[00:00:00] Bridget Scanlon: Welcome to the Water Resources Podcast. I am Bridget Scanlon. In this podcast, we discuss water challenges with the leading experts, including topics on extreme climate events, over-exploitation and potential solutions towards more sustainable management. Hi everybody. delighted to have Dr. Lenny Konikow on the podcast today.
Lenny is the Editor-in-Chief of Groundwater Journal and is an emeritus scientist at the USGS, following a 42-year career. As a research hydrologist, Lenny has received numerous awards from the hydrologic community and was inducted into the National Academy of Engineering in 2015. The citation was for “modeling of coupled groundwater and surface water flow and of solute transport in groundwater”. He's also a fellow of the American Geophysical Union and, received the Meinzer Award from the Geological Society of America. One of his recent books is titled Groundwater Resource Development Effects and Sustainability that was published within the Groundwater Project. Lenny, it might be nice if you could give the audience a brief review of the topics that you focus on in your research.
[00:01:22] Lenny Konikow: Okay. I've had a long career in hydrogeology and groundwater research, and the main focus of my work has been the development and application of groundwater flow models and solute transport models, both theoretical numerical developments as well as application to field problems. I've been very interested in assessing the predictive accuracy of groundwater models as well as the so-called validation process, which usually is not valid, usually constitutes false advertising, but that, that's a different podcast. I've also been interested in groundwater - surface water interactions, long-term groundwater depletion on a national and global scale, as well as the broader issue of groundwater sustainability.
[00:02:14] Bridget Scanlon: Thank you so much, Lenny, and I really appreciate your taking the time to talk with us today. I think people are becoming increasingly interested in groundwater as a resource, in recognizing the importance of groundwater. As we are subjected to intense droughts and surface water depletion, groundwater can provide a buffer against the surface water resource issues. You have conducted a lot of work collating data across the US for many different aquifers, quantifying depletion and also areas where groundwater resources have been increasing over the past century. Maybe you can describe a little bit about that work.
[00:02:56] Lenny Konikow: Sure, be happy to, and thanks for inviting me to participate in this. Basically, you know, the overview is that groundwater represents a huge reserve or reservoir of freshwater in the United States and globally, and it's present to some degree almost everywhere. It's certainly the largest stock of liquid freshwater on the planet. So groundwater really has a great potential for assisting in the adaptation to climate extremes and climate change.
As traditional surface water resources become scarcer and more strained, locally available groundwater resources may offer an alternative source, especially for domestic use, as well as an alternative storage reservoir for periods when there's excess water. If groundwater development and use is going to increase, we really should do careful planning to provide optimal management of the resource, as well as it will require detailed hydrogeologic assessments to find out where the groundwater could be developed with the least costs and environmental consequences, and the consequences are a major issue from pumping groundwater out of a well has consequences and these need to be evaluated as a trade off or a cost in the overall water supply picture. We want to minimize environmental consequences and really try to assure that groundwater use is sustainable and can be relied on to provide water security for an indefinite period into the future.
Keeping that in mind, groundwater is not a limitless resource, but it's always a valuable resource. In some places, groundwater really behaves as a renewable resource, and sustainability of development can be more easily achieved. But in other places, it's more of a non-renewable resource and development of groundwater is essentially mining the groundwater, it's not renewable. In the extreme cases, we could call this fossil groundwater. Anywhere that an aquifer is overexploited or overdeveloped, the consequences will eventually come back to haunt us. And this can be in the form of depletion of surface water resources. Drying out of springs, dry wells, the increased cost of pumping, or the increased cost of drilling deeper wells to replace the ones that have dried out.
[00:05:56] Bridget Scanlon: It's really nice that you collected the data and you summarized data, which was a huge effort that the USGS had all these regional models of major aquifers in the US from beginning in the 1900s when they originally developed some of these aquifers, they did think that it was a limitless resource. I mean, for example, in California drilling wells, they were artesian. They flowed at the surface. They thought they would never run out. So over time then we began to realize that it's not a limitless resource and we need to manage it better. So we're fortunate in the US that the USGS and various state agencies have invested a lot in developing regional models and monitoring networks to evaluate the fate of groundwater and the evolution of these groundwater systems. Maybe you can describe the results of some of those modeling and monitoring analysis and the different aquifers in the US and how they compare with each other.
[00:06:55] Lenny Konikow: Right. 40 years or so ago, or maybe longer, the USGS started a program called the RASA program, which is Regional Aquifer System Analysis program. And we, the survey, started analysis in depth, including simulation model development. Almost all of the largest aquifer systems in the US and this generated a wealth of data and improved understanding. Along this time in the late 1980s to 1990 or so, more than 30 years ago, I really began my interest in groundwater depletion and this possible connection to sea level rise.
At that time, many people were beginning to be concerned. Depletion of groundwater resources, excessive water level declines, and water table declines in some areas, such as Arizona, the Southern High Plains, Central Valley, California, and so on. And. A whole other different circle of scientists, oceanographers, were also interested in warning about sea level rise and it's possible connection to climate change. There were very few people that I was aware of that were making any connections between the two, and I really was not. Anyone who had done any studies to integrate all the different groundwater assessments throughout the nation and throughout the world, to try to estimate how much total cumulative groundwater depletion was occurring.
So I kind of decided to start doing an inventory based largely on published studies and data to assess long-term changes in groundwater storage, that is groundwater depletion, and one of my reasons was to see if cumulatively over a long period of time, these added up to such a large volume of loss of groundwater that it might actually have some impact on or contribution to sea level rise to the maximum extent possible.
I try to base my estimates on direct measurements of water level changes in aquifers and on calibrated simulation models for large aquifer systems where the calibrations of the models are typically keyed to observed changes in water levels or heads, as observed in wells. I wanted to start my study long enough ago that we could really get at long-term trends and anthropogenic effects of groundwater development.
Kind of getting back to the bigger picture, what I did is I started with 1900 and went up through 2008, as close as I get to the time that I was doing the study. So we're looking at a time period of more than a hundred years. What I found was that in the US the total groundwater depletion was approximately a thousand cubic kilometers. That's a lot of water. It's for a comparison, it's about twice the volume of water contained in Lake Erie. That's how much groundwater has been lost from the subsurface aquifers just in the United States.
I also look globally, there were several systems aquifers, large aquifer systems around the world for which at least some data were available to assess long-term groundwater depletion. And my best estimate on a global picture was that there was about 4,500 cubic kilometers of groundwater depletion during this 108 year study period. And the overall accuracy probably in the US with as better data available was on the order of plus or minus 10 to 15%, and globally was probably closer to plus or minus 20%.
And that basically about the best we can do. There's a lot of uncertainty in the storage properties of aquifers and just all the fact of how representative you observe water level changes are and so on. So there's definitely uncertainty there in the US. The largest volume of depletion occurring in a single aquifer system is in the High Plains aquifer, whereby from 1900 to 2008, totaled to approximately 340 cubic kilometers, and that's continued to increase even though we might say, well, that can't possibly continue, but it is continuing and it's continuing at at a fairly high rate by now. By 2020 or more recently, it's probably closer to 400 cubic kilometers, just this one aquifer system. You could say the depletion almost equivalent to the volume in Lake Erie. The lake area is a little bigger, other very large depletion volumes, but not quite as large as in the High Plains occurs in the Central Valley of California. The alluvial basins of Arizona, mostly central and Southern Arizona and the Mississippi Embayment Aquifer. These are large systems and there's large amounts of groundwater use and there's large storage losses in these systems. The High Plains aquifer, as you know, you've studied it, is a very large system and it's not this one number doesn't do it justice. There's a lot of variation. The most severe depletion occurs in the southern and central High Plains, Kansas, Oklahoma, and Texas and New Mexico. And in the Northern part, Nebraska for example, there's been almost no long-term net change in groundwater storage on average, in Nebraska, for example, in some areas near the South Platte River, there's actually been a long-term rise in water levels due to diversion of surface water irrigation, which increases recharge above the natural rate. In other parts of Nebraska, there is depletion, but on the whole, it's not a significant total change in volume of groundwater in Nebraska.
[00:13:51] Bridget Scanlon: So Lenny, I mean, in Nebraska, I mean it's maybe not because they're pumping less or whatever, but they also have high recharge from the Sand Hills. And then maybe 30% of the irrigation is coming from surface water, which helps to recharge the aquifer. But the soils in the rest of the High Plains, the central and southern High Plains are very clay rich and the recharge is very low. So similar levels of pumpage depleting the groundwater because I know from our work in the Texas part of the High Plains, they were pumping about 10 times the recharge rate in the southern part of the central high Plains. So it's the soils and the texture and the recharge rates and all of these things that impact the sustainability of the use of groundwater in the High Plains.
[00:14:39] Lenny Konikow: Absolutely. The present rates of groundwater use in the Southern high plains clearly is not sustainable, so there will be changes. These changes can come through conservation efforts through changes in water management and policy or changes will happen because mother Nature will dictate that these changes will happen. So, but clearly the present rates are unsustainable in the US.
It's interesting that there are two very large aquifer systems in the Northwestern US, both of these volcanic rock systems where there's been a very significant net rise in the water table. In the long term, these increases in groundwater storage in these systems arise principally because in the 20th century, surface water has been diverted for use in irrigated agriculture. And early on it was applied with a flood irrigation method, although that's been changing to sprinkler irrigation and other more conservative methods, but still, resulted in large applications of water, two large areas, agricultural areas, which increased the recharge, which caused water tables to rise. So there's a net increase in storage over this long-term period. What we did in looking more detailed at both of these systems, one of the Snake River Plain Area system, the other being the Columbia Plateau. In both of these systems, there's been a reversal of the trend in the last couple of decades due to increased groundwater use, and this increase has been substantial. So during the last couple of decades, there's been a net depletion of groundwater in storage in the systems, although not enough to offset the increases that occurred in the 20th century. So comparing today's groundwater in storage with 1900, there's still more groundwater in these aquifers than there was in 1900, but the trend is that the reserves are being slowly depleted,
[00:17:05] Bridget Scanlon: Right, I think surface water irrigation can recharge groundwater, and I think sometimes when people say, we need very efficient irrigation and Claudia Faunt from the USGS in California said, you know, we need inefficient surface water irrigation because it can recharge the groundwater and it's not a net loss, and we need very efficient groundwater irrigation systems. And just as you described in the northwest, the Snake and the Columbia, I mean the Thousand Springs in the Snake River Plain, you know, was evidence of discharge from the Thousand Springs increased over time as, as the, the aquifers built up. And I think Alan McDonald and his team found similar things happening in the be IndoGangetic Plain when they had the canal irrigation there early on and the net increase of about 350 cubic kilometers, right. But in the last couple of decades, declined because of increasing groundwater use. I guess the increasing groundwater uses, as you say, groundwater is pervasive. They can access it anywhere. They can drill a well locally and access groundwater, whereas it takes a lot more management maybe to have surface water irrigation system.
[00:18:22] Lenny Konikow: Right with groundwater, it's available right where you need it, and you control the pumps and so it's available when you need it. You don't need expensive conveyance structures, pipelines or canals to get the water to you. So even where the canals exist today from prior projects, there's always constraints and the canals can only carry so much water. So there's limitations on what farmers can get. So they typically need more water or want to irrigate more land, so they'll make the investment in drilling a well. And then they have more water available to irrigate their crops and basically lead to more successful, crop yields.
[00:19:09] Bridget Scanlon: You know, I think that's really important that this connection between surface water and groundwater, many people don't recognize that, it's unfortunate that in many areas regulated separately or managed separately, but they're one and we need to recognize that to optimize their use and to try to make it more sustainable.
[00:19:32] Lenny Konikow: Absolutely.
[00:19:33] Bridget Scanlon: So you mentioned the sea level rise contribution of groundwater depletion. So I think you mentioned that globally there was about 4,500 cubic kilometers of depletion and did that include the 1000 cubic kilometers from the US. That included it. Yes. So the US is about a quarter then of the global depletion, and then how much of the sea level rise then would that contribute and how has it changed over time?
[00:20:02] Lenny Konikow: Basically the, question is, can it contribute to sea level rise? And that gets to what happens to all the water that's pumped out of wells? Where does it go? Well, it's applied to the land. It's used for crops, it used for water supply. Much of it evaporates or transpires from agricultural use. Some of it runs off into streams and rivers. Some of it infiltrates back into the aquifer. We could look just at the net plumage and not rather total pumpage? What we believe is that much of this water will travel through the atmosphere if it's evaporated or transpired or travel through river systems. But the ultimate sink, for most, the great majority of the groundwater that's pumped and used ultimately would have to be in the oceans where it can accumulate. Okay.
And that it becomes part of the overall hydrologic cycle. So how much could that 4,500 kilometers contribute? Well, the simple way to estimate that is spread that total volume of groundwater depletion over the surface area of the oceans, and you find that it would account for a sea level rise of about 13 millimeters over the 20th century into the present time.
That's something on the order of 10% of the observed sea level rise. Now, if you look at the rate of sea level rise since 1990, the oceanography community believes that the rate of sea level rise has increased to something on the order of 3.1 millimeters per year compared to an average rate in the 20th century of about 1.8 millimeters per year.
So the rate of sea level rise has increased, but the rate of groundwater depletion during this same time has also increased, and over the last decade or two would be equivalent to about 0.4 mm/yr over the area of the oceans. So just on that basis, groundwater depletion in the recent decades can balance about 13% of the recent sea level rise.
So I believe that groundwater depletion is a small, but non-trivial, contributor to sea level rise. It should be accounted for in the balance of volumes of water in the ocean. And groundwater depletion really represents a transfer of water from the continents to the oceans. And can be viewed that way.
Of course, there's a counteracting process in terms of water transfer from continents to oceans, and that's the construction of dams and large reservoirs behind the dams. During the 20th century, a lot of water was held back from running off into the oceans, but during the latter part of the 20th century, the volume. Additional surface water storage on the continent has generally diminished or is stabilized while the greater rate of groundwater depletion has still been increasing. So it's relative importance of groundwater continues to be a factor.
[00:23:35] Bridget Scanlon: Yeah, that's really important to consider and much of your work Lenny, you emphasize how when you pump water from the ground, the source of that water changes over time, initially maybe from storage, and then other sources. Can you describe that, and I think you describe it really well in your recent book in The Groundwater Project. And the groundwater myths, I guess would be another topic of interest.
[00:24:03] Lenny Konikow: Yeah, the water budget myths. Water budget myths as it was described by John Bredehoeft and by others. Okay. So yeah, I guess we could get at the question, what is the source of water pumpage? Well, and this was really described very well by the very famous groundwater hydrologist CV Theis in a classic 1940 paper. So all of these principles really have been well defined for more than 80 years. They're just not well recognized by the vast community of hydrologists, groundwater hydrologists generally do, but, it's still even among groundwater scientists, it's sometimes a fuzzy concept. And what CV Theis pointed out in this paper is that when you pump a well, initially most of that water comes out of storage in the aquifer, you're depleting the groundwater in storage in the aquifer. As water flows into the well due to head declines related to pumping, when you pump water, it lifts water out of the well, the water level in the well goes down. You get drawdown. That drawdown spreads into the aquifer and spreads out over time and distance. And this results in changes in hydraulic gradient. The cone depression that results from this really reflects the volume of water depleted from that pumping. Over time, that cone of depression spreads and has other consequences.
So as the water table declines, groundwater from aquifer transpiration may decrease because the water table drops below the root zone. In some areas, that cone depression may reach surface water sources, and so it may reduce the discharge or base flow to streams, or it may actually reverse hydraulic gradients between the stream and the aquifer and cause induced infiltration.
These effects other than the storage depletion as a whole can be called capture because it reflects the fact that the well is pumping water that otherwise would've gone elsewhere. It's capturing the water that would've flowed to streams or discharged to lakes, to wetlands or to springs, be consumed by plants or springs or submarine groundwater discharge in coastal areas.
So all of these really reflect captures. So the basic principle elucidated by CV Theis in his 1940 paper is that groundwater is balanced by a combination of storage depletion and capture over time. Initially, storage depletion is the primary factor or source of water to wells and over time. That balance between capture and storage depletion shifts so that over time capture as the cone of depression spreads out through the aquifer, capture becomes a larger and larger fraction of the water derived by pumping a well.
Ultimately, the rate of storage depletion could stabilize, and at that point, there's no additional depletion of storage. Water levels do not continue to drop, and all of the pumpage from the well is balanced by capture. If that happens, and it doesn't happen in all systems, can't happen everywhere. But when that does happen, the rate of pumpage can continue indefinitely in time and from strictly a hydraulic perspective, that pumpage is sustainable, as long as it's not negatively impacting the surface water too much,
Well, that you know how much is too much. It will impact surface water. That is it. We cannot use strictly a hydraulic definition for sustainability because that ignores the consequences on surface water and on streamflow, right.
So determining sustainability of groundwater really requires some subjective judgment calls, okay? Because you have to incorporate an assessment of these environmental consequences of pumpage in deciding what a sustainable level is or should and it may be quite a bit less than what you could physically continue to pump out of a well. And the effects can include land subsidence, wells going dry, springs drying up, surface water being diminished, and so on. So this then gets into a broader issue of how do you trade all of these consequences against the need for water security. Right? You know, where do you draw the line and who draws the line?
And this, this is a right, this is a complicated issue, and inevitably will bring in policy makers, legislators, and lawyers. Right. You know this, this can get very messy and involve litigation, and we've certainly seen cases where one state has sued another state because they believe they're not getting their fair share of surface water because the upstream state, there's a lot of groundwater usage, which is affecting surface water and depleting the stream flow before it gets across the state line.
And in general, and you know, interstate cases go directly to the US Supreme Court in general. The courts, including the Supreme Court, has recognized the fact that pumping groundwater can indeed diminish or deplete surface water resources. So that legal basis has been established.
[00:30:53] Bridget Scanlon: So I guess you're referring to Kansas and Colorado,
[00:30:58] Lenny Konikow: Right, that's one. There are other cases, but, in that case, again, it's a complicated case and I'm not aware of all the issues, but the basic outcome is that Kansas won the case for the most part, and, Colorado had to make better efforts to meet their obligations of surface water flows to the state line what goes into Kansas, and that involved some controls, management and limitations on groundwater use in the Arkansas River Valley of Colorado.
That made some people unhappy that's a concern, right? Yeah. Without doing anything. Then downstream users were unhappy, and concerned. So there's no way to make everybody happy, and you have to find some way to balance all the complexities.
[00:31:51] Bridget Scanlon: Yeah. I have a couple of questions that I'd like to ask, related to this interaction between groundwater and surface water.
So to summarize what you said with groundwater development before you put in any wells, the recharge to the aquifer balances discharges to surface water bodies, springs, lake, rivers as baseflow. And then when you put in a well, then initially it takes it from aquifer storage, and then the cone of depression then expands and captures it from other sources.
[00:32:23] Lenny Konikow: Yeah, yeah. One of the things Theis pointed out in his paper, or you could gather from reading it, is that when you look at the balance between well pumped storage depletion, and capture, what you realize in what Theis pointed out, the natural recharge to the system does not enter into the equation. It does not enter into the balance in terms of the effects of pumping the well and limits on the pumping of the well and what balances well recharge does not enter into it because of what you were just getting at. There is natural recharge and there's natural discharge before the well is drilled. When you start pumping, you disturb the balance between natural recharge and natural discharge, and it's made up by some combination of a depletion of the amount in storage, an increase in recharge, and or a decrease in discharge.
So it's really those two factors. The increase in recharge and decrease in discharge that constitute capture and really have to be evaluated. Yes, the actual amount of recharge should not be considered as a primary factor in assessing limits or sustainability of groundwater development. And that's really, that's really the water budget myth that Bredehoeft and others get into.
[00:33:53] Bridget Scanlon: Right? So when you look at the numbers, then in the US we are very fortunate in that we have estimates of how much people pump. And then we also have estimates of how much groundwater discharge occurs. So when you look at the US as a whole and you look at all the studies that you evaluated, how much is groundwater depletion relative to capture and how much they pump?
[00:34:21] Lenny Konikow: Okay, that's certainly an interesting question. And to assess that we really have to have some good idea of how much is being pumped. Fortunately, the US Geological Survey every five years publishes basically a census of water use in the United States. It's updated every five years, and it includes a reasonably good estimate of how much groundwater use occurs in the US on average during each five year period.
And, the most recent assessment, 2015 indicated: withdrawals of fresh groundwater in the US totaled about 315 million acre feet, which is about 400 cubic kilometers a year. Again, compare that to lake area. That's more than 80% of the volume of water in Lake Erie pumped every year across the US.
Okay, and we could assess how much of that pumpage is balanced by storage depletion versus how much is balanced by capture. And in my studies, I came up with an assessment of groundwater depletion. Over that 108 year period with estimate included estimates of annual depletion for each year in that period. And I did my assessment for approximately the 40 largest aquifer systems in the US in which there was some change in groundwater storage.
There was some other system in which we looked at where there was a negligible change, long term change in groundwater storage, so I didn't include that. But in terms of overall effects in the US, the depletion volume compared to the pumping, it turns out that only about 15%, one five percent of the groundwater pumpage is balanced by storage depletion in aquifers.
And that's, I think, is a little surprising to a lot of people who've thought it's probably a much bigger number. But that implies that 85% of the pumpage has to be balanced by capture in one form or another, and that I think is surprising to many people, particularly hydrologists, who have not been focused on groundwater studies.
There's right a lot of assessments that assume that all or most pumped water is balanced by storage depletion, and that's simply not the case. And I would argue that, the numbers that we got, for the US which again is, is probably reasonable. There's certainly uncertainty there, but it's probably good within plus or minus 10%, maybe plus or minus 15%.
So that 85% being balanced by capture I think is a pretty reliable, fairly reliable number. And you can ask how does that apply to the rest of the world? And I would argue that in terms of sampling, numbers in the US are reasonably representative of the entire world in the, you know, in terms of a sampling issue.The US pumpage is a large fraction of the global pumpage, you know, maybe 20% something on that order. So, you know, it's a big sample. In the US we have the full spectrum of climatic conditions. We have the full spectrum of geologic and hydrogeologic conditions for the aquifers, a full range of depth to water or to the water table from very shallow to very deep and a full spectrum of types of water use.
It's really, almost a perfect example for the entire world in terms of a sampling problem. So I would argue, those figures of 15% depletion, 85% capture are pretty representative for the world as a whole.
[00:38:32] Bridget Scanlon: I would like to remind the listeners that our podcast speaker today is Dr. Lenny Konikow, who is an emeritus professor at the U S G S Geological Survey. So Lenny, I'd like to shift a little bit now to the GRACE satellite data and the information we have obtained from it on water storage changes and how that relates to groundwater in the US and elsewhere. So, you know, you and I have been collaborating recently on trying to interpret that and I just maybe preface it with the GRACE satellites, monitoring changes in gravity, the Earth's gravity and the major cause of changes in the Earth's gravity at monthly time scales are changes total water storage from the atmosphere to the Moho, and then we have to figure out how much of that is groundwater. And then I think the value of GRACE also is extremely important for areas where we don't have the wealth of information that we have in the US from monitoring and modeling programs. So you and Bill Alley wrote the paper “Bringing Grace down to Earth” where you looked at what GRACE can provide, but also the limitations for water managers and things like that.
[00:39:46] Lenny Konikow: GRACE is a wonderful tool, and I have every reason to believe that it could provide accurate and amazingly precise estimates of changes in gravity.
So as a hydrologic tool, it has to be viewed as one of many tools in our toolbox, and it has to be used with some proper care as any other tool. The GRACE satellites started collecting data in the early 2000s. So one immediate limitation is you can't go back before that and make any assessments of long-term changes that may have occurred over the 20th century.
So that's clearly one limitation. The other major factor is what you had mentioned. One of the major factors to keep in mind is that transforming the estimates of total water storage change into groundwater depletion or groundwater storage changes involves some idealizing assumptions and complicated calculations, and it's not that simple.
And that introduces, you know, some errors. And I've seen published studies where they've made some gross misstatements and errors about how much groundwater pumpage and groundwater and the causes of groundwater depletion, have occurred in some large areas, not because there's anything wrong with the GRACE data, but because they haven't accounted for other hydrologic processes and factors, such as changes in soil moisture over large areas and natural base flow recession during drought periods when the water table naturally declines.
And base flow discharge will exponentially decrease in time, and you can attribute these changes to groundwater pumpage. So we've seen areas where such mistakes have have been made. On the other hand, as you've been involved in these studies, we've looked at some systems where the GRACE data or the estimates of groundwater depletion from GRACE do not agree with very detailed calibrated groundwater model estimates of depletion in close examination, at least in one well-known case that, that you've published on shows that the GRACE data were more accurate. The interpretations from GRACE data were probably more reliable than the well calibrated groundwater model, and the case specifically is the Mississippi Embayment aquifer, which is a large system. There's a lot of groundwater use. A lot of it is for rice production and which requires a lot of water. So there's a lot of groundwater pumpage and groundwater use really accelerated in the early 1970s as irrigation, particularly for rice, grew. So there was certainly depletion associated with it. Concerns about the aquifer led to the Geological Survey developing a large model of the Mississippi Embayment Regional Aquifer systems (MERAS). This was published in a professional paper. It indicated a very large volume of groundwater depletion. I don't remember the number exactly, but it was certainly more than 150, maybe 200 cubic kilometers of groundwater depletion over the long term.
This was surprising for an area that's in at least a humid or semi humid climate, and it turns out reexamination by the USGS and by others. It was recognized that the original model, and I'm not criticizing the original model really, but they, they did a nice job, but they only included the 40 largest surface water features, river streams in their model.
In fact, a closer look, including by the USGS showed that there are more than a thousand smaller streams and water features that are well connected with the aquifer system. And the great bulk of these were not included or represented in the simulation model. Now, the simulation models are great for numerical accuracy and they perform a perfect mass balance with an error of less than 0.1% typically in the water mass balance. So the amount of pumpage was specified, that was known, and so the model then has to account for it. And if there's not a possibility of representing the full scope of capture, the model will assume or balance taken from storage depletion. And so I know they're working on a better model now that incorporates finer resolution and many, many more surface water features. And I look forward to seeing the, revised USGS model study. But that's an example where analysis of GRACE data was really valuable for demonstrating that a down to earth model needed improvement. So, it works both ways. GRACE data is a powerful tool. It could be used, it could be misused, you know, in each case has to be examined. Yes. Another factor with the GRACE data is that it has a pretty large footprint, the scale, a pixel of information, you know, the footprints on the order of a hundred thousand square kilometers.
You know, with the the latest satellites, maybe it's a little smaller than that, but many aquifers of concern that are critical for small communities or even large communities. The aquifers are on the order of hundreds to thousands of square kilometers, an aerial extent, and the GRACE data provides a big picture. It does not provide any details on what's going on at a scale that the water managers in these smaller systems need in order to better manage their systems. So there is a limitation in that sense, and it's a very big limitation. It may be in the future that we'll be able to get much better resolution, but that's still far off into the future.
[00:46:19] Bridget Scanlon: Yeah, I don't think they'll be able to get much higher resolution, Lenny, because that's a function of the elevation of the satellite and, and there's a lot of atmospheric drag and other things that they have to count. So it's a trade off between the lifespan of the satellite and the resolution, you know, so I'm not sure that they're going to get much higher resolution in the future, but I think that's a good, summary of the linkages then between GRACE.
And I think because it's so visual what they show with GRACE, that it captures the public's interest and and they see these animations and stuff. So things like the Central Valley region and other areas, in IndoGangetic Basin, and then decision makers and policy makers can seem to get it more readily than what we get from the detailed analysis. So I think it plays a role in helping move towards more sustainable groundwater management.
[00:47:15] Lenny Konikow: Right, right. Well, one more thing, in terms of sustainability of groundwater resources and the interaction between surface water and groundwater, there are two just excellent. US Geological Survey circulars that are available.
One is by Tom Winter and Associates circular number 1139 and circular 1376 by Barlow and Leake. These reports are available for free downloading. From the USGS websites and, I'll point out on the two highly relevant eBooks published by the Groundwater Project. I'm the co-author of one of them that are available for downloading at no cost at the website, gw-project.org.
So if you go there, you'll find a, a great resource for many, many books and information at no.
[00:48:09] Bridget Scanlon: So Lenny, you know, we haven't had time to get into the management aspects, but your understanding of groundwater surface interactions will parlay into managing these resources conjunctively, and then also managed aquifer recharge and many other things.
But unfortunately we don't have time, to discuss it, but I really appreciate your taking the time to visit with me today and to describe your work, and I am a huge admirer of your tireless efforts to collate so much data from different sources to try to provide an understanding of groundwater systems and to educate all of us on the nuances of the implications. So thank you so much Lenny.
[00:48:52] Lenny Konikow: Thank you, Bridget, for inviting me and thank you for hosting this really valuable series of podcasts and I look forward to seeing how the series develop and listening to all the other speakers that you invite to participate. So thank you very much.
[00:49:09] Bridget Scanlon: Okay, take care. Bye-bye.
[00:49:11] Lenny Konikow: Bye-bye.
