How does water dowsing work? | Science Questions with Surprising ...

How does water dowsing work?
When an object, such as a marble on a roof ridge, is in a state of unstable equilibrium, a small movement (such as the breeze nudging the marble to one side) becomes a large movement (such as the marble shooting off one side of the roof). This event could be misinterpreted as a powerful, hidden agent on one side of the house drawing the marble towards it. The belief in water dowsing arises out of this type of misinterpretation. Public Domain Image, source: Christopher S. Baird.
In the sense that it finds underground water, water dowsing does not work. Water dowsing involves the claim that a person can locate underground sources of water without using any scientific instruments. Typically, the person that is dowsing holds sticks or rods and walks around a property in the hopes that the rods will dip, twitch, or cross when he walks over the underground water. The dowsing rods do indeed move, but not in response to anything underground. They are simply responding to the random movements of the person holding the rods. The rods are typically held in a position of unstable equilibrium, so that a small movement gets amplified into a big movement. The movements of the rods do not seem like they are coming from the small vibrations in the dowser's arms, since these vibrations are so small and the rod's movements are so large. From the false assumption that the movements of the rods are not coming from the small random vibrations of the dowser's arms, people then make the illogical leap that the movements must therefore be caused by something powerful that is out of sight, i.e. underground water. Since successfully locating underground water can save a farmer the trouble of digging several wells that end up dry, and since scientific approaches can be expensive, there is a strong incentive for people to want water dowsing to work.
Unstable equilibrium describes a state where all the forces on an object cancel out but the slightest deviation from the point of equilibrium causes the object to fly off. For instance, if you place a marble on exactly the top edge of a sharply-ridged roof, the marble will sit there motionless since the forces pulling it down either side of the roof cancel out. However, if the slightest breeze blows past the marble, it will give the marble a small bump toward one side of the roof. The forces will no longer cancel and the marble will shoot down one side of the roof. Since the marble was in a state of unstable equilibrium, gravity was able to amplify a small movement invisible to humans (the bump from the gentle breeze) into a large movement (the marble rolling down the side of the roof). To the naked human eye, it looks like a power agent exists only on one side of the house and is drawing the marble towards it. If we didn't understand the concept of unstable equilibrium, we may be tempted to say that there is underground water only on the one side of the house which pulled the marble down that side. Belief in water dowsing operates on this type of misunderstanding.
In many areas of the world, water dowsing seems to really work. In such areas, the location that the dowser points out indeed leads to a productive well. However, such areas of the world have so much groundwater close to the surface that any location will yield a productive well. The situation is like filling a box with only green socks and then asking a magician to close his eyes and use his magic powers to find a green sock in the box. If a system is secretly rigged for 100% success from the start, any method we use will seem successful. The U.S. Geological Survey states, "The natural explanation of ‘successful' water dowsing is that in many areas underground water is so prevalent close to the land surface that it would be hard to drill a well and not find water. In a region of adequate rainfall and favorable geology, it is difficult not to drill and find water!"
The belief in dowsing typically involves the misunderstanding that underground water consists of large underground rivers flowing through caverns. In this thinking, one spot on a farm would be a good location to drill a well because it is lined up with the underground river, whereas another spot 20 feet away would be a bad location since it misses the underground river. In reality, most underground water does not flow in rivers but instead flows everywhere through the tiny pores and cracks in the rocks. In any climate that receives a moderate amount of rainfall, you will always hit water if you dig deep enough. Therefore, the question to ask is not, "What spot on my farm has water underneath it?" Every spot has water underneath it. The right question to ask is, "How deep will I have to dig to get below the water table?" Another important question to ask when drilling a well is, "Does my ground contain the right kind of rock that will release its water fast enough to fill my well?" Even if there is water in the ground, a dense rock with small pores may release its water too slowly to be useful.
Various controlled scientific studies over the last hundred years have repeatedly found that water dowsing does not work. For instance, 30 "expert" dowsers were invited to Kassel, Germany in 1990 to have their abilities tested in a study organized by James Randi. Pipes carrying flowing water were buried underground at known locations and the dowsers were tested as to their ability to determine if water was flowing through the pipes. All failed to do better than random guessing. In the book Carl Sagan's Universe, edited by Yervant Terzian and Elizabeth Bilson, James Randi describes the tests:
While we were there we designed a series of tests, as I have done in many countries around the world, to test the forked stick or the pendulum or the coat hanger wires or whatever. Some people do it with their hands. And we did it in Kassel, Germany, two years ago, a very definitive set of tests, and, of course, it proved that the law of averages works quite well, but dowsing doesn't.