Dr. Thomas Harter on the importance groundwater – a critical source of drinking water and drought insurance for 40% of our planet’s agriculture
Dr. Thomas Harter, Robert M. Hagan Endowed Chair in Water Management and Policy at the University of California Davis, gave this presentation on groundwater earlier this year at the Beckman Center in Irvine as part of the National Academies of Science Distinctive Voices series.
Dr. Thomas Harter began by noting that with the drought getting a lot of media attention, water is certainly on everybody’s mind. “We’ve seen all these pictures of the empty reservoirs or of reservoirs with long bath tub rings and no snowpack to fill them this year, and not really having seen a lot of rain for the third, fourth year in a row, seems spooky to many of us,” he said. “Orchards are going dry, fields are being fallowed, people run out of water, and especially in our rural communities with small community water supply systems, their straw no longer is deep enough in the groundwater systems and they are forced to haul water in from distant places and paying a significant amount of their income for just drinking water.”
“The drought has no end in sight; it’s not just captivating California but much of West,” he said, noting that while the last two or three years individually have not been as low as the record drought in the 1920s or the 1977 drought, when one looks at a running totals for precipitation, the last three years have indeed actually broken records in the measured history of California. “We’re at the lowest level we’ve ever been in the cumulative rainfall record for three years here in California, and it’s that three year record that you will learn tonight is really important to our groundwater resources, and not just year to year variability.”
“We all know that California is kind of a dry state; we have wet winters and dry summers, we have a wet north and a relatively dry south, and we have this disconnect in space and in time between where rainfall happens and where water is used,” said Dr. Harter. “Rainfall happens in the winter; we use it in the summer. Rainfall happens in the mountains; we use it in the valley. It’s fall in the north, we need it in the center and in the south. We’ve built a huge infrastructure around this.”
He then gave some overall numbers for water use. California has about 10 million acres of irrigated agriculture which uses anywhere between 27 and 35 million acre-feet; most of that is in the Central Valley, but also in the Imperial Valley and Coachella Valley, the coastal basins, and a few of the inland basins. Urban users in Central and Southern California use about 8 million acre-feet, about one-quarter of what agriculture uses. About 45 million acre-feet is allocated for wetlands and dedicated streamflows.
“To get the water there, we’ve built a large network of canals, reservoirs and dams,” he said. “The reservoirs and dams, they are at the foothills of our mountain ranges, not just the Central Valley but also mountain ranges along the coast where we keep that spring runoff from our mountains to hold it over for the summer and maybe for the summer after that, if it’s a dry year. We’ve built a large network of canals that’s connecting our northernmost mountains, all the way down to San Diego, and we can actually redistribute water throughout the state to fix that disconnect between where water occurs and where water is being used. At that very center of this infrastructure sits the Sacramento-San Joaquin Delta, which is really the Grand Central Station of water in California.”
Mr. Harter said he is deviating from that story and focusing on California’s hidden resource of water which is groundwater. He said people talk about groundwater as if it is flowing in arteries hidden in the subsurface, but groundwater is really more like what you have at home in your flowerpot, only bigger. “You stuff it with dirt, take your watering can and pour water in it, and the water disappears,” he said. “Where does it disappear into? Not into arteries, not into a cavern in that flowerpot, – it just disappears into that pore space between all the dirt particles that are in your flower pot.”
In California, the groundwater basins are much the same, only much larger, he explained, presenting a geologic map of the state. “Instead of a flowerpot, you have a bathtub and what you see here in color are the bathtub walls, the ancient rocks of California,” he said.
“Sediment fills the bathtub and it’s in that sediment fill that we find most of our groundwater. Granted, there’s quite a bit of groundwater in the fractures of these rocks as well, and any of you who owns a home in any of the foothills with a domestic well knows that it’s enough to kind of get water for a house, but to get water for a big city or for agriculture, we need these large sand and gravel aquifers that fill these bathtubs all around California.”
He presented a cartoon graphic of a groundwater basin, noting that California’s groundwater basins hold hundreds of millions of acre-feet of water, much more than any reservoir in the state. “Over time as rivers have come out of the mountains and flow over these sediments, they lose water into these sediments which fills up the groundwater,” he said.
“Rainfall goes onto the soil and to the degree it’s not used by plants, it percolates down into the sediments and fills these sediments up with groundwater,” he said. “Eventually it all comes out at the lowest point in the sediment surface in our valleys which is usually where our rivers are.”
“When we fill up this groundwater basin some more, then we create a slope in this water table surface which lets the water move from the higher places of water table to the lower places of the water table,” he said. “Groundwater moves essentially to these rivers, so the balance is the recharge coming in from rivers and from rainfall and then groundwater discharging to the river at the bottom of the valley, the large river that ultimately drains all of this into the ocean.”
The development of the turbine pump in the early 20th century meant we were able to extract large amounts of water from these aquifers, Mr. Harter said. “So mostly in the summer, we create these large cones of depression around these wells; we turn them off in the fall, and as a result with recharge coming in from streams and rainfall, the water table recovers, and we go with this, back and forth,” he said. “In this way, groundwater really becomes a storage reservoir and it becomes part of the solution to carrying over water that’s available in the winter for use in the summer. It’s another reservoir.”
He presented a slide of a landscape water budget in the Tulare Lake Basin showing the month-to-month water budgets for the landscape in a normal water year, noting that above the zero line are water inputs to the landscape, and below the zero line is what is lost from that landscape. He explained that the light blue color is rain, medium blue is irrigation from river water, and dark blue is groundwater extraction for irrigation; red is what is used by the plants,, and orange and yellow is what percolates down to the groundwater from streams, rivers, and what’s left over from irrigation.
“We have rain in the winter and we irrigate with groundwater and stream water in the summer,” he said. “That rain in the winter leads to a lot of recharge, both from streams but also from the landscape, and the same with irrigation in the summer, it actually creates a significant amount of recharge to our groundwater aquifers. That’s part of that groundwater balance.”
In a wet year, there’s even more recharge happening because there is more rain and less irrigation, he said. “Especially in the early summer, we almost see no irrigation; most of the irrigation happens in the later summer, so there is a surplus of water that’s ultimately not actually being used in this particular year.”
In dry years, there is little rain, and the irrigation now comes from the groundwater basin with very little recharge to balance the extraction, he said. “Not only do we save water that’s being recharged in the winter for summer use, but we also take that water that’s being recharged in wet years and use it in dry years, so these groundwater basins are actually large enough storage reservoirs for long term storage and use.”
He then presented the Southern Tulare County Water Budget over the past 30 years, noting that supply is on the right side and demand is on the left. “We see that in some really dry years like 1977 drought, in this particular part of the Central Valley, more than half of the water use comes from groundwater, whereas in really wet years like mid 1990s, it might be as little as 20 or 25%.”
It’s the same on a statewide basis, he said. “In wet years, we use 12 to 15 million acre-feet of groundwater,” he said. “Last year, I’m estimating that we probably used well over 20 million acre-feet if not 24 million acre-feet of groundwater. And that’s how are groundwater basins are used as storage basins.”
We also use our groundwater basins as a conveyance facility, figuratively speaking, Mr. Harter said. “If you look at this particular part here in the Tulare County, there are a large number of different water districts that all have different water rights for surface water,” he said. “Some have a lot of surface water available, and they irrigate a lot. … They tend to add quite a bit of recharge to groundwater, whereas this district in the middle, which has little access to surface water, instead pumps a lot of its irrigation water from the subsurface. As a result, groundwater that piles up under this districts flows into this district. And so this district actually gets about 70,000 acre-feet of water every year through underground flow across this groundwater basin, essentially delivered.”
Mr. Harter said colleagues have done modeling exercises looking at the Central Valley as a whole. “Between these larger groundwater basins, we see groundwater exchanges that are on the order of tens to hundreds of thousands of acre-feet each year, so there’s a lot of groundwater transport happening as well.”
“So we go back and forth in this system, and where it gets a little bit more critical is when the balance between what we recharge into this groundwater basin and what we take out of this groundwater basin is out of whack, especially during extended droughts,” he said.
He then presented a slide, showing the hydrograph from 1950 to 2010 for a well in Tulare County. He pointed out the 10 feet of drawdown in this well in 1977, and the drought in the late 1980s and early 1990s where the well lost 10 feet of water level each year. “In the drought in the early 2000s, we even lost 15 feet on this particular well, so it’s no surprise that this well probably has similar amount of water level changes now in this most recent drought,” he said.
Other wells have more drawdown depending on pumping behavior, construction, and on how the aquifer exactly looks like where the wells are, he said. “This particular well during these droughts typically shows water level drops of about 40 feet from year to year, with big drops happening typically in the summer and then some recovery in the winter.”
“What happens during a drought is that you pump it down in the summer, but then the recovery in the winter that happens in the average or wet years, doesn’t happen in these dry winters,” he said. “We go down through the summer, we don’t recover in the winter, and we go down again in the next summer, and that’s what we see during drought.”
Mr. Harter said that when the lowest water levels measured in the 20th century (typically in the mid 1950s in Southern and Central California) compared to current water levels, we are well below where we were at the record lows in the 20th century, especially here in the southern part of the Central Valley, although not as much in Southern California.
“People are now estimating that on the order of 130 MAF of groundwater in total has been lost,” he said. “This is about three to four times as much as we have surface water reservoirs in California that we’ve lost mostly out of the southern part of the Central Valley over the last 100 years.”
The consequences of overpumping
Mr. Harter said that while there is still a lot of groundwater left, there are many consequences to groundwater depletion that are felt long before groundwater is actually gone.
Sea water intrusion
Groundwater normally is pressurized with a water table typically near the surface of the land, he explained. Under these conditions, groundwater flows usually in the direction of where water table surface is sloping; under natural conditions, it’s sloping from the land to the ocean, and that makes fresh groundwater flow out toward the ocean. But when we put pumps in on the inland side, that water level surface actually declines and that means that now the direction of flow is in the other direction, which pulls in salt water.
“In some places where this has gone unchecked for decades and where our local water agencies haven’t really done anything about like in the Salinas Valley, this has led to seawater intrusion that goes now 8 miles inland,” he said.
Cost of pumping
“Another consequence of this tremendous drawdown in groundwater levels is the cost of pumping, because as the water levels are drawn down, each year the cost of pumping goes up,” he said. “Some of the wells fall dry because they are not deep enough, and that’s then really the big cost. Drilling a new well and spending $50,000-$100,000 for a small domestic well and $1-2 million for a large urban well or an irrigation well that has to be drilled to 1000 or 1500 feet which is no small feat. Colleagues of mine have estimated that last year, the cost to agriculture, the drought impact to agriculture has been on the order of 2.2 billion dollars. About 20% of that is due to the cost of drilling new wells and pumping and lifting that water and extra 20, 40, or 100 feet.”
Subsidence is another impact that can occur from overpumping of groundwater. Mr. Harter explained that the sediments are made up of sand and gravel as well as clays. When the aquifer is formed and the sediment materials are being laid down, the clay particles are typically deposited at the bottom; the clay particles don’t quite touch each or maybe just a little bit.
“As the clay particles are buried under evermore deposits, there are two forces that act on these,” he said. “On one hand, they want to squish together because they are holding this huge sediment pack on top. On the other hand, there’s water between them, which is pressurized; there might be 100-200 feet of water above them, (a lot of pressure), so that water pressure actually keeps those clay particles apart. But as we lower the water table, we decrease the pressure without decreasing the sediment overload, and that squishes these clay particles together. They cannot be pushed back apart once they get pushed together.” He noted that on the map, all the areas shown in red have significant amounts of clay in their sediments.
“We can see significant amounts of subsidence if water levels drop below levels we have seen historically, which is the case in the southern part of the Central Valley,” he said. “The graphic is looking at almost 100 miles across the whole width of the Central Valley, and there are many areas in yellow and red that have seen more than 6” of land subsidence over the entire landscape. Some of these areas in the 20th century have already seen as much as 30 feet land subsidence. In some areas, they are starting to go for that record again with huge consequences to slopes in canals, the bridges that go over canals, and the water infrastructure in this region.”
Surface water depletion and impact to groundwater-dependent ecosystems
Overpumping of groundwater can impact springs and vegetation that is associated with springs. “We have a lot of these groundwater-dependent ecosystems that depend on springs all around California, mostly in the mountain ranges,” he said. “Springs are really suffering in this drought; our wetlands around the Delta and in the Sacramento Valley and out in the desert are suffering from this drought as well. They have lots of areas of wetlands that are groundwater dependent and more importantly, the rivers, now instead of receiving water that used to come to them, are now losing water that’s going to these wells that we’re pumping for irrigation and for urban water uses, and many of these rivers in Central and Southern California as a result have gone dry.”
Sustainable groundwater management
So how do we deal with all of these conflicts? “Drought is not new to California,” Mr. Harter said. “The current drought may be the largest we have seen in the last 120 years, and combined with some of the climate change effects, it may have bigger impacts on California than any of the droughts before, but drought has been part of the California landscape since the state was founded. As a result, we’ve actually have established groundwater rights. We have a correlative rights doctrine which under our court system, states a clear philosophy of how we divide up groundwater in the state of California.”
“What the correlative rights doctrine says is that the right to pump groundwater that can be replenished is correlative to the size of landowner’s parcels and to the properties of the aquifer, so in other words, all the landowners across a groundwater basin share the replenishable part of that resource in correlation to the size of their land property,” he said. “Nobody has a right to empty out our aquifers. This is going back to court decisions that have been handed down in 1908 and have been since confirmed in many court decisions.”
“We have a constitutional mandate to put water to beneficial uses, so we can’t just waste water,” he added.
There are many special districts, especially in Southern California, that deal with groundwater and groundwater management, and counties, under their police power, can terminate exports of groundwater and can manage the transfer of water, he said.
And there’s always the courts, Mr. Harter pointed out. “When people fight about groundwater and they can’t agree on something between them, they can go to court and the court use the correlative rights doctrine to divide up groundwater,” he said. “In Southern California, we’ve seen many court cases that have led to a basin adjudication, which is a process where all landowners send their attorneys to court, they divide up water in the court, and decide who gets how much water. We’ve seen many of these adjudications here in Southern California.”
As groundwater resources have continued to be depleted over the last 30 years, each drought has yielded a little bit more of legislative action, Mr. Harter said. “The late 1980s, early 1990s drought yielded AB 3030 which for the first time, suggested that locals do groundwater management plans and provided a framework for these groundwater management plans,” he said. “In the early 2000s with the next drought, the state went one step further and said, if you want money from us for your water projects, you better have a groundwater management plan, and so that was SB 1938. And with the current drought, which from a groundwater perspective, really has been going on since 2007 with a one-year interruption in 2011, we have finally managed last year, to pass a sustainable groundwater management bill.”
The Sustainable Groundwater Management Act makes it the policy of the state that groundwater be managed sustainably for long term reliability and multiple economic, social, and environmental benefits, he said. “The statute lays out how that is being done; the first step being that Groundwater Sustainability Agencies be formed in all high and medium priority basins. Everything that’s orange and yellow within the next two years has to have groundwater sustainability agencies that then implement groundwater sustainability plans.”
Mr. Harter explained that groundwater sustainability plans are a form of communication among many different stakeholders in the basin that they all have a stake in how groundwater is being used; they contain descriptions of the groundwater basin and monitoring activities and include processes for accountability and review. “A lot of this has been part of groundwater management plans in the past; what the new law does is it empowers these agencies to actually do enforcement,” he said. “They can prohibit pumping and they can make charges for groundwater pumping as has been the case for some special districts; this makes it possible for all these groundwater sustainability agencies to engage these much more rigorous management activities. It is integration with surface water management, integration with water quality management, and it also for the first time makes a linkage between groundwater management and land use, which drives all of the groundwater pumping.”
“So in essence, it’s a data collection, monitoring, and assessment processes; it is management of the demand side, management of the supply side, and stakeholder engagement and management,” he said. “You want to manage that bank account, you want to increase how much goes into that bank account and if there’s not enough a balance, you want to decrease how much comes out of the bank account, and a lot of that requires engaging your stakeholders and managing your stakeholders. Those are the four key tasks of these groundwater sustainability agencies. And nobody has more experience with these processes than Southern California.”
Many urban water districts in Southern California have engaged in groundwater management practices for the last 50 to 80 years because it was the first place where groundwater basins started to empty out when large scale groundwater pumping happened, he said. “Many of the impacts that I described started to occur as early as the mid 20th century, one being pumping leading to sea water intrusion,” he said. “So what these agencies have done, like the Water Replenishment District and Orange County Water District, they have build seawater barriers where they inject groundwater to build up this water table that prohibits sea water from intruding into the groundwater basin. … They have managed to keep sea water intrusion at bay unlike what the Salinas Valley has seen.”
“At the Central Basin in Los Angeles County, they’ve seen dramatic drawdowns in the 1940s and 50s, and through a combination of injecting groundwater near the coast, developing additional local surface water storage, and by bringing in water from Northern California, Los Angeles County just like Orange County has managed to recover its water levels to something that’s much closer to sea level.”
There are groundwater banks in the Central Valley, Mr. Harter said. “Kern County Water Bank now stores 2.5 MAF of water for long-term storage and Southern California has about 400,000 AF of water stored in its groundwater basins for taking out during dry times,” he said. “A lot of this water is stored by putting it into basins and letting it infiltrate from these basins, and then pumping it out during the drought.”
“In Orange County, there is the Groundwater Replenishment Project which takes wastewater, puts it through an extensive process and reverse osmosis process and now supplies about 25% of the recharge to the groundwater basin.” He noted that over the decades, Orange County has developed a wide variety of options to recharge the groundwater: stormwater recharge, recharge from the Santa Ana River; imported water, and the Groundwater Replenishment Project.
“Over the water district area as a whole, water levels since the 1950s have significantly recovered; yet you do see we have management going on,” he said. “Every drought we have seen, the 1977 drought, the 1990 drought, the 2000 drought, and the most recent drought, we draw down on the bank account, on this large storage reservoir and we hope to recover in wet years.”
He then presented a map of the state, showing the water balance by region. He noted that groundwater is shown in purple. “In Southern California, more than one-third of the water comes from groundwater; they’ve also developed a significant amount of recycled water and water reuse in addition to local projects to make up for mostly urban demands of water,” he said. “The Central Valley has mostly agricultural water demands, and in the southern part, groundwater is a big part of it. In the northern part, much less so. And if you go to the north coast, groundwater plays a relatively small role; there it’s streamflow that’s the biggest part and that needs to be protected.”
“In some parts of Northern California, like the Scott Valley here in Siskiyou County, there is salmon running in the stream through the valley, competing in the summer for the water with agriculture,” he said. “Agriculture is pumping out of this groundwater basin that also supports the stream and the streamflow in the summer, long after the snow has melted off of these mountains around it, so what we’re doing is starting to manage water that’s available in the winter, putting it into the groundwater basin, which is one of the bathtubs filled with 100-200 feet of sediments. We try to fill these sediments up with as much water as we can by doing recharge projects to support that streamflow better in the summers while also allowing irrigated agriculture to take water out for irrigating crops.”
“The Sustainable Groundwater Management Act that we passed last year is built for a long time frame,” he said. “The agencies have to start implementing processes, and as it took Southern California decades to get where it is today, it will take many of these water districts and water agencies in the rest of California decades to get to sustainability.”
“The Sustainable Groundwater Management Act is not a drought management act, so if this drought is going to continue, there are going to be other measures that we’ll be looking at both locally and at a statewide basis,” he noted.
Groundwater quality protection
California also has groundwater quality issues, Mr. Harter said, noting that Boeing has been engaged for 30 years in groundwater contamination cleanup. “We have done lots of work over the last 3 to 4 decades to deal with industrial solvents, with gasoline spills, with paint and other industrial processes that leach chemicals into our groundwater, all of which typically are point sources and create small or larger plumes on the order of tens of acres, maybe hundreds of acres in size, contaminating drinking water wells mostly in urban areas,” he said. “But our much larger groundwater quality problem are naturally occurring contaminants such as arsenic, naturally-occurring hexavalent chromium, arsenic, and uranium.”
We also have nitrate problems in some of our irrigated agricultural regions, affecting as many as 5% of wells statewide, he said. “In some regions, it may affect as many as 40% of domestic wells, and some studies now show that has major impact on drinking habits of people, primarily in disadvantaged communities where sugary drinks are leading to obesity.”
“Nitrate is not only a problem in California, it’s a problem throughout agricultural regions in the U.S. with areas shown in red having nitrate in excess of drinking water limits in their production aquifer,” he said.
He then presented a map of nitrate-contaminated wells in California, noting that the nitrate contamination includes areas in Southern California that used to have a significant amount of agriculture. “Two counties in the Central Valley have more than 40% of their domestic wells contaminated with nitrate above drinking water limit,” he said.
“Much of that really has been the result of a historic increase in our crop production that has been essentially made possible by the use of synthetic fertilizer,” he said. “Also the animal and dairy herds, which are putting out huge amounts of nitrogen as part of their manure. If you take the total nitrogen that’s excreted and that’s applied together, that nitrogen far exceeds what is being harvested in our fields, and the difference converts to nitrate which then percolates into groundwater.”
“We’ve seen significant increases in the last 30 to 40 years in nitrate and our prediction is that the increase will lead to large amounts of domestic well exceedances of nitrate throughout some areas in the Central Valley at rates of 30 to 40%,” he said. “The only way to get the nitrate out or the salts that sometimes come with it is through expensive treatment. The estimates are on the order of $50-$100 million per year that still has to be spent on systems that currently are not able to provide adequate treatment on the nitrate contamination.”
“Much of this action, both the groundwater sustainability action and the nitrate action, is really pointing towards agriculture, and agriculture is squarely coming in to focus,” Mr. Harter said. “But agriculture also has huge opportunities. They are a big part of our groundwater supply management, and they can be a part of that groundwater supply enhancement. They have to be part of doing groundwater quality improvement and drinking water protection. But at the same time, we have to do agriculture in an economically viable way.”
“So how to do this? That’s the challenge of today and the future, much like the Superfund and the Clean Water Act have been challenging companies like Boeing and other industrial companies 30 and 40 years ago with how to deal with that,” he said. “That’s going to be a revolution for agriculture and a change for agriculture that will not come easy.”
A closing perspective on agriculture and water use
California grows a lot of the nation’s food, some of which are only grown here. “99% of US walnuts are only grown here, 99% of almonds are grown here, and some of our vegetables are grown for a large part for the entire US in this region, and on and on it goes,” he said. “There’s a water footprint associated with that … A quarter pound of beef has a water footprint of about 375 gallons, and a quarter pound of chicken has a water footprint of about 72 gallons. There is a distinct difference in the water footprints on the foods that we eat, especially between beef and chicken and between meat and vegetables.”
He presented a pie chart showing the average American’s water footprint, noting that the red portion represents what Americans pay for their water bills. “The largest part really is actually our diet. A significant part is the energy and transportation, much of this being cooling water used in our nuclear power plants and our coal power plants, but the diet is a big part of our water footprint.”
“Agriculture is not disconnected from us, and it’s not disconnected only in California but it’s not disconnected from what we eat around the globe,” he said. “40% of agricultural production, whether its food, feed, fiber, biofuels happens on the 20% of irrigated lands that we have around the globe in the US, the Mediterranean, the Middle East, Northern India and China. That’s where we have irrigated agriculture that looks much like irrigated agriculture in California and that’s where we grow 40% of our food. That’s why figuring out how we can manage agriculture – it’s water use and its water quality impacts – is important for our global food security.”
“As a result of food production with groundwater, we have seen huge increases in groundwater production, not only in the US, but very much so in India, China, Bangladesh, and some places in Latin America,” he said.
“These huge increases in groundwater pumping have led to groundwater overdraft, not only in California in the southern part of the Central Valley, but also on the Ogallala aquifer in the Midwest, where we see consistent drawdowns of the water table, and in Texas it’s now forseeable that this aquifer is going to be essentially dry before the end of the century.”
“This is a map showing the world relative to population size and if you look at where we do irrigated agriculture, it is an important piece for our global population, so figuring this out and doing this is very important for our long term food security,” he said.
“You can be part of this because you make decisions on what you eat, where your food comes from, and whether your food comes from a place where there’s enough water to grow that food, and where there is management that protects groundwater resources as well as surface water resources from water quality degradation,” he said. “We don’t have all of the answers to those questions , and part of the challenge for us on the science side will be to provide you with some of this information. Part of the challenge for agriculture will be to be more transparent in how they impact the environment with their production system, but you make that happen by asking questions, much like you made it happen for Boeing by asking questions and asking for Boeing to be more sustainable. I think we can do the same with agriculture and agriculture is much more responsive to consumer questions than it is perhaps to regulations.”