Many people don’t realize how simple it is to make an estimate of probable drop in groundwater from groundwater pumping. Start with the fact there are about 326,000 gallons per acre-foot, a common measurement used in flood irrigation.
A medium-size agriculture pump can pull 1,000 gallons per minute, so it will take 326 minutes to yield enough water to cover an acre a foot deep. That is about five and a half hours, but let’s say six, meaning you can pump about 4 acre-feet in a single day.
How much will that much pumping lower the water table?
It depends on how much water will drain from the saturated rock beneath the surface and how large an area will be affected. Groundwater modelers estimate 10 percent to 20 percent of the rock volume contains drainable water. But that percentage varies widely from place to place, depending on how much clay is in the soil and how much is made up of coarser sediments. For an estimate, I assume 15 percent of the rock can be drained.
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In that case, the water table will drop about 7 feet for every acre-foot of water pumped per day – if the water is withdrawn from only 1 acre. If only 10 percent of the rock formation can be drained, then the drop would be 10 feet in that one acre. With 4 acre-feet, the drop for that 1 acre would be about 28 feet in a single day.
Of course, as soon as the water table drops around the well, water from elsewhere in the aquifer flows laterally to replace it, thereby causing a more modest drop to spread out away from the well to distances that depend on time and the density of the sediments in the aquifer. (Technically, this is called “specific yield.”)
If one well pumping 1,000 gallons per minute is drawing from an aquifer covering 1,000 acres instead of only 1, the average drop would be only 0.026 feet per day – or about a third of an inch.
But in a year it would be 10.2 feet – if nothing happened to replace that water (rain or lateral flows from other aquifers).
The foothills get only a little over a foot of rain per year and groundwater recharge is smaller because there’s more runoff on sloping ground. But lateral underground flow from rivers and reservoirs nearby can be significant. As the foothill pumps draw more water, the flows from surface water – rivers and reservoirs –can be significant. Further, many large wells in the foothills pull considerable more groundwater than 1,000 gallons per minute, some up to 2,500 gallons per minute. So the drop in the water table can be much more than the 10 feet per year.
Almond trees planted in the foothills might be pulling as much as 4 acre-feet from groundwater per year.
This raises the question: Should the state’s new rules on groundwater sustainability apply now in the foothills, when the water table might be dropping significantly, or should we wait 20 years to prove what can be reasonably calculated now?
It seems obvious that almond growing in the foothills is unsustainable over the longer term. Meanwhile, many people with smaller and shallower wells near those large wells can see their wells go dry.
In the Valley, where flood irrigation has been common, the situation is more complex.
In our calculations the estimated draw-down was averaged for an area. But water requires a gradient to flow to a well, so there would be a cone of depression – though different in shape from the ones usually pictured, because the rocks have quite different permeability in vertical and horizontal directions.
There has been much comment about the unwillingness of farmers to report how much groundwater they’re pumping, but that need not be a problem. We can make a rough estimate. It is widely recognized that there is a minimum of water needed for a particular crop, say 22 inches for tomatoes but 36 to 42 inches for almonds. Hence, a calculation seems feasible.
It is not a mystery. There is no reason a map showing estimated groundwater draw-down near large wells could not be done with publicly available data.
The estimates will only be approximations, but even with accurate pumping data there is no way to readily estimate the area affected by a specific well without detailed data on other nearby wells.
The removal of groundwater has been widely recognized as a problem. But you cannot remove groundwater that does not exist. Groundwater must be recharged, but the methods of doing that have had far less research than is needed. That lack of investigation must be corrected.
This is just one more example of the state’s lack of recognition of the facts of hydrology. Public understanding and pressure is the only way to correct the situation.
Vance Kennedy is a retired U.S. Geological Survey scientist who lives in Modesto.