Using Satellites to Map Changes in Water Availability

(Dr. Jay Famiglietti) With the current generation of satellites, we can now track changes to water availability into water storage all over the world. The pattern that emerges is one that is quite compelling in the following ways. Imagine a global pattern where the wet areas of the world are getting wetter. That’s the High Arctic and the Tropics; the high latitude parts of the world and the low latitude parts of the world. Those areas are getting wetter. The arid and semi-arid parts in the middle are getting drier. We see the wet areas of the world getting wetter and the dry areas of the world getting drier. Embedded in those drying mid-latitudes are these hot spots for water scarcity which basically represent the world’s major aquifer systems which also coincide with the world’s major food producing regions. We see that those aquifers are being depleted at unsustainable rates. The satellite that we’ve been using in our group, which is called GRACE which stands for gravity recovery and climate experiment, we are actually weighing water mass changes. I like to say that GRACE works like a scale in the sky. There are two satellites. They orbit at about 400 kilometers. They’re over the poles. They chase each other around; they are separated by about 200 kilometers. They’re flying in a line over the poles, and of course, the earth is spinning it. There are satellites, which are sweeping over the earth. What happens is those two satellites they act like a scale because they’re responding to gravitational variations in earth’s gravity field. What does that mean? It means the following. When those satellites fly over a region that has gained water mass because of a big flood, say flooding in the upper Missouri, that region has gained water weight. It exerts a slightly greater tug on those satellites that are 400 kilometers up in the air, pulls them down towards the earth just a little bit, a few millimeters. Likewise, when those satellites fly over place like you had a drought here few years ago, this region lost a lot of water weight. This region exerts just a slightly less of a gravitational tug on the satellites, and they float just a little bit higher in their orbit just a few millimeters. We track the position of those satellites incredibly accurately to a thousandth of a millimeter who’s less than that, set high, greater than that, so we call up the sub-micron level. By keeping track of the ups and downs of the satellites, it’s like keeping track of the surface of a scale. It allows us to map out the regions underneath where those measurements are taken that are gaining or losing water mass on a monthly basis. We can create these incredible maps and these hot spots or these aquifers just pop right up. If we had to rank Middle East and the North China Plain or in India North Western India are among the aquifers being most rapidly depleted. But here in the United States the Southern part of the Ogallala and the Central Valley in California, we’re right behind those top ranked aquifers in terms of depletion rate. Over half of the world’s major aquifers are being depleted at a very rapid clip. Most of these aquifers, the ones that are being depleted, support irrigation for the world’s major food producing regions. There is no question that it takes a lot of water to grow food. We need to get some basic parameters. How much water do we have? How much do we need and the needs now need to include not just food production. But we have to leave some water behind for the environment, we have to think about energy production and how are those changing. How’s the supply and how is the demand changing over time? I think that many of these problems are going to be solved in the United States anyway; state-by-state or region-by-region. When you look at Kansas and Nebraska and Oklahoma and look at the Ogallala, we’re producing a lot of grains, a lot of corn. Just using water from that region but it supports the whole world, it supports the whole country and the same is true for California. We may be needing some help from other states. But we use more than we have available to us. We have to get a grip on slowing the rate of depletion if not stemming the rate of depletion. Can that be done? Probably. A lot of it depends on what the climate is like and what the commitment is to building the infrastructure to get that recharge. It takes a lot of water to grow food. We use more than we have and we’re making up the difference from groundwater, and it’s not been replaced. But what the satellite allows us to see is that this is a global problem and let’s remember that population is growing. We’re going to have more and more challenges ahead to produce food for growing population while that ground water supply is dwindling.

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