The exceptionally dry conditions along with the recent passage of groundwater legislation has focused the state's attention on its groundwater resources. In this presentation, Groundwater and Surface Water Interactions Under Water Shortage, Thomas Harter, faculty and cooperative extension specialist with UC Davis, gives an overview of groundwater and how groundwater works, where groundwater is in California, and how groundwater is managed in California.
“Groundwater is a very big part of our water supply,” began Mr. Harter. “Groundwater is about 40% of the industrial, urban, and agricultural water supply in average year – that’s about 16 MAF.”
“Let’s start with the question, what is groundwater,” he said. “Groundwater is very mysterious to many people. We can’t see it. Some people think of it as water in arteries flowing around, some people think of giant reservoirs that are somehow filled underground in big caverns, but groundwater is like the water you would pour in a flower pot that just disappears in your soil – it fills the pore spaces between the soil particles that are in your flower pot.
Our groundwater in California is much the same way; it fills the pore space between sand and gravel pieces, even between small tiny clay particles and silt particles, or it can fill the fissures and cracks in the rocks that make up our big mountain ranges in California,” he said.
Most of the groundwater is in between the sand and gravel sediments which are present in the valleys we have in California, he said, presenting a map of the groundwater basins in California. “I like to think of these groundwater basins as big giant bathtubs that are filled with dirt and the walls are the mountain ranges that we have – the Sierra Nevada to the east, and the Coast Ranges on the west, and they continue under these valleys and they hold the sediments that have deposited in these valleys over millions of years.”
He then presented a slide of a cross section of one of the valleys, explaining that the gray represents the mountain ranges of fractured bedrock and the blue are the basins which are filled with sediments – clay, silt, sand, and gravel. Over time, the pore spaces between them have filled with groundwater.
“Groundwater is water that completely fills the pores in these sediments; the top of the zone that’s completely filled with sediments is called the water table, and above that we have the unsaturated zone or the vadose zone,” he said. “Rainfall and streams with water will recharge this groundwater,” noting that the diagram depicts an unconfined aquifer, which means that the water table is not restricted in how it moves up and down.
“Over time, these basins are filled up with groundwater,” Mr. Harter said. “Eventually, when this bathtub gets to be full, where this groundwater shows up is at the lowest part of these valleys; that is usually where we have a stream, and that’s where groundwater typically connects with our streams.”
As the water from rain comes down the mountain or fills the streams, and it starts flowing over the sediments, it will recharge the groundwater basin, and as a result, there will be discharge from groundwater into our streams, he said. “Groundwater flows from where we have high water tables towards where we have low water tables,” he said. “It might not always move directly downhill. In some cases, it can well up because it’s under the influence of the pressure from the high water table, and it will well up into the stream from below.”
This is the natural way our system works, with groundwater being replenished from streams and rainfall that’s not used by the plants, and then discharging into streams, he said.
The subsurface is very complex consisting of many different types of materials that can be divided in terms of coarser materials that we can use to pump groundwater and the finer materials that hold a lot of water but don’t move it much. “These fine materials we refer to as aquitards and the coarse materials we refer to as aquifers,” he said, noting that the aquifers are shown in blue on the slide and the aquitards are shown in gray. “What typically happens is that the sediments in our groundwater basins are a complex sequence of fine material, aquitards, and coarser material, which are our aquifers. They are unconfined closer to the mountains and as we go to the center of the valley, we typically have these clay layers, or aquitards, that then confine our aquifers. So just keep in mind that the subsurface has a very complex pattern and we can’t pull groundwater out of every part of the subsurface, but there is a significant amount of groundwater.”
“In the 1930s, 1940s, we began to pump significant amounts of water from these groundwater basins in many areas in California,” he said, presenting a graphic of a typical groundwater well. He explained that a well is essentially a borehole into the ground; the well drillers then sets a well casing, which is slotted or screened in certain places; there is a filter pack around the casing that excludes the finer materials from the water that is pumped out of the well.
There are many of these wells, and as they take water out, they create what we call a cone of depression, which is when the water table around the wells is lowered,” he said. “The more we pump, the larger the cone of depression.”
In the Central Valley there are literally hundreds of small domestic wells, many tens of thousands of agricultural irrigation wells, and several thousand public water supply wells that pull groundwater out of the Central Valley aquifer system. “These colors indicate how diverse the sediments are that these wells are penetrating,” he said, presenting a slide from the USGS. “Only the red and yellow parts are really sandy and carry lots of water. The blue and purple parts are much, much tighter and don’t carry as much water.”
“Typically we pump our groundwater in the summer for irrigation and in urban areas, we use much of our water in the summer, much of that for irrigation of lawns,” Mr. Harter said. “Over the winter, it rains, so we pump less in urban areas; we don’t pump at all in agricultural areas, and we have more water in rivers that can recharge these groundwater basins near the mountains. We have rainfall, some of which might be recharging the groundwater, and the groundwater will table will recover and go back to its original state. And this goes back and forth, year after year.”
During long periods of drought, there is much less water coming out of the mountains and much less recharge occurring, so much of the water that we use for irrigation, both in urban areas and in agricultural areas, comes from groundwater, he said. “On this graph on the left side, you have the different water uses, including irrigated agriculture here in the lighter green color. On the right side, this graph from the Department of Water Resources shows where all of this water that we’re using is coming from for the years 1998 to 2005,” he said. “What I want you to focus on is the groundwater, shown in purple, which is showing how much groundwater is being pumped to supply water on the left side. In a wet year like 1998, it might be as little as 12 MAF statewide; in a very dry year where we rely predominantly on groundwater to supply water resources, surface water resources are much less and we may be pumping as much as 20 MAF of water from our groundwater reservoirs.”
“One of the things that has happened in the last 10 – 15 years is that we have had a lot of dry years, and as a result, looking forward from 2004 to 2010, we’ve had many years where we have used large amounts of groundwater, closer to the 20 MAF, and not so much down to the 12 MAF,” Mr. Harter said.
As a result, especially between 2007 and 2009, groundwater levels have declined in the Sacramento Valley and more so in the San Joaquin Valley, with the largest amount in the Tulare Lake basin, he said. “There, water users rely predominantly on groundwater, and in some of the Central Coast basins, we have predominantly groundwater supplying the overall water supply used for irrigation and urban water.”
“Now there are some dynamics in this groundwater use,” he said, presenting a graph of water use the Tule River Basin. He said that you can think of the graph as a water budget for the root zone of the agricultural landscape, noting that the graph shows an annual cycle going from October till September. “There’s a lot of water applied to our agricultural landscape in the summer between June and September, and much of that in a dry year is from groundwater, the darkest color. In a wetter year, much of the water that gets applied in the summer comes from surface water, the lighter blue colors and the medium blue colors, and less so from groundwater, which is the dark color.”
In wet years, a significant amount of water is precipitated onto the agricultural landscape and being recharged into groundwater, with much of that recharge happens in the spring and in the summer months, with much more so in a wet year like 1998, and very little in a dry year like 1977, he said.
“As a result of introducing agriculture into the Central Valley in the early 20th century, we have significant amounts of recharge into groundwater aquifer, much more so than under natural conditions, and the recharge out of these agricultural landscapes makes up a significant amount of the recharge into our groundwater basins,” Mr. Harter said.
“Some years it’s more; some years it’s less. The further south we go, the more we vary how much groundwater we use. Some wet years, very little groundwater use, and in dry years, the groundwater use can double, triple, or even quadruple,” he said, noting that this is a groundwater basin in the southern part of the Central Valley.
“We draw down these water tables; they may recover over the winter. They hopefully recover,” he said. “In some cases, we may be taking more water out of the groundwater basin then we’re putting in, and in that case, water levels will decline permanently. Part of that consequence is that now we’re losing the connection to streams, which if they are not supplied by reservoirs, may ultimately dry out, especially during the summer period. This is the situation that we refer to as overdraft, when the total amount going into groundwater is less than the amount of water that we’re pumping out through our well network.”
Mr. Harter then presented a graph of the drought history from the beginning of the 20th century through 2010 for both the Sacramento River and the San Joaquin River. The wet periods are shown in blue and green, and dry periods are in orange and brown. “In the last 15 years, we’ve certainly had a good share of drier years which has led to significant water level declines in many aquifers in California, but in particularly in Central California, the Central Valley and some of the coastal aquifers.”
He then presented a graphic showing total change in groundwater storage in the Central Valley between 2005 and 2010 which includes the drought years from 2007 to 2009. “Most of the impact is to the Tulare Lake Basin in the southern part of the Central Valley; primarily on the east side of the Tulare Lake Basin, but also on the margins of the San Joaquin Valley and the margins of the Sacramento Valley, we have local areas where we have drawn down water storage in our groundwater basins significantly. In total, the DWR estimates that drawdown to be on the order to 5 to 13 MAF between 2005 and 2010.”
“As a result, the depth to groundwater is significantly deeper in 2010 than it was in the early 2000s,” he said, showing a map of the Central Valley from about Fresno to Bakersfield. He noted that the blue and green represent areas where the water table is between zero and 100 feet below ground surface; in yellow are the areas where the water table is between 100 and 150 feet below the ground surface, and in red are the areas that have a water table that is more than 200 feet below ground surface, and in some areas, more than 300 feet below ground surface. “This is one of the most extreme situations that we have here in the state,” he said.
He then presented a graphic from the Department of Water Resources which shows the change in water level between spring of 2012 and spring of 2013, noting that on this particular map, the wells in yellow have seen relatively little change, and the wells in brown have seen a change in water levels up to about 10 feet. “There are many wells down in the southern parts of California and in some parts of the northeastern California where we’ve seen significantly more than 10 feet of water level change between spring of 2012 and spring of 2013,” he said.
He then presented another graphic, noting that the scale had changed from the previous graphic. The yellow wells are those wells with a drop in levels from 0 to 25 feet; those wells in orange had a drop of 25 – 50 feet, and in red are those that dropped by more than 50 feet. “This gives you an idea of that water level drawdown during a drought period where there’s no recovery of water levels in the winter, and the magnitude of how much the water levels may change in this year. We’re seeing water level changes that are typically on the order of 0 to 10 feet, maybe 20 feet further north, and down south there are many areas that will see water level declines that are exceeding 40 feet.”
“As a result of the long term use of groundwater, we’ve actually created a significant amount of groundwater storage in the Central Valley,” he said, presenting a graph from the California Water Plan draft for 2013. “Much of that storage is in the San Joaquin Valley and in the Tulare Lake Basin along the east side. In total, the Department of Water Resources estimates that we have created storage space that we could fill up again that is on the order of 20 to 40 MAF, roughly the equivalent of the amount of surface water storage that we have available in this state.” He pointed out that the storage space could be used for conjunctive use of surface and groundwater, or groundwater banking, where water is stored in wet years for use in periods of drought.
“As a result of this long term decline in groundwater levels in some of these areas, we have also seen subsidence, which is perhaps one of the most economically destructive impacts,” he said. “Much of these subsidence has happened on the west side of the San Joaquin Valley in the Tulare Lake Basin and some on the east side of the Central Valley with land surface levels declining by as much as 30 feet between the 1920s and the 1930s when we first started pumping, and the 1970s when the SWP became available and much of the water supply, especially on the west side of the southern Central Valley, switched to surface water supplies.”
“But with surface water supplies tighter in the last ten years for a number of reasons, we have started to increase the amount of groundwater pumping, and as a result, water levels continue to decline,” he said, pointing out that in the graph, the blue line indicates the land surface elevation. “That land surface elevation has also begun to subside again, very much in sync with the changes in water level elevations, and that could pose significant problems for infrastructure that we have in these regions, including the many canals that all graded according to a certain slope. There is damage that comes from this subsidence phenomena that needs to be addressed, and would be very costly to repair.”
He then briefly discussed groundwater rights. “California has what is called a correlative rights doctrine,” he said. “That means that landowners are not owners of the groundwater that is below them, but they do have what’s called a usufractory right. They have the right to use that groundwater, and that right is correlative to other land around the particular property, the properties of the aquifer, and the amount of water that’s available. It’s a legal doctrine that was developed in the early 20th century and that governs the overall way we share groundwater within a groundwater basin among many users.” He also noted that there is a constitutional mandate to use all water for beneficial use, so one cannot use water to waste it.
There are many special districts that manage water for many different purposes: water districts, water districts, irrigation districts, water conservation districts and many others, he said. Counties have under their police power the right to control groundwater exports out of county.
In Southern California, many groundwater basins experienced overdraft in the early to mid 20th century, and as a result, the basin has been adjudicated. He explained that with basin adjudication, experts look at how much recharge there is into a groundwater basin, and the amount of replensihable water is divided up and everyone is given a numerical right to how much groundwater they can extract. He noted that the basin adjudications cover most of Southern California, with only one in Northern California.
The State Water Resources Control Board is the agency that oversees groundwater management in the state, and it’s the courts that ultimately decide if there’s multiple parties that don’t agree on who gets how much water, he said.
Some basins are managed through groundwater management plans which came about in the early 1990s, he said, presenting a map showing where groundwater management plans were in force. “About 42% of groundwater basins in California have some form of a groundwater management plan,” he noted. “In lighter green colors are the groundwater management plans that have been prepared under the particular structures of SB 1938, which cover about 32% of all groundwater basins. Those adhere to significantly tighter management practices and assessments than some of the older groundwater management plans that were done in the 1990s.”
“Groundwater management plans include many different pieces,” he said. “For one, there’s a planning process. They define governance structures, agreements on goals, and reporting and enforcement. Ideally, there’s cooperation among stakeholders. In preparing for a groundwater management plan, organizations identify and engage participants and stakeholders; they develop structures to involve stakeholders, and they try to facilitate disputes, trying to resolve disputes, avoiding going to court.”
A big part of a groundwater management is collecting information about the groundwater basin – to find out what is there, to collect data, to do the analysis, and to have a portfolio of management practices that can improve on groundwater conditions in that particular basin, he said.
California has a minimum list of elements that go into a groundwater management plan which includes a basin description, a description of the governance, specific management objectives, monitoring, the collection of data, and some form of accountability and review built into the groundwater management process.
The current challenge is with enforcement mandates, he said. “Who actually tells a groundwater pumper that he or she needs to stop pumping if there is overdraft, who decides that there is overdraft, and under what conditions do we stop groundwater pumping and who makes those decisions, so there are a number of challenges that we have,” he said. “There are also challenges with respect to integrating groundwater management with surface water management, which now happens under Integrated Regional Water Management Plans, which also have a governance structure, as many groundwater management plans are now part of IRWMPs.”
“One of the biggest political questions is what the role of the state is, and what the role of local and regional agencies is,” Mr. Harter said. “The SWRCB recently emphasized that it is interested in pursuing a primarily local regional management approach to groundwater management, but sill it has an oversight role, and defining that oversight role will be something that we’ll be looking in the next few months and couple years.”
- California Statewide Groundwater Elevation Monitoring (CASGEM)
- California Department of Water Resources Drought Information Page
- U.C. Davis groundwater information page
- San Joaquin Valley Agricultural Drainage, page from the Department of Water Resources
- California DWR Drought Workshop, January 2014
For more information …
This post is derived from a webinar from the series, Insights: Water & Drought Online Seminar Series, produced by the University of California Agriculture and Natural Resources.