WaterUnderground

# Water Underground

## What is the volume (in kegs) of groundwater is stored on earth?

Last week I gave a ‘blue drinks’  presentation for a networking evening for the Victoria chapter of the Canadian Water Resources Association entitled “How much groundwater is on earth?” based on our paper from Nature Geoscience last year. Since the night was hosted at Philips Brewery, an awesome local brewery (who makes Blue Buck, the perfect blue drink, and lots of other great beer), I decided to calculate how many kegs of groundwater we have on earth or said another way “what is the volume (in kegs) of groundwater is stored on earth?

So this blog post is a skill-testing question for all the nerds out there – answer below in the comments knowing:
a keg is 58.7 liters = 5.87e-11 km3 so there are 1.7 e+10 kegs in a km3.

Hint it is more than 1.7 e+10 kegs…. and one person during the evening got it almost correct.

## Research mini-conference in fourth year groundwater class

Fourth year and graduate students led a fun mini-conference during class in Groundwater Hydrology (CIVE 445, Civil Engineering at University of Victoria) yesterday. Local consulting and government hydrogeologists joined, making the students both nervous and excited to be presenting to professionals with up to forty years of groundwater experience. The presentations were the culmination of a term-long independent group research project – they also write a research paper (which is peer-reviewed by their classmates). And the mini-conference culminated in beers at the grad club, unfortunately drinking beer brewed with surface water.

It seemed like a win-win-win for everyone. The students loved meeting and presenting to, and being grilled by, the people who had mapped the aquifer they were modeling or asked if their model is based on any real data. The practitioners loved seeing the new ideas and enthusiasm of the students. And I loved seeing the interaction and learning.

For any prof reading this, here is a description of the Group Research Project and the conference poster:

## Happy world water day from water underground!

We hope you have a great world water day! May you sip a cool, refreshing glass of renewable groundwater and feel excited to make the world a better, more sustainable place.

## WTF of the WTF method

by Tara Forstner, University of Victoria

I recently wrote a term paper for one of my graduate classes on the limitations of the water table fluctuation (WTF) method, and I have to say, WTF!

Techniques using groundwater level fluctuations as a means of calculating recharge are very common. With observation well hydrographs and precipitation data, this method can be applied quite simply, requiring no field work or data collection. Although, this is definitely not the method to end all recharge methods for a number of reasons. As a newbie hydrogeologist studying the WTF method, the application of the method quickly became convoluted based on its limitations and uncertainties.

My term paper focused mainly on the WTF method as described by the classic papers by Healy and Cook (2001), and Cuthbert’s novel estimation of drainage (Cuthbert, 2010) and straight line recession (Cuthbert, 2014).  Here is a list of the three most important things I learned:

• Developing a good conceptual model of the region is essential for the success of this method, as large uncertainties entail if effects of pumping, proximity to surface water bodies, water table depths, and geology are not considered. With the water table fluctuating based on several factors, it becomes essential to investigate possible influences.
• The WTF method has two main approaches; (a) to solve for a time series model of recharge, or alternatively, (b) to calculate a long term average recharge value from the groundwater recession constant. The time series approach is best used to observe fluctuations of recharge in response to precipitation over a smaller temporal scale compared to the long term average recharge value calculated from the groundwater recession constant.
• Simply ‘plugging in’ the values or using computer programs to estimate drainage recession constant could seriously warp the ‘real’ recharge value. Mark Cuthbert mentioned to me in a discussion that he still prints off the hydrographs and often plots the groundwater recession by hand in order to help visualize the groundwater recession before taking a computing approach.

In closing I thought I would share one of my silly ‘WTF!?’ moments and that ‘oooooohhh’ moment that follows once I figured it out. In Healy and Cook (2002), the formula for recharge is written as R = Sy dh/dt, and later in Crosbie (2005) as R = Dh Sy and Cuthbert (2010) as R = Sy dh/dt + D. There are two things that tripped me up with this method. Firstly, the meaning of the symbols R and Dh varies slightly between papers which is easy to miss, and recharge is either calculated as a rate or a value over a specified time. Secondly, the approach in deriving the groundwater recession constant is also different in all three papers, and should be chosen on the basis of the conceptual model.

So alas, the WTF can definitely have it’s ‘WTF!?’ moments, however when the method, possibilities, and limitations are properly understood, this method has the potential of providing a cost effective and non-invasive approach in deriving recharge values.

## Deep challenges: China’s ‘war on water pollution’ must tackle deep groundwater pollution pathways

by Matthew Currell, School of Engineering, RMIT University, Australia

As part of its recent ‘war on pollution’, the Chinese Central Government released a major policy on water pollution control and clean-up, called the ‘10-point water plan’ in 2015. The plan aims to deal once and for all with China’s chronic water quality problems. China’s water quality deficiencies became widely recognised around the turn of the millennium, following publication of seminal works by Ma Jun, Elizabeth Economy and other local and overseas environmental campaigners. It is now widely acknowledged that chronic exposure to water pollution in China has contributed to the emergence of hundreds of cancer villages, where rates of particular types of cancer that are linked to water pollution far exceed normal population-wide averages. In addition to agricultural pollution and domestic wastewater, in many regions the pollution has resulted from industries that are part of multi-national supply chains, meaning international factors have played an important role.

In a recent review paper published in Environmental Pollution, my colleague Dongmei Han and I compiled data from official Chinese government reports to provide a snapshot of the current status of water quality in China’s major river basins, coastal waters and groundwater systems, including shallow unconfined and deeper confined aquifers (Figure 1). The results are sobering, showing that despite some recent progress, about a third of China’s river monitoring stations and more than 60% of sampled groundwater wells are seriously polluted. These data agree with an internal Ministry of Water Resources report that was briefly made public in early 2016, which showed that more than 80% of the more than 2000 monitored shallow groundwater wells in northern China’s plains areas contain serious pollution and that the aquifers they monitor are unfit to supply drinking water.

Figure 1 – Status of water quality in China based on recent government statistics. a) Surface water, ranked according to the 6-class water quality classification standard. b) Groundwater, ranked using the 5-class system in 6 sub-areas of China, including shallow and deep groundwater. Overall percentages of sampled stations/wells in each water quality class are shown as large pie-charts; percentages in yellow and red on small pie-charts indicate proportion of samples in the lowest two classes (IV & V) for shallow and deep groundwater, respectively. Both maps have been overlain with the locations of known ‘cancer villages’.

In addition to the government data, we also targeted the research literature and compiled as many datasets as possible reporting concentrations of nitrate in shallow and/or deep aquifers throughout China. Compiling these data from over 70 different sources provides greater local detail about the severity of groundwater pollution (Figure 2). We chose nitrate as an ‘indicator pollutant’ because it is widely measured, easy to detect and highly water-soluble. The presence of nitrate in a sample is often an indicator that other pollutants may also be there. The results indicate that all shallow aquifers sampled contain nitrate above the typical natural background level (approximately 1 mg/L nitrate-N or 4.5 mg/L nitrate as NO3 ion), indicating some degree of pollution. Of these 36 aquifers, samples from 25 contained nitrate concentrations exceeding the US EPA maximum contaminant level (MCL) of 10 mg/L nitrate-N. Worryingly, all but one of 37 deep or karst aquifers examined contained nitrate above the background level, while 10 of these aquifers had samples above the MCL. In five of the shallow aquifers and four of the deep aquifers, median nitrate concentrations also exceeded the MCL, meaning half of all wells in the aquifer pump groundwater with nitrate levels exceeding the maximum safe level. We also compiled groundwater stable nitrogen isotope values of the nitrate where they were available. These isotope data help to identify the major sources of nitrate pollution such as chemical fertilizers, soil nitrogen, manure and domestic wastewater, as each potential source can have a unique isotope ‘signature’. Nitrogen isotopes can also provide evidence of microbes breaking down pollution; this is important when considering whether the nitrate will naturally degrade, or if engineered clean-up strategies are required.

Figure 2 – Nitrate concentrations in groundwater from major groundwater systems in China: a) Location map of the 52 study areas from which data were compiled; b) & c) Boxplot distributions of nitrate concentrations (as N) in shallow and deep groundwater throughout China. Boxplots show median, inter-quartile range and 10th and 90th percentile values. Data is compared to the United States Environmental Protection Agency maximum contaminant level (10 mg/L) and a background concentration of 1 mg/L Nitrate-N (equivalent to approximately 4.5 mg/L nitrate as NO3- ion).

Perhaps the issue of greatest concern from our review was the observation that in addition to being ubiquitous in shallow groundwater (as is perhaps expected in areas of intensive agriculture or wastewater pollution), nitrate pollution also frequently appears in deep wells (drilled to >100m below the surface) throughout China. Normally, the time taken for water to reach these confined aquifers is long, and much of the deep groundwater in China has been dated using radio-isotopes, which indicate that it was recharged thousands or tens of thousands of years before the present. The presence of nitrate above natural background levels in these groundwater bodies suggests that pollution is undergoing rapid ‘bypass flow’ (e.g. taking short-cuts) from the surface into deep aquifers.

The Chinese Ministry of Water Resources has made public statements indicating it believes that China’s deep aquifers are safe drinking water sources, isolated from surface pollution effects due to natural geological barriers (called ‘aquitards’ by hydrogeologists). However, our data call into question this assumption. A similar finding was recently made by a group at the Chinese Academy of Sciences, who conducted a geochemical survey of tap water from various sites around Beijing. Most of Beijing’s water supply plants pump from deep groundwater wells around the city. The survey found that a significant number of samples contained nitrate and other pollutants, consistent with our findings that contamination is reaching deep aquifers through short-cut pathways. The most likely explanation is that polluted water is flowing from shallow depths down preferential conduits, such as poorly constructed or badly maintained wells, and bypassing natural geological barriers (Figure 3).

It is estimated that over 4 million wells have been drilled in China’s northern plains alone since the groundwater boom of the 1960s and 1970s. However, only a fraction of these are registered with the government or maintained. Clearly, a program to identify and plug leaking and abandoned wells is needed to stop further pollution of China’s precious deep groundwater reserves.

Figure 3 – Mechanism by which faulty wells can allow shallow contaminants to bypass into deep aquifers, compromising water supply safety. China has millions of unregistered wells that may act in this way, and depends on deep aquifers for much of its drinking water.

We hope that our research highlights the scale of China’s water pollution challenges, and can help the public and policy makers better understand the extent and mechanisms of groundwater pollution – a problem which is causing serious human health effects. While addressing the problem of pollution in deep aquifers will be difficult, it is too important a task to ignore, as these aquifers supply drinking water to millions of Chinese people.

Currell, M.J., Han, D., Chen, Z., Cartwright, I. (2012). Sustainability of groundwater usage in northern China: dependence on palaeowaters and effects on water quality, quantity and ecosystem health. Hydrological Processes 26: 4050-4066.

Currell, M.J., Han, D. (2017). The Global Drain: Why China’s water pollution problems should matter to the rest of the world. Environment: Science and Policy for Sustainable Development 59: 16-29. http://dx.doi.org/10.1080/00139157.2017.1252605

Han, D., Currell, M.J., Cao, G. (2016). Deep challenges for China’s war on water pollution. Environmental Pollution 218: 1222-1233. http://www.sciencedirect.com/science/article/pii/S0269749116310363

Peters, M., Guo Q., Strauss, H., Zhu, G. Geochemical and multiple stable isotope (N, O, S) investigation on tap and bottled water from Beijing, China. Journal of Geochemical Exploration 157: 36-51. http://www.sciencedirect.com/science/article/pii/S0375674215300030

## What is a hydrogeologist?

Hydrogeologists are a diverse group, in part because we come to this discipline from so many different paths.  We come from different academic programs in engineering, geological sciences and environmental sciences.  These differences in backgrounds create a diversity of perspectives, which enriches hydrogeology and allows for dynamic collaborations.  Engineers and geophysicists are known for bringing quantitative skills to hydrogeology, while geologists shine in problems involving stratigraphy, structural geology and embrace uncertainty.  Geochemists and environmental scientists are often stronger in contaminant hydrogeology.  However, each of these backgrounds also have their deficiencies.  This is underscored by looking at programs in civil engineering and geology, which are two of the most common undergraduate degrees among hydrogeologists. Aside from foundational math and science courses the first years of these programs, they usually only share an elective course in hydrogeology.  A review of hydrogeology courses covered by Gleeson et al. (2012)  showed that aside from a few topics, these courses vary substantially in their content.

Hydrogeologists are often found crossing streams wearing ghost-buster backpacks (or so it seems from here)

This is further complicated by how professionals are licensed in many jurisdictions, which is often based on these academic programs rather than whether someone has the capacity to practice hydrogeology.  Engineers are required to have engineering fundamentals in areas such as statics, dynamics, and engineering design, along with competency is areas such as structural and transportation engineering for civil engineering. Geologists receive professional registration based on core competencies in subjects such as mineralogy, sedimentology, paleontology and structural geology.  Registration for fields more closely aligned with hydrogeology, such as environmental geoscience and geological engineering may consider hydrogeology as a core requirement.  In general, this means that somebody registered as a professional engineer or geoscientist might be a hydrogeologist but they also may have very little knowledge of hydrogeology.  Environmental scientists and similar fields might be better prepared to practice hydrogeology in some instances but professional registration is not as common.

Maybe this involves graduate school?  Many practicing hydrogeologists have advanced degrees.  These programs are often designed to give a broad base in hydrogeology and typically deliver material in:

• physical hydrogeology
• chemical/contaminant hydrogeology
• geochemistry
• numerical modeling
• field techniques

Additional material on porous media, geotechnical engineering and hydrology are frequently also covered.  Anyone with a background in these areas is probably a hydrogeologist.  However, there are still some grey areas.  Can someone who doesn’t understand numerical models be a hydrogeologist? What about someone who has never done field work?  Where to draw the line is unclear and may differ substantially based on who is asking the question.  However, if the goal is to promote competent practitioners and researchers in hydrogeology, the traditional paths through engineering and geoscience may be less than ideal.  The requirement of knowledge outside hydrogeology at the expense of core knowledge may be holding us back. On the other hand, a great number of us did not enter university with the goal of becoming a hydrogeologist and maybe we need these more traditional programs as gateways.

What most hydrogeologists working really looks like (from here)

## Groundwater and Agriculture: Tapping the Hidden Benefits

By: Sam Zipper, Postdoctoral Fellow, McGill University/University of Victoria

When people think of groundwater in agricultural landscapes, pumping and irrigation are usually the first thing that comes to mind. However, groundwater can have a more subtle but extremely important impact on crop production when we decide to leave it underground:

When there is shallow groundwater beneath an agricultural field, some of the water creeps upwards from the water table, which increases the soil water available in the root zone of crops. This helps areas with shallow groundwater perform better during drought than areas where the water table is deeper, and is known as a ‘groundwater yield subsidy’; however, if soils get too wet, crop roots can’t breathe, which leads to a ‘groundwater yield penalty’:

Figure 1. Diagram showing how shallow groundwater can help or hurt a crop, and how differences in soil texture or weather conditions impact that relationship. Source: Zipper et al. (2015) Water Resources Research

In a recent study, we found that this effect was largest in coarser grained soils, which drain water much more quickly, but the water table had to be very shallow to have a positive effect. Furthermore, as described in the video, the groundwater yield subsidies during dry years were big enough to outweigh the groundwater yield penalties in wet years at the fields in south-central Wisconsin that we were studying. This means that agricultural drainage systems (such as tile drains) which are designed to lower the water table might inadvertently be making crops more vulnerable to drought, even as they improve performance during wet years.

From a broader perspective, this signals that groundwater recharge – which is conventionally thought of as beneficial from a water supply perspective for replenishing depleted aquifers – is not always a good thing:

Figure 2. Impact of groundwater on different ecosystem services. Source: Booth et al. (2016) Ecosystem Services.

Depending on the goal, groundwater can provide an ecosystem service, ecosystem disservice, or both at different times of the year! From a hydrogeology perspective, this means it is important to understand not just how much groundwater recharge is occurring, but how the entire hydrological cycle interacts with ecosystems and the benefits societies derive from them.

Additional videos associated with the project are available on YouTube. These videos were produced by the University of Wisconsin-Madison Water Sustainability and Climate project.

References

Booth EG, SC Zipper, CJ Kucharik, SP Loheide II (2016). Is groundwater recharge always serving us well? Water supply provisioning, crop production, and flood attenuation in conflict in the Yahara River Watershed, Wisconsin, USA. Ecosystem Services, 21, Part A:153-165. DOI: 10.1016/j.ecoser.2016.08.007

Zipper SC, ME Soylu, EG Booth, SP Loheide II (2015). Untangling the effects of shallow groundwater and soil texture as drivers of subfield-scale yield variability. Water Resources Research 51(8): 6338-6358. DOI: 10.1002/2015WR017522.

## How did our planet get its water?

Post by WaterUnderground contributors Elco Luijendijk and Stefan Peters from  the University of Göttingen, in Germany.

After my first ever scientific presentation, someone in the audience asked a question that caught me off guard: “Where does the groundwater come from?”.  “Ehm, from rainfall”, I answered. The answer seemed obvious at the time. However, we did not realize at the time that this is actually a profound question in hydrogeology, and one that is rarely addressed in hydrology textbooks and courses: “How did our planet get its water?”. To find out how far science has come to answering this question I (EL) joined up with a geochemist and meteorite expert (SP) to write this blog post.

We are lucky to live on a planet of which ~71% of the surface is covered with water, located mostly in rivers, lakes, glaciers and oceans at the surface and as groundwater in the shallow subsurface. Liquid water sustains life on our planet and seems to play a critical role in plate tectonics. And incidentally, it also to gives hydrogeologists something to study. Liquid water is so important in sustaining life, that the search for life on other planets in our solar system or beyond always focuses first on finding planets with liquid water.

Not only do we have abundant liquid water, we seem to have just the right amount. Compared to our direct planetary neighbors, Mars and Venus, we are extremely lucky. On the surface of Mars, at present, water mainly occurs as ice, whereas tiny amounts of water vapor are present in the Martian atmosphere. Venus also has minute amounts of water vapor in the atmosphere, but its blazingly hot surface is entirely devoid of water. In contrast to Mars and Venus, some objects in the solar system that are further away actually have too much water. Take for instance Enceladus, a moon of the planet Saturn, at which an icy crust overlies a 10 km deep water ocean. The amount of water on Enceladus is so large that it causes a wobble in the rotation of this moon, which is one of the reasons why this large volume of water was discovered in the first place. Clearly Enceladus is great for ice-skating, but probably not for sustaining land-based life similar to humans.

Figure 1: From left to right, Venus, Earth and Mars. Which one would you like to live on? Source: ESA (link) .

So how did we on Earth get so lucky?

It turns out that this depends on which scientist you ask. There are two theories:
Theory 1: The major building blocks of the Earth contained water from the start. This water then accumulated at the surface of our planet (by “degassing” from the mantle) and formed the oceans and the hydrosphere.
Theory 2: The major building blocks of the Earth were bone dry, and most of the water was delivered by comets and water-rich asteroids some time after most of our planet’s mass had formed by accretion.
So far, scientists do not have reasons to discard either of these theories, but there are two important arguments in favor of water being delivered after most of the planet had already formed:

Earth formed in a hot region of the solar system from which molecules with “low” condensation temperatures such as water had largely been removed before planetary accretion started (Albarède, 2009). Secondly, the ratio of heavy to light water in Earth’s oceans is similar to that of water in some comets and asteroids (Hartogh et al., 2011). Although you may not have noticed this when you last opened your water tap, a very small fraction (0.016 %) of the water on our planet is heavy, because it contains an extra neutron. The similarity in heavy water composition between asteroids and comets and Earth’s oceans does not prove that water on Earth was delivered by comets, but it certainly is consistent with this scenario. To make matters more complicated, however, the recent European space agency mission Rosetta to the water-rich comet 67P/Churyumov–Gerasimenko found that it has a very different ratio of heavy to light water than our oceans, which certainly complicates the debate.

Figure 2 Comet 67P/Churyumov-Gerasimenko losing water (and dust) as it gets closer to the sun. Source: ESA

Interestingly, neither theory can directly explain why our direct planetary neighbors, Mars and Venus, are so dry compared to Earth. So is it possible that these planets once were similar to Earth, and contained more water in their early days than that they do now?

Due to the high surface temperatures at Venus, any liquid water near the surface would immediately evaporate and diffuse into the atmosphere of the planet as a gas. We know that due to the lack of a protective magnetic field on Venus, solar winds continuously erode the atmosphere of the planet. If Venus had abundant water in the past, such erosion by solar winds would therefore have effectively stripped water from the planet’s atmosphere. Similar to Venus, Mars also does not have a protective magnetic field, but the temperatures and pressures at the Martian surface are significantly lower than at Venus’ surface, allowing water to be present at the surface as ice. In fact, Mars may have had a denser atmosphere in the past that allowed liquid water to be present at the surface. Nowadays, erosional features such as channels are the dry witnesses that water indeed once occurred as a liquid on the surface of the planet.

Figure 3. Dry channels (in inverted relief) in the Eberswalde delta on Mars as seen by NASA’s Mars Global Surveyor (link)

As a summary, we have an idea on why our planet was lucky enough to keep large amounts of water compared to Venus and Mars. However, do we know how our planet got its water in the first place? Unfortunately we are still not sure. There is hope though: we keep getting closer to the answer thanks to recent research on the composition of water on our planet and comets and asteroids in the solar system. So stay tuned, there’s a good chance that science will be able to answer this question in the coming years…

References
Hartogh, P. et al. (2011), Ocean-like water in the Jupiter-family comet 103P/Hartley 2, Nature, 478(7368), 218–220.
Albarède, F. (2009). Volatile accretion history of the terrestrial planets and dynamic implications. Nature, 461(7268), 1227-1233.

## Crop kites

Post by WaterUnderground contributor Mikhail Smilovic. Mikhail is a PhD  candidate in the Department of Civil Engineering at McGill University, in Quebec.

Crops use water for photosynthesis, absorbing nutrients, and transpiration, or the plant-equivalent of sweating. A crop may experience water-stress if the soil surrounding the roots is not adequately wet, and this stress will affect the crop differently depending on the crop’s stage of growth. Irrigation is the watering of plants to ultimately avoid such water-stress.

Non-irrigated crops are more vulnerable to intervals of dry and hot weather, and the increasing unpredictability of a changing climate will further complicate other crop management tools, such as choosing different cultivars (the particular variety of crop, some which may deal with certain stresses in an improved way) or changing planting dates.

Irrigated crops do not experience water stress (they may in fact experience water stress under a non-perfect irrigation system, but forgive this for now), but the water is necessarily derived from somewhere else. This somewhere else may also experience water withdrawals from municipalities, industry, and other agriculture. The source of water may be underground, or water from a river, lake, or spring, but a connection between both underground and surface waters shares with us that water removed from a system somewhere will have a response somewhere. This somewhere may very well be an ecosystem. Irrigation may also be costly related to the abstraction, transportation, and on-farm distribution.

Between non-irrigated and irrigated is a curious place where we can increase the resiliency of our agricultural systems to periods of drought and heat with limited irrigation, while allowing crops to experience well-timed water stress. Agricultural productivity or yield is determined as the amount of crop produced per area of land, say 3 tons/hectare for wheat. When water is a limiting factor, we would be sensible to also consider water productivity, that is the ratio of crop yield and water use, or, the amount of crop produced per drop of water. The practices of limited irrigation, also known as supplemental or deficit irrigation, makes an effort to increase this water productivity.

This space in-between non-irrigated and irrigated, however, has been often poorly explored or simplified. Crop kites is a novel tool to determine and quantify the potential agricultural and water productivity associated with different irrigation practices. This is important for regions interested in shifting investments into or away from irrigation, as well as for researchers interested in evaluating limited irrigation practices as initiatives to establish food and water security, both currently and with changing climates.

A first thought might be, if a crop uses three quarters of the water than it would under ideal conditions, does the crop produce three quarters as much as the crop under ideal conditions? In fact, the answer depends very much on when this water is used.

Let us take the example of winter wheat in northern Africa. Winter wheat can be broadly characterized into five different growth stages. We can illustrate water use throughout the season with the following figure:

Water use is represented by the bottom blue colour, and the associated deficit is represented with the upper orange colour – the top line of the shape is the amount of water the crop would use under ideal conditions on the associated day. This example shows a 0, 10, 20, 30, and 40% deficit occurring in stages 1 to 5 respectively, representing a 78% water use across the entire growing season as compared to ideal conditions. Understanding both the amount of water used and when the water was used, we are able to determine the associated yield, for this example, we reach 68% of potential yield.

Now, what if we were to simulate the yield using all reasonable water uses and all reasonable distributions of the timing of this water use? The resulting shape is our crop kite, with each point associated with a water use distributed throughout the growing season in a particular way:

This shape illustrates the incredible range of yields associated with each water use; for example, 80% of potential water use relates to between ~20 and 90% of potential crop yield.

Water distributed through canals are often delivered according to a schedule, and not necessarily related to growth-stage sensitivities or actual weather. From the crop kite we can derive estimates on how the crop yield will be affected by adopting certain irrigation schedules. We elaborate on this with three examples: S1) water use is distributed to optimize yield, S2) the deficit is distributed evenly across all growth stages, S3) water is used preferentially for the earlier growth stages. The resulting crop-water production functions are illustrated in the following figure:

Although the first schedule optimizing for crop yield may be in line with the motivations of the irrigating farmer, it is often an unreasonable assumption for farmers delivered water according to predetermined schedules, but may be appropriate for farmers irrigating with a privately owned well. Evaluating the potential of supplemental irrigation necessitates estimating the ability of farmers to manage both the amount and timing of irrigation applications. Otherwise, non-reasonable assumptions may be used to evaluate and over promise estimates for agricultural production, with the fault not in the practice of limited irrigation, but in the criteria used to evaluate the system.

Crop kites demonstrate the wide range of water use-crop yield relationships, and can be used to evaluate the potential of limited irrigation to shift both food and water security.

Mikhail Smilovic is a PhD candidate at McGill University and the University of Victoria . Mikhail’s work investigates the interplay between foot security, water resources, and energy, and evaluating and integrating initiatives that increase agricultural production while reducing demands on water resources.

## Limits to global groundwater use

Post by WaterUnderground contributor Inge de Graaf. Inge is a postdoc fellow at Colorado School of Mines, in the USA.

Groundwater is the world’s most important source of freshwater. It supplies 2 billion people with drinking water and is used for irrigation of the largest share of the world’s food supply.

However, in many regions around the world, groundwater reserves are depleting as the resource is being pumped faster than it is being renewed by rain infiltrating through the soil. Additionally, in many cases, we are still clueless about how long we can keep drawing down these water reserves before groundwater depletion will have devastating impacts on environmental and socio-economic systems. Indeed, these devastating effects are already being observed.

The most direct effect of groundwater depletion is the decline in groundwater levels. As a direct impact, groundwater-pumping cost will increase, so too will the cost of well replacement and the cost of deepening wells. One of the indirect consequences of declining water levels is land subsidence, which is the gradual sinking of the surface. In many coastal and delta cities, increased flooding results in damages totaling billions of dollars per year. Next to this, declining groundwater levels lead to a decrease in groundwater discharge to rivers, wetlands, and lakes, resulting in rivers running dry, wetlands that are no longer sustained, and groundwater-dependent ecosystems that are harmed.

Over the past decades, global groundwater demands have more than doubled. These demands will continue to increase due to population growth and climate change.

The increase in demands and the aforementioned negative effects of groundwater depletion raise the urgent question: at what time in future are the limits to global groundwater use reached? This is when and where groundwater levels drop to a level where groundwater becomes unattainable for abstraction, or that groundwater baseflows no longer sustain river discharges.

In my PhD research, I predicted where and when we will reach these limits of groundwater consumption worldwide. I defended my dissertation last year April at Utrecht University, in the Netherlands.

Where and when are the limits reached?

Results show that many large aquifer systems are already highly depleted, especially for intensively irrigated areas in dryer regions of the world, like India, Pakistan, Mid West USA, and Mexico (see Figure 1). New areas experiencing groundwater depletion will develop in the near future, such as Eastern Europe and Africa. Future predictions show that some areas, like the Central Valley, and the High Plains Aquifer, partly recover when more recharge will becomes available. Notwithstanding, environmental groundwater demands will increase as to buffer more irregular streamflow occurrences due to climate change.

Figure 1: Estimated groundwater depletion (1960-2010) in [m], masked for aquifer areas, and zooms for hotspot regions, which are the intensively irrigated regions of the world.

In 2010, about 20% if the world population lived in groundwater depleted regions, where groundwater dropped below the economical exploitable limit. As a rule of thumb: the economic limit is reached when groundwater becomes unattainable for a local farmer, which is approximately when the water level drops to 100 m below the surface. In 2050, 26% to 36% of the world’s population will live in areas where the economic exploitable limit is reached (see Figure 2). Evidently, this persistence and increasing level of groundwater stress will impair local development and generate tension within the global socio-economic system.

Figure 2: First time that groundwater falls below the 100m limit.

Global-scale simulations

To answer my main question, I studied the effects of groundwater abstractions on river low flows and groundwater levels worldwide, as well as which trends in river low flow frequency and groundwater level change can be attributed to groundwater abstractions.

I used a newly developed physically based surface water-groundwater model to simulate i.a. river flows, lateral groundwater flow, and groundwater-surface water interactions at a high resolution (approx. 10×10 km) at the global scale. Total water demands were estimated and account for agricultural, industrial, and domestic demands. I simulated groundwater and surface water abstractions based on the availability of the resource, making the estimate reliable for future projections under climate change and for data-poor regions where we do not know how much groundwater or surface water is abstracted. Next, I developed a global-scale groundwater model. I estimated alluvial aquifer thickness worldwide, as no data at the global scale is available (see Figure 3). Aquifer thickness is one of the parameters you need to estimate groundwater flow and storage.

Figure 3: Estimated alluvial aquifer thickness. White areas are mountain regions, where no aquifers are simulated.

Simulations were done for the recent past and near future (1960-2050) and the results include maps and trends of groundwater heads, groundwater fluctuations, and river discharges.

In conclusion, most of our water reserves are hidden underground and most of our groundwater abstractions rates exceed groundwater renewing rates, leading to depletion. The growing demand and the expected climate change bring our groundwater reserves under mounting pressure. More than two-thirds of all abstracted groundwater is used for food production. Every year the world’s population is growing by 83 million people.

Improving our knowledge about how much water we can use in the near future while avoiding negative environmental and socio-economic impacts is therefore extremely important. A study like this contributes to the knowledge gap and can help guide towards sustainable water use worldwide to overcome potential political water conflicts and reduce potential socio-economic friction, as well as to secure future food production.

Want to read more? Check out the recent AGU press release or if you have more time… read my papers on dynamic water allocation (click here), development of a global groundwater model (click here, or here), or read my PhD thesis (here).

Author Inge de Graaf receiving her PhD degree from her advisor, professor Marc Bierkens (at Utrecht University, Netherlands). Note Tom Gleeson’s bald head in the lower left…