Water to the desert
This image was captured by the Earthkam camera on the International Space Station during a flight over Saudi Arabia earlier this year. The town of Wadi Aldwasir has an estimated population of about 70,000 inhabitants. It is in south-central Saudi Arabia, surrounded mostly by deserts. However, it sits in an ephemeral river channel – when it does rain, water flows through the gap in the hills at the southeast and enters the desert near the city. You can see salts visible in this photo sitting in that channel.
The presence of a river channel in a desert setting often leads to elevated groundwater levels. Groundwater will form in this setting when water flows in these channels – water will literally leak out of the bottom of the river and enter open porespace in the ground below.
Water leaking from a river will keep the level of groundwater fairly shallow, and shallow groundwater is in an area that humans can reach with drills and pumps. Each of the green circles in this image is a “Central pivot irrigation system” – an irrigation system pulls water from the ground at the center of the circle and the sprinkler assembly rotates around the center, watering the ground. Each of the green circles at this site is water being pulled from the ground and used to water crops.
Saudi Arabia is one of the heaviest consumers of groundwater in the world, but groundwater resources aren’t easily renewed, even here. It’s a desert; the river isn’t always flowing. Water that is pulled out of the ground can have been there for anything from months to millennia. In a desert like this, it likely will take a long time for water pumped from the ground to be replaced.
Satellite measurements have shown that Saudi Arabia has drawn down their groundwater supplies as fast as anywhere on Earth. Groundwater pumping is limited by energy costs – the deeper the water is, the more energy it takes to pump the water out of the ground. Saudi Arabia of course is a land of cheap energy, in this case supplied largely by the sun, so they’ve had the resource to get the water from the ground easily. But, the longer pumping goes on, the deeper groundwater gets and the more expensive it gets. Plus, deeper and older groundwater is often saltier and less useful for irrigation. Groundwater pumping can be used as a resource to grow crops, but eventually that resource will dry up.
In fact, if you look closely at this frame, there is a cluster of circles turning grey. These are likely irrigation systems that have been abandoned – either water wasn't shallow enough to run them or pumping the water became too expensive to manage those fields.
-JBB
Image credit: NASA/ISS http://images.earthkam.org/main.php?g2_itemId=799010 Read more: http://bit.ly/1IPEipg https://www.jpl.nasa.gov/spaceimages/details.php?id=PIA20077_ _
