Instruments on the Landsat satellites capture images in the visible spectrum, but they also take images in wavelengths invisible to the naked eye. Landsat's thermal imager captures land surface temperature data. As farmers irrigate fields, water evaporates from the soil and transpires from plants' leaves. The combined process is called evapotranspiration. Evapotranspiring water absorbs energy, so farm fields consuming more water appear cooler in the thermal band. Landsat-based evapotranspiration measurements provide an objective way for water managers to assess on a field-by-field basis how much water agricultural growers are using. The measurements have even been used to help settle water rights conflicts in court.
In this animation, the first image shows an agricultural region in Idaho on August 14, 2000, captured in the visible spectrum. The round, green circles are irrigated farm fields. The second image, using Landsat's thermal band, depicts cooler and warmer areas. Irrigated fields appear cooler because evapotranspiring water absorbs energy and cools the fields. The thermal image dissolves into a map of evapotranspiration, created using the METRIC tool.
This thermal band image shows the same agricultural region in Idaho on the same day. The thermal band - Landsat's band 6 - captures data on land surface temperature. As agricultural growers irrigate farm fields, water evaporating from the soil and transpiring from the plants' leaves absorbs energy and cools the fields.
GCMD keywords can be found on the Internet with the following citation:
Olsen, L.M., G. Major, K. Shein, J. Scialdone, S. Ritz, T. Stevens, M. Morahan, A. Aleman, R. Vogel, S. Leicester, H. Weir, M. Meaux, S. Grebas, C.Solomon, M. Holland, T. Northcutt, R. A. Restrepo, R. Bilodeau, 2013. NASA/Global Change Master Directory (GCMD) Earth Science Keywords. Version 126.96.36.199.0