Irrigation--Twinkle, twinkle, little laser

by Biodun Iginla, Tech Analyst for BBC News, the Economist, France 24, and allvoices.com. Research by Moira Tamayo and Xian Wan Jan 21st 2009
From Economist.com A NOVEL WAY TO SAVE WATER
Samer NaifA large aperture scintillometer, of course HOW much and how often should farmers water their crops? The invention of the automatic sprinkler gave farmers the power to act upon the answers to those questions. It did not, though, provide the answers themselves. Most farmers still make the call based on instinct or err on the side of caution and switch the sprinklers on at fixed intervals. Unfortunately, both of these approaches risk wasting water, and in some parts of the world that is a scarce resource. California, for example, has suffered a drought for the past two years and just experienced the driest spring in recorded history. Water bills have risen, but even that has not curbed demand sufficiently. Many parts of the state have therefore limited the use of water, to the detriment of crops and lawns. Jan Kleissl and his colleagues at the University of California, San Diego, think they may be able to help. Dr Kleissl’s idea is to use lasers to detect the amount of moisture in the air above the crops, and then use this information to decide when they need to be watered. His system, known grandiosely as a “large aperture scintillometer”, consists of a laser on one side of a field, a telescope on the other, and a lot of clever computing to interpret what the telescope sees. What that computing is trying to measure is how much the laser beam twinkles as it passes across the field. Stars twinkle because air currents driven by the evaporation of water cause their light to shimmer as it passes through the Earth’s atmosphere. (To avoid this twinkling, large observatories are sited on remote, dry mountaintops in places like Chile.) On a smaller scale, the light from Dr Kleissl’s laser also shimmers. The more water that is evaporating from a field, the more the laser twinkles. This, when properly interpreted, tells the field’s owner how damp the soil is and thus whether he needs to switch the sprinkler on, and for how long. The idea was originally military, as many technologies are. Twinkling limits the accuracy with which weapons can be aimed. It has since been adapted for use in airports, to measure conditions over runways. Dr Kleissl’s innovations are to modify the procedure so that it can use off-the-shelf lasers, and also to improve the software. The result is a system expected to cost between $25,000 and $50,000 when it is commercialised. That price obviously puts large aperture scintillometers out of the reach of the average farmer in, say, India. But for a big farm or golf course in a drought-prone area like California or in one of the man-made oases in the Gulf, it might be just the ticket.
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