We only have ONE hydrologic system, dammit!
School kids know that (it's in all their textbooks), geologists, geophysicists, geomorphologists, environmental scientists and ecologists, climatologists and meteorologists, resource scientists, most engineers, and a hell of a lot of people on the street know that. It is, however, a fact that seems to have escaped multitudes of politicians, policy-makers, regulators, at least one US presidential candidate, and, of course, commercial enterprises - whether out of profound and inexcusable ignorance or under the influence of vested interests and lobbies is open to question.
But it remains a fact. The entire hydrologic system - rain drops, clouds, springs, creeks, rivers, lakes, snow, ice, run-off, evapotranspiration and groundwater - is interconnected. Mess about with one bit and other parts will be affected - it's complex, but it is one single system.
The title of this post is a quotation from John Wesley Powell, a voice of knowledge and rationality that we could benefit from today - more of that, quite possibly, in a future post. Over 120 years ago, addressing an audience of (booing) vested interests in irrigation at any cost, he said:
When all the rivers are used, when all the creeks in the ravines, when all the brooks, when all the springs are used, when all the reservoirs along the streams are used, when all the canyon waters are taken up, when all the artesian waters are taken up, when all the wells are sunk or dug that can be dug, there is still not sufficient water to irrigate all this arid region. I tell you, gentlemen, you are piling up a heritage of conflict and litigation over water rights, for there is not sufficient water to supply these arid lands.
This occasional series, A Reverence for Rivers, is dedicated to Luna Leopold and perhaps I should add John Wesley Powell. In a the previous episode I quoted from a piece, written over sixty years ago by Leopold and his colleague Harold Thomas:
There are enough examples of streamflow depletion by ground-water development, and of ground-water pollution from wastes released into surface waters, to attest to the close though variable relation between surface water and ground water.
Man has coped with the complexity of water by trying to compartmentalize it. The partition committed by hydrologists—into ground water, soil water, surface water, for instance—is as nothing compared with that which has been promulgated by the legal profession, which has on occasion borrowed from the criminal code to term some waters "fugitive" and others, a "common enemy." The legal classification of water includes "percolating waters," "defined underground streams," "underflow of surface streams," "water-courses." and "diffuse surface waters"; all these waters are actually interrelated and interdependent, yet in many jurisdictions unrelated water rights rest upon this classification.
This jurisdictional and regulatory problem of compartmentalization of water resources contrary to the facts of the way the system works has given rise to many of the profound problems we face today. So I thought it might be helpful to set out some of the facts of the relationships between ground and surface waters and the consequences of ignoring them - bear with me, this may be a summary but it's not going to be short!
First, the interested reader can do no better than to go (as is so often the case) to the USGS. Published nearly 20 years go, Circular 1139 is titled Ground Water And Surface Water: A Single Resource. The synopsis is as follows:
As the Nation's concerns over water resources and the environment increase, the importance of considering ground water and surface water as a single resource has become increasingly evident. Issues related to water supply, water quality, and degradation of aquatic environments are reported on frequently. The interaction of ground water and surface water has been shown to be a significant concern in many of these issues. For example, contaminated aquifers that discharge to streams can result in long-term contamination of surface water; conversely, streams can be a major source of contamination to aquifers. Surface water commonly is hydraulically connected to ground water, but the interactions are difficult to observe and measure and commonly have been ignored in water-management considerations and policies. Many natural processes and human activities affect the interactions of ground water and surface water. The purpose of this report is to present our current understanding of these processes and activities as well as limitations in our knowledge and ability to characterize them.
USGS Circular1376, published in 2012, incorporated material from 1139 and proceed to document in considerable detail Streamflow Depletion by Wells—Understanding and Managing the Effects of Groundwater Pumping on Streamflow. Again, a straightforward summary:
Groundwater is an important source of water for many human needs, including public supply, agriculture, and industry. With the development of any natural resource, however, adverse consequences may be associated with its use. One of the primary concerns related to the development of groundwater resources is the effect of groundwater pumping on streamflow. Groundwater and surface-water systems are connected, and groundwater discharge is often a substantial component of the total flow of a stream. Groundwater pumping reduces the amount of groundwater that flows to streams and, in some cases, can draw streamflow into the underlying groundwater system. Streamflow reductions (or depletions) caused by pumping have become an important water-resource management issue because of the negative impacts that reduced flows can have on aquatic ecosystems, the availability of surface water, and the quality and aesthetic value of streams and rivers.
Both of these publications should be required reading for anyone, anywhere who is even remotely involved in water management and policy-making.
So let's start with a simple and fairly self-explanatory image from the USGS:
Ground-water flow paths vary greatly in length, depth, and travel time from points of recharge
to points of discharge in the groundwater system.
Groundwater, streams and wells all interact and influence water flow, but the rates of that flow vary over several orders of magnitude, from days to millennia. The architecture and physical characteristics (permeability and so on) of the aquifer system are always complex in reality and the distribution of lower permeability (confining) layers will dramatically influence the direction, amount, and rate of flow.
In places where the water table "outcrops" - i.e., intersects with the surface - there will be a spring (as at Havasu Falls, the image at the head of this post, ultimately feeding the Colorado River), a lake or groundwater feeding the flow of a river through its bed and banks. The relationship between the water table and a body of surface water is critical - a stream can be "gaining" flow from groundwater discharge or "losing" it by flow into the water table (depletion):
Groundwater discharge into streams and rivers is commonly the major contribution to their flow which is only augmented by rainfall and surface runoff.
Different segments of a river or stream may be typically gaining or losing and this will vary with the season - if the water table falls, the river may become "disconnected" from the water table, or, in times of flood, the river level may rise higher than the water table and effectively charge storage in its banks until the level falls back to normal.
All these variations occur quite naturally depending on time and place, and the system may be stable for long periods of time. But start pumping groundwater in the vicinity of a river and the system is de-stabilized. As the caption to this next USGS illustration describes, "In a schematic hydrologic setting where ground water discharges to a stream under natural conditions (A), placement of a well pumping at a rate (Q1) near the stream will intercept part of the ground water that would have discharged to the stream (B). If the well is pumped at an even greater rate (Q2), it can intercept additional water that would have discharged to the stream in the vicinity of the well and can draw water from the stream to the well (C)."
To quote again from the USGS:
The first clear articulation of the effects of groundwater pumping on surface water was by the well-known USGS hydrologist C.V. Theis. In a paper published in 1940 entitled "The Source of Water Derived from Wells," Theis pointed out that pumped groundwater initially comes from reductions in aquifer storage. As pumping continues, the effects of groundwater withdrawals can spread to distant connected streams, lakes, and wetlands through decreased rates of discharge from the aquifer to these surface-water systems. In some settings, increased rates of aquifer recharge also occur in response to pumping, including recharge from the connected surface-water features. Associated with this decrease in groundwater discharge to surface waters is an increased rate of aquifer recharge. Pumping-induced increased inflow to and decreased outflow from an aquifer is now called "streamflow depletion" or "capture."
So there is a critical interaction between groundwater and surface water by complex flows that take place over days, centuries and millennia, and wells can cause major changes to that interaction - streamflow can be severely depleted, water tables lowered and aquifer water volume maintenance totally disrupted.
For here is the first of what USGS Circular 1376 intriguingly documents as "common misconceptions." That is:
Misconception 1. Total development of groundwater resources from an aquifer system is “safe” or “sustainable” at rates up to the average rate of recharge.
This is fundamental, because we hear all the time that aquifers are being "mined" as a result of volumes pumped exceeding the recharge rate from rainfall up in the hills - it's really not as simple as this. Because streams and rivers are frequently depleted by losing water to an aquifer, particularly when wells are pumping from that groundwater supply, it is this source that is the dominant way in which an aquifer is recharged, not by water from the hills.
The sources of water to a well are reductions in aquifer storage, increases in the rates of recharge (inflow) to an aquifer, and decreases in the rates of discharge (outflow) from an aquifer. The latter two components are referred to as capture. In many groundwater systems, the primary components of capture are groundwater that would otherwise have discharged to a connected stream or river in the absence of pumping (referred to as captured groundwater discharge) and streamflow drawn into an aquifer because of the pumping (induced infiltration of streamflow)...
Reductions in aquifer storage are the primary source of water to a well during the early stages of pumping. The contribution of water from storage decreases and the contribution from streamflow depletion increases with time as the hydraulic stress caused by pumping expands outward away from the well and reaches one or more areas of the aquifer from which water can be captured. At some point in time, streamflow depletion will be the dominant source of water to the well (that is, more than 50 percent of the discharge from the well) and after an extended period of time may become the only source of water to the well. The time at which streamflow depletion is the only source of water to a well is referred to as the time to full capture.
So, if you choose to define and manage, through regulation and policy, groundwater and surface water as two entirely separate systems, you choose a recipe for disaster - you are more often than not severely double-counting and thereby overestimating your resources and you are not managing the system at all. And yet this is exactly what California, Arizona and, indeed, all the arid States of the Western US (not to mention other States and other countries) have been doing for more than a century. This has been superbly documented in ProPublica's piece, "Less Than Zero: Despite decades of accepted science, California and Arizona are still miscounting their water supplies." That analysis, part of their extensive "Killing the Colorado" series of reports, has also been summarised in the New York Times:
John Bredehoeft, a leading hydrogeologist and former director of the federal government’s Western states water program, bluntly emphasized the importance of basic honesty in counting water.
“If you don’t connect the two, then you don’t understand the system,” he said. “And if you don’t understand the system, I don’t know how in the hell you’re going to make any kind of judgement about how much water you've got to work with.”
Until state officials do, it seems unlikely that there will be any real solution to managing the Southwest’s strained water resources for the future.
But there's another, fundamental, problem - if you can't measure, monitor, document, you can't manage - and the data are essentially not there to measure, monitor and document. For example, California, again from the New York Times:
Although the state has not updated the surveys in the last three decades, the Department of Water Resources recently reported that across most of the state groundwater levels have dropped 50 feet below historical lows, with levels in many areas in the San Joaquin Valley more than 100 feet below previous historic lows.
In California, the state’s water agency has said that the failure to account for how groundwater withdrawals affect the state’s rivers is a major impediment to a true accounting of its resources. In April , authorities reported that less than half of the state’s local water agencies had complied with a 2002 law that made them eligible for state funds only if they set up groundwater management plans and determined if a connection between surface water and groundwater existed. That connection does not exist uniformly and varies depending on local geology. Only 17 percent of the state’s groundwater basins had been examined.
Indeed, California still doesn't require that water pumped from underground be measured at all, much less factored into an overall assessment of total water resources; it’s merely an option under a new law signed last September.
California’s new groundwater legislation does require local water authorities to come up with sustainable groundwater plans, but they don’t have to do that until 2020, and they don’t have to balance their water withdrawals until 2040.
To all intents and purposes, the only way we have to analyse these issues is through computer modelling - very clever and sophisticated, but, at the end of the day it is modelling, not analysis of real-world data. But assembling meaningful real-world data is not easy on a local scale and the time-frame of aquifer behaviour is a long one. Make your way through USGS Circular 1376, and you will find several regional-scale analyses of groundwater/streamflow interaction based on the statistical analysis of data, and the authors document in detail the challenges of field studies, stating that
Statistical studies such as these can be used in general to evaluate the large-scale effects of basinwide pumping on streamflow reductions. They cannot, however, account for the specific effects of pumping at individual wells, nor can they help with understanding how specific management actions might affect future depletion. Such analyses require the use of analytical or numerical models.
So we are largely stuck with modelling. It is, nevertheless, a powerful tool, and several examples are presented in the USGS document. Since we are particularly interested in water and arid lands, here is a very relevant - not to mention fascinating, sobering, and perhaps surprising - example. The situation, that of a hypothetical desert-basin aquifer with a through-flowing river along the east side of the basin, is summarised here (remember that 1 acre-foot of water equals 1233 cubic meters):
There is natural recharge from the west and, under natural conditions, inflow from the river to the aquifer in the north and outflow from the aquifer to the river further downstream. Two wells are drilled into the aquifer, one five miles from the river, the other ten. Each well then pumps at a rate of around 750,000 cubic meters per year, and continues to do so for 50 years when pumping then ceases. The analysis treats each well separately - they are not both pumping at the same time. So let's consider the effects of streamflow of the well closer to the river (but still five miles away). What we are looking at are plots of the resulting increase in flow from the river into the aquifer, decrease in flow into the river from the aquifer and the total changes to river flow (depletion):
Over the period of pumping, the river's flow is decreased by almost 25%, depriving downstream users of water. After pumping stops, the river's flow begins to recover - but note the timescale: the river has still not returned to its natural flow rate after 150 years. And nearly half of the total volume of depletion will occur after pumping stops. The effects of the more distant well are only slightly more modest - but they last much longer and the maximum depletion occurs several years after the well stops pumping.
Now this "hypothetical desert-basin aquifer" may remind you of somewhere - the Ranegras Valley of Arizona, perhaps? Where the Saudi conglomerate, Almarai, are growing alfalfa for export back home? Except that this hypothetical example only covers the effects of one well pumping: Almarai operate 18 wells, each one capable of pumping 4 or 5 times the lonely well in our hypothetical desert valley, and they have received permission to drill 8 more.
Now of course there is no flowing river in the Ranegras Valley, but there is Brouse Wash, ephemeral, yes (although prone to flash flooding), but clearly with water not far below the surface, given the vegetation and the occasional stock pond visible on Google Earth:
So the Brouse River probably flows just below the surface of the wash, and, if it flows at all, will not be doing so much longer as its water supplies the alfalfa.
The Brouse does, I believe, eventually supply modest volumes of water to the Colorado, and, interestingly, while I was researching and preparing this post, Matthew Miller and his colleagues at the USGS published a paper titled "The importance of base flow in sustaining surface water flow in the Upper Colorado River Basin", summarised by Science Daily. The research covered the upper Colorado basin (i.e. upstream from Lee's Ferry and the Grand Canyon) where 90% of the river's flow originates and the work documents that 60% of that flow is provided by groundwater. Subsequently, more than 80% of that groundwater is lost, by evapotranspiration and diversion for irrigation, before it reaches the lower river (where, amongst other problems, Lake Meade has recently been reported to be at its lowest level ever). The abstract for the paper puts it very succinctly:
The Colorado River has been identified as the most overallocated river in the world. Considering predicted future imbalances between water supply and demand and the growing recognition that base flow (a proxy for groundwater discharge to streams) is critical for sustaining flow in streams and rivers, there is a need to develop methods to better quantify present-day base flow across large regions....
Our results indicate that surface waters in the Colorado River Basin are dependent on base flow, and that management approaches that consider groundwater and surface water as a joint resource will be needed to effectively manage current and future water resources in the Basin.
Or, as Matthew Miller himself is quoted as saying, "In light of recent droughts, predicted climate changes and human consumption, there is an urgent need for us all to continue to think of groundwater and surface water as a single resource."
It couldn't have been put better - if you have made it through this post, thank you! And many thanks, as always, to the USGS.