Groundwater’s Safe Yield

How would you determine the volume – in gallons, cubic feet, or acre-feet – of water in a regularly-shaped reservoir? If the reservoir was rectangular, you would need to know the length and width of the reservoir, plus the depth of the water in storage. Then, just multiply these three dimensions to determine the volume. Pretty easy.
How would you determine the volume of water in a groundwater basin? Hmmm. Not so easy. While we might be able to measure the surface area of the aquifer, we can’t see what’s happening below the surface. Plus, our volume here is filled with three things: water, soil, and air. So once we determine our total volume, we need to subtract out the non-water portions of the volume. Let’s take a look at these two complicating factors, and how we deal with them to determine a reliable assessment of water in the ground.
How can we “see” below the surface in an aquifer? “Drill, baby, drill!” We need wells to determine how deep our aquifer is. These don’t need to be water production wells – we can also use “observation” wells or exploratory wells. The more the better, because lots of weird things can happen below the surface, and go completely unseen – even with lots of wells. As an example, we might have a huge rock formation in the middle of our aquifer. This volume would hold little or no water. And our wells may miss it completely, leaving it invisible to us. So even though it’s an imperfect process, it’s the best means we have to look below the surface.
In addition to finding the depth of our aquifer, our wells will tell us about the three contents of the aquifer. Samples at various depths in each well are analyzed to determine what type of soil (gravel, sand, silt, clay, etc.) is present. Sand and gravel have a lot of “void” space that is filled with water (below the water table) or air (above the water table.) Silt and clay have far less volume available for water.
Now we have the information we need to estimate the volume of water in our aquifer. Is this an accurate value? Possibly not, because we can’t drill wells everywhere.
Another tool that we use to manage our groundwater is the “Safe Yield.” Instead of telling us how much water is in our aquifer, this tells us how much we can “safely” withdraw each year. In this case, “safe” means that pumping at or below this rate will not cause the level of the water table to drop. This really measures how much water nature puts back into our aquifer. This comes mostly from precipitation, which varies from year to year. So the safe yield is a long term tool: we can expect water levels to go up or down each year, but if we withdraw water at the safe yield, we will see no long-term decline in our aquifer’s water level.
How do we determine the safe yield? By pumping, and measuring our pumping rate and the water levels in our aquifer over a long period of time. If our water levels don’t decline, we know we have not yet reached the safe yield. If our water levels show a continuing decline, we are exceeding the safe yield.
An inexact science? Perhaps. But until we can find a way to measure the precise volume of an aquifer, the safe yield is usually the best tool for us to manage our groundwater.

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