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A vast alluvial fan unfolds across the desolate landscape between the Kunlun and Altun mountain ranges that form the southern border of the Taklimakan Desert in China’s Xinjiang Province. At its broadest point, the fan is about 60 kilometers long and 55 kilometers wide. The left side is the active part of the fan. Water flowing down from the mountains in the many small streams appears blue in this 2002 image taken by the NASA Terra satellite. Vegetation appears red and can be seen in the upper left corner of the image.

Farmers take advantage of water at the foot of the fan to irrigate small fields. The “lumpy” terrain at the top of the image is composed of sand dunes at the edge of the Taklimakan, one of the largest sandy deserts on Earth. Shifting sand dunes, some reaching as high as 200 meters, cover more than 80 percent of the desert floor.

The NASA Earth As Art collection features stunning images of Earth from five U.S. Earth-observing satellites. Image courtesy of NASA.

When water falls on Earth in its liquid or frozen form, it continues its path on the water cycle. What happens immediately after water reaches the ground will depend on many factors such as the type of soil, vegetation or degree of urbanization. If the water falls in an urban area, for example, the impermeable surfaces keep the water from infiltrating into the ground, and the sewer systems are efficient at moving the water away.

The USGS estimates that 70% of rainfall will return to the atmosphere through evaporation or transpiration. The remainder will eventually enter streams or rivers and begin to move back to the ocean. How long it takes to enter the ocean may also depend on the weather when it landed. If it landed on a glacier as snow or ice, for example, it could take years to make its way to the ocean. If the water landed in an east or west coast city, it could make its way the ocean quickly.  The USGS estimates the amount of water flowing in streams in the U.S. at 1,200 billion gallons per day.  

Note: This lesson is based on the NASA Viz App module The Water Cycle: Following the Water.

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