IT'S ironic that just when Earth-monitoring satellites are needed more than ever to address the food and freshwater demands of a burgeoning global human population, we face an impending gap in coverage by the Landsat program. A series of Landsat satellites has been continuously in orbit since 1972, collecting an invaluable time sequence of global imagery that records decades of land-use and land-cover changes. The recent decision by the US Geological Survey (USGS) to provide Landsat imagery free of charge has led to an explosion in applications, enabling unprecedented study of global deforestation, changes in cropping systems and irrigation practices, and conversion of land from its natural state to managed or urban use. Addition of a thermal infrared channel to the Landsat series in 1982, with Landsat 4, enabled monitoring of not just land use but also water use. Evaporation of water from the soil and transpiration by plants cool the land surface and generate a detectable thermal signal.
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