The Prime Challenges of Measuring Groundwater in California’s Central Valley Region

Any time there is a drought, groundwater pumping needs to be increased to adjust for the losses experienced from a lack of water on the surface. This is a common occurrence in the Central Valley region of California, an area that stretches several hundred miles from south of Bakersfield to Redding. The area also happens to be at the heart of California’s massive agricultural industry, meaning a constant supply of water is absolutely necessary.

There have been problems with overdrafting from the Central Valley aquifer for decades. Recent research by scientists at the University of California, Los Angeles and the University of Houston have made it possible to get more accurate with quantifying how much groundwater is being pumped.

These researchers’ studies show that over the last five years or so, in which droughts have been a persistent issue in the region, the state withdrew approximately 9.5 cubic miles of groundwater in California. This comes out to approximately seven times the amount of water you’ll find in the largest reservoir in the state, Lake Shasta.

The study was published in the journal Geophysical Research Letters. It also featured estimates that, during a drought that lasted from 2007 to 2009, the valley had another 4 cubic miles of groundwater withdrawn from its aquifer.

While these numbers themselves are not especially surprising, the real news of these findings is that the state needs to find a way to replenish the groundwater resource if it is going to keep pumping at the current rate. While there is a lot of groundwater, it’s not going to last forever, and eventually the well will run dry.

The Central Valley groundwater basin

The basin from which the groundwater is being drawn is the largest single source of water in the region. Some researchers believe it could hold more than 1 billion acre-feet of water.

Groundwater sources can get naturally replenished by water from rain and irrigation, as well as other surface water sources that soak into the ground. However, dry years can result in the state pumping anywhere from 5 to 7 million more acre-feet of groundwater than can naturally be replenished. In a wet year, the aquifers are usually only refilled by between 1 and 5 million acre-feet.

This means the state needs more above-average wet years than drought years, which just hasn’t been the case in recent years. This current drought has lasted five years, and with climate change being a constant looming presence, it is expected that these types of lengthy droughts will only become more frequent.

Another challenge is that measuring the amount of groundwater being lost during the drought is difficult, because we can never be completely certain how much is there to begin with. One method used is to look at changes in gravity, which NASA occasionally does by using satellites to detect gravitational changes in a region. But because the resolution of the system is so large, the readings might extend beyond the valley.

Simply put, the state needs to figure out how it can better preserve its groundwater resources, as that water might not always be there to fall back on during drought years.

For more information, contact a surveyor specializing in groundwater in California.