Even in the desert there is water, if you drill deep enough. thumbnail: http://svs.gsfc.nasa.gov/images/gallery/Landsat/Wadi As Sirhan compare_gal.png

Above the Earth, Below the Surface: Landsat’s Role in Monitoring Water Quality
Safeguarding freshwater resources is crucial, and while scientists use a variety of ground-based techniques to gauge water quality, the Landsat program has provided water quality data from orbit for decades.