over the U.S. The system is expected to become particularly relevant as climate change presents challenges for growers in areas such as the High Plains and the southwest U.S.

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