KENNEDY: Water mining in the county foothills

June 29, 2013 


(Bart Ah You / The Modesto Bee) Vance Kennedy on his farm in 2006

BART AH YOU — Modesto Bee Buy Photo

A major problem is developing in the foothills east of the flood-irrigated soils in the valley.

Because almonds are as profitable as they are, and flood- irrigated farmland is so costly, there is an ongoing effort to plant almonds in the foothills east of the flood-irrigated farmland. That has the potential to cause major problems for ranchers, who have long used such land for grazing.

In order to understand the potential problems, one needs to understand the water situation in the foothills.

Almonds need about 30 inches of water, or more, to survive well. Average rainfall is in the range of 12 to 16 inches per year, of which perhaps half penetrates to recharge ground water. That means that about 2 feet of water, in addition to rain, is needed from a supplemental source. That source is usually pumped groundwater.

That groundwater has accumulated over many thousands of years, and cannot be rapidly replaced. That means that old groundwater is literally being "mined."

As the water is pumped, out, the water table drops. How rapidly it drops is related to how much water-saturated open space between grains can be drained in the rocks involved. That open space can be as little as 10 percent. Hence, when 1 foot of irrigation water is removed, the water table can drop 10 feet. Since about 2 feet of water, in addition to rain, is needed for almonds, that means that the water table can drop about 20 feet each year. So, in 10 years, the water table will be at perhaps 200 feet, if there is no other source of water.

I have been told that some large wells are already at 400 feet. At some point the cost of pumping will be so high that it will no longer be economically feasible to continue watering the trees, and the trees will die. The land will be useless for anything else because water for grazing animals will be too costly to provide. From society's standpoint, it will be an environmental disaster. Something needs to be done as soon as possible.

What happens to the water table at a distance from the well? In order for the well to continue accessing groundwater, there is what is known as a cone of depression that develops around the well. As the water level drops at the well, water surrounding the well flows toward the well, drawing water from increasing distances. Neighboring properties will contribute to that well, causing water tables there to drop, and hence hurt the neighbor. The time required for the well to affect the neighbors depends on the distribution of the permeability of the rocks involved.

Generally, lateral permeability is considerably greater than vertical permeability in sedimentary rocks. Hence, it may be easier for water to flow laterally from adjacent properties than vertically within the specific property itself.

Many states have groundwater laws related to situations described above. California has no such laws. A summary of how other states have handled such problems is available. A water lawyer told me that the county supervisors have the power to regulate wells. Hence, they are the proper people to address what already is a major problem and promises to be much worse in the future.

There are problems beyond volume. Deeper groundwater tends to have higher dissolved salts. So, as almond growers in the foothills reduce drip irrigation, there is little flushing of accumulating salts in the soil. Hence, over time, the soils will tend to become saline. When plants take in water, they preferentially exclude salts from their roots, which can add to the problem of salt accumulation.

Kennedy is a retired hydrologist who farms north of Modesto. He frequently speaks at Modesto Irrigation District board meetings and other places where water is on the agenda.

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