"GROUNDWATER . You can't see it, but millions of Californians depend upon it as a vital source of water for their homes and businesses." Those are not my words but the introductory words for the promotional video that appears on the SGMA (Sustainable Groundwater Management Act) website1 . The statement is a rather simply worded way to convey the obvious: that groundwater is vital to our livelihood here in California, and especially important to honoring and sustaining our agricultural heritage. While groundwater provides needed sustenance for numerous rural, as well as urban communities, it is agriculture that demands the lion's share of the resource.

Central Valley agriculture turns to groundwater pumping whenever drought or poor rainfall years reduce the availability of surface water deliveries. Cries for construction of more reservoirs to capture and store more stormwater and snowmelt so that canals can be filled with more water are always being heard. Yet, apart from the Smith River, all river systems in California are currently dammed2 . The current reservoir storage capacity in California is approximately 50 million acre-feet. Compare this to 850 million acre-feet, which is the estimated groundwater storage capacity in California. According to my calculations, groundwater resources are 17 times more plentiful than surface water resources.

Indeed, groundwater is a wonderful resource for California; and because it is so plentiful, it has been pumped mostly without regard to its sustainability. It helps that "you can't see it" because, in that way, it may seem to be an unlimited resource. But it isn't. Many large water basins are over-drafted, meaning that more water is pumped from them than is being replaced by natural means. In some cases, such as parts of the Central Valley, the overdraft has caused land subsidence.

Groundwater pumping in the Central Valley began in the 1920s, and land subsidence of over 25 feet was measured in some locations as far back as 19703 . That's truly scary, not only because the land and everything on it dropped about as much as a first down in football over that 50-year period, but because the aquifer also got squashed by that much. And now, 50 years later, how much more has it sunk? With subsidence, no means of restoration can restore that groundwater capacity. It's lost forever.

Not all groundwater basins cause that kind of land subsidence as they are over-drafted. For instance, the Paso Robles groundwater basin, which is currently critically over-drafted, remains intact and can hopefully be replenished by natural (or augmented natural) means if the overdraft situation can be reversed.

Subsidence, i.e. sinking land, can damage water-moving infrastructure that relies on gravity.

SGMA to the Rescue

The Sustainable Groundwater Management Act was enacted in 2014. Medium-priority and high-priority water basins were identified throughout the state, as well as critically over-drafted basins at one extreme and low-priority basins at the other extreme. The critically over-drafted basins, as well as those ranked medium- and high-priority, were assigned over 250 different management zones which were/are to be over-seen by individual Groundwater Ser vice Agencies (GS As). Each GSA has been tasked to develop a Groundwater Sustainability Plan (GSP) to be approved under SGMA by the California Department of Water Resources (DWR ). By 2020, GSPs were submitted for the critically over-drafted basins and, by 2022, for the medium- and high-priority basins.

GSPs must consider six sustainability indicators: Declining groundwater levels, surface water depletion (from shallow groundwater zones), degraded water quality, reduction in water storage capacity, land subsidence and seawater intrusion. If plans are accepted, then the local GSA implements the GSP; otherwise, the basin is turned over to the State Water Board as a backstop, which nobody wants because the water board regulations will be overly generic and heavy-handed with management compared to that of a local GSA that knows its specific situation better than a state-wide agency and is better able to connect with its constituents.

The beauty of SGMA is that it provides for local management of groundwater resources, with oversight by the DWR . The DWR also claims to provide some funding for projects to implement the GSPs, but from what I've heard (and it's not comprehensive), that funding may not be that easy to obtain. The DWR requires annual reports from the GSAs and implements five-year evaluations of each GSP to determine progress. GSAs have the authority to impose local fees.

Monitoring groundwater levels is not terribly difficult. Groundwater levels can be monitored in existing wells, or specific monitoring wells can be installed at key locations within each basin. Drilling monitoring wells isn't cheap, though. Water depth can be measured by hand with several techniques, including pumping air into a plastic tube whose outlet has sunk to a known depth in the well column. The pressure required to clear the tube is equal to the water pressure of the column of water above the outlet. Well companies often install these "sounding tubes" in the wells and use hand pumps to measure water levels. Similarly, this process can be automated for continuous monitoring. A San Luis Obispo-based company produces "The Well Bubbler," which automates this process. Similarly, a pressure transducer can be dropped down a well to a known depth to measure the water column. My company has set up many wells in the North and Central Coasts for continuous monitoring, primarily using The Well Bubbler system. The DWR can supply funds for some of these efforts to monitor groundwater levels.

The Well Bubbler offers a non-invasive way to measure well depth. From this real-time data you can infer speed of recharge and monitor overdrafting, which can damage your well pump. 

Monitoring Grower Water Use

This is where it gets dicey and a bit controversial in my opinion, if not downright infuriating. Unlike SB88, which requires the use of flowmeters by water users to measure water diversions, SGMA is fuzzier and more generic about how GSAs may monitor water users' pumping volumes.

I was sitting in on a local GSA meeting a few months ago as I had the opportunity to do a few times in 2023. The meetings are public, and at least this particular one is typically available on-line during the meeting. I'm not going to mention the actual GSA because I respect all the board members and think they are doing a fantastic job and genuinely have sustainability in their hearts and minds; however, I was dumbfounded to learn that remotely sensed evapotranspiration (ET ) methods were going to be used to quantify groundwater pumping volumes per property within the GSA's purview.

I had been previously under the impression that growers (and residences) would be required to monitor their pumping volumes with flowmeters, much like under SB88. I had heard grumblings about the inaccuracy of flowmeters, their maintenance and grower resistance to their use and reporting requirements. In truth, if a flowmeter is installed according to spec, it will be accurate. And while mechanical flowmeters do wear out over time, the more current electromagnetic meters have no moving parts and, therefore, require minimal maintenance. On the other hand, I can't argue against the fear that growers will be resistant to flowmeters on their wells, and things could get ugly for a while if growers are required to report their actual pumping volumes. Indeed, the GSA does have the authority to impose such a requirement, unpopular as it would be.

Remotely Sensed ET

Two sources of ET data are being considered to estimate consumption: OpenET and Land IQ. OpenET is a resource that according to its website4 , is led by NASA, the Desert Research Institute (DRI) and the Environmental Defense Fund (EDF ), with in-kind support from Google Earth Engine. The OpenET model is available at no cost to the user and currently covers the 17 westernmost U.S. states, with plans to expand its coverage further. Land IQ5 is a private company, based in Sacramento, that provides many different ser vices in the environmental realm, but their Land IQ ET product is the ser vice that appears to be most readily adopted by GSAs. Land IQ focuses on California, namely 3.3 million acres in the Central Valley, but is expanding with GSA demand to other parts of the state.

Both services have data sets that can be measured for specific locations on a grid or field, but Land IQ can aggregate its data over districts and regions, hence its attraction to local GSAs. OpenET's spatial resolution is 30m2 (0.22 acres) whereas Land IQ's resolution is a much sharper 10m2 (0.02 acres). Both OpenET and Land IQ use ground-based, gridded weather data sets that include parameters, such as solar radiation, air temperature and humidity, wind speed and precipitation. Both ser vices also use satellite data to provide surface temperature and surface reflectance information. Both use Landsat, which provides updated imager y ever y 16 days, as well as Sentinel, which provides imager y ever y five days. Other satellites are also used. Note that clouds and smoke will interfere with the measurements, so those images are excluded.

OpenET employs six different ET models by using the ava ilable data; and while each of the model results may be obtained from the system, most commonly, an ensemble average is used, with automatic removal of any outliers. Three of the models generated by OpenET use remotely sensed shortwave reflectance and thermal imager y, another uses remotely sensed surface reflectance and crop-type information, and two others are further simplified models. Land IQ is less transparent about the models it uses but takes ground-based measurements of actual ET using eddy covariance (most accurate), as well as surface renewal/energy budget methods (less accurate than eddy covariance) to calibrate its model on a consistent basis.

It appears that most GSAs are moving toward the Land IQ service, which to me seems like it may have both an accuracy and a precision edge over OpenET, if only because it is focusing on California and is continuously calibrating itself.

But it is still only a measurement of consumptive use-not water application.

Flowmeters remain the only tried-and-true way to quantify water use. 

Is ET Enough to Save Water?

My objection to using ET, as a surrogate to flowmeters, has been partially allayed by investigating these services. I reassured myself by looking at some output of OpenET and seeing that ET measurements do not fall to zero, during the winter, when vineyards (and most orchards) have no leaves on them. Indeed, there was ET occurring then, so the model can consider non-crop vegetation, as well as surface evaporation.

But does it truly capture a grower's water use? No, it does not!

ET measures consumptive water use, not applied water. Land IQ's brochure states this clearly. If a grower over-irrigates (i.e., applies more water than the consumptive use), the consumptive water use estimate doesn't change. Indeed, a grower could over-irrigate, causing runoff or deep percolation, and ET would remain unaffected. Does this matter? I think so. For one, it provides no incentive for a grower to irrigate more efficiently because their water use report will remain largely unaffected, regardless of what they do to conserve water.

What about frost sprinklers? They are a heavy user of water, and the ET model will report nearly nothing from them because there is very little vegetation present during spring frost season. Likewise, how about winter drip irrigation, a necessity for many growers in regions with little winter rainfall, especially during drought. Again, this water use is real but will not be captured by the remotely sensed ET model.

Where does the excessive water go? That is a little less clear. Some of it evaporates. The rest seeps into the ground below the root zone. The claim is that the deep percolation water just goes back into the groundwater. That may be the case in some but not definitely all cases as there is not always a solid hydrological connection between surface water and groundwater. Aquifers are commonly a few hundred feet below the surface (some are tens of feet and some many hundred feet below the surface). The vadose zone is the unsaturated zone between the surface and the capillar y fringe immediately above the aquifer. Excessive irrigation may eventually return to the aquifer, but it may just as likely run off into streams or just linger there for decades until the next big flood event.

So I contend that using ET alone is insufficient to encourage and reward more efficient irrigation of crops by growers and that without monitoring of actual use, we may find ourselves in the same predicament we are currently in. To measure actual water use, we need flowmeters ideally or at least pump power consumption records. It would be a shame if the only way we can reduce demand for a water basin is to take acres out of production because that's not the only solution. I know from firsthand experience that most growers could reduce their irrigation applications without productivity losses. I know that because I live in that world.

But the fact is that as the regulation is currently written, ET is an allowable method for consumptive use estimation. I had a brief email exchange with Timothy Parker, principal hydrogeologist for the DWR and co-author of the report "Hydrogeologic conceptual model in Paso Robles6 ," about my concerns regarding remotely sensed ET as a surrogate for water usage monitoring. He agreed with me about my concern and said that he had argued that very point with colleagues years ago during the development of SGMA . The adoption of ET over that of flowmeters was largely a political one, in his opinion, as requiring flowmeters would probably have meant SGMA's failure to be put into law. What a shame.

So for now, we must live with it and hope that our groundwater basins can be managed sustainably without knowing actual water draws. At least we'll be monitoring the aquifers more consistently and frequently than we had been in the preceding decades; so if we don't see progress, which I fear could be the case, GSPs will be eventually forced into metering actual flow.

The longer we let our groundwater basins decline, the longer it will take to bring them into sustainability. Realizing the actual effects on groundwater storage could take a decade or longer of groundwater level monitoring. And who knows how many years, after that, to enact the needed changes. It's time we growers take responsibility for our water use, demonstrate our water efficiency, improve our water efficiency and not simply talk about sustainability. Prove it-measure it!

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Also be sure to check out our latest blogpost on groundwater recharge.

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