Drawdown and Pumping Off Explained
Your water well contractor will sometimes use big words to describe what is going on in your well. This is an attempt to explain some of those terms.
Pumping off is a term that we use to describe when the pump sucks air. This is caused by the pump moving more water than the well provides. Many people think that the water in a well is a large unlimited pool of water. In the Austin area (and in most of the world), that is not the case. Water flows through cracks, veins, porous rock, gravel, sand, or sandstone. The flow of water through those layers is limited by several things. Number one is the porosity. That is the size of the holes. If the holes are large, then there will be little restriction. If the holes are small, then the water does not flow as fast. This is like water pipes that carry the water through the aquifer. The second determining factor is the surface area of the production area. If you only have a few feet of production zone, then water will not flow into the well as fast as it would if you had 100 ft. of production zone. Also, a 6” drilled hole will have much less surface area than an 8” hole.
The third determining factor is the pressure that the water is under. The less pressure the water is under, the slower it will flow into your well. While the porosity and surface area are unchanging, the pressure is always changing. We measure the pressure in two ways. One is the static water level. This is the water level when the well has recovered fully from any pumping. This is a good measurement of the pressure that is in the aquifer as a whole. Another measurement that we use is the drawdown level. This is the water level in the well after a significant amount of water has been taken from the well. This usually occurs within an hour for small residential wells. As demonstrated in the picture, the porosity in the production zone slows the flow of water from the aquifer into the borehole. This creates a cone of depression. If you measured the water level in your well during pumping, the level would be lower than the static water level. If you had a monitor well a few feet away, you would see that the water level would be slightly higher in the formation just outside the well. As you go farther from the well, the water level rises until you get far enough away that the water level is back to static level. The distance that it takes to get back to static depends on many factors, but mainly porosity. The higher the porosity, the flatter the drawdown curve.
When a driller drills your well, he will estimate the flow of water. This is always an estimate based on the conditions that he sees on that day. But, there are many things that he can’t see. He can’t see how the water pressure will change during droughts. He can’t see if there is a pinch in the aquifer that will slow the flow of water during extended demand. He also can’t tell what is going to happen to the water level after years of use. He can’t see into the future and tell how many other people are going to use (and sometimes abuse) the aquifer. All of these things have an effect on your wells capability.