Crop monitoring from space

Landsat 7 records thermal images that can be used to estimate evapotranspiration. Landsat imagery courtesy of NASA Goddard Space Flight Center and U.S. Geological Survey

The USDA Agricultural Research Service, funded by the National Oceanic and Atmospheric Administration (NOAA) and the National Aeronautics and Space Administration (NASA), is developing a computer model to map evapotranspiration data from Landsat satellites, which can be used to remotely assess crop health.

The US Geological Survey Landsat satellites have been in orbit since 1972. These satellites, which take an image of each point on the Earth’s surface once every 16-18 days, have thermal and optical sensors to collect land imagery. This is used by agriculture and forestry researchers and global change analysts to assess changes in vegetation, soil moisture content and mineral deposits.

The model that the USDA scientists are creating (Atmosphere-Land Exchange Inverse model; ALEXI) takes data from the Landsat satellites, along with lower resolution imagery of vegetation from NASA’s Terra and Aqua satellites. Temperature data can be used to estimate levels of soil evaporation and plant transpiration. By identifying anomalies in the evapotranspiration levels, ALEXI can identify regions of drought. Researchers have compared the ALEXI drought-mapping software with the traditional methods of hydrologic mapping, and found that remote-sensing with satellites gives more detail and is more accurate.

There are many potential applications of the data obtained from ALEXI. By monitoring areas of drought, it will be possible to predict the levels of water stress that crops are under and hence enable a prediction of crop yield. Currently, information from field weather stations is used to inform irrigation scheduling. However, the new remote-sensing method could give evapotranspiration measurements that are specific to individual fields. This would be particularly useful in less developed countries where there aren’t already networks of weather stations, and would enable farmers to be more accurate with their irrigation. ALEXI’s data can also be used to improve weather forecasting because evapotranspiration affects likelihood of storms and flooding.

There is still quite a lot to be developed over the next two years of the ALEXI project. At the moment it’s very time-consuming to produce regular information about individual fields; in order for these satellites to be used for real-time monitoring of evapotranspiration and thermal patterns, more frequent imaging is required. Only one of the six launched Landsat satellites (Landsat 7) is still in operation, but Landsat 8 is scheduled to launch in January 2013, which should speed up data acquisition and allow routine mapping of crop production areas.

Source: Comis, D. (2012) A new way to map drought and water use worldwide. Agricultural Research 60 (2): 4-7.

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