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Groundwater organic matter: carbon source or sink?

Groundwater organic matter: carbon source or sink?

Post by Andy Baker, Professor researching groundwater, caves, past climate, organic carbon and more at the University of New South Wales, in Australia.

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We know a lot about the carbon cycle, right? Increased carbon dioxide emissions since the Industrial Revolution have perturbed the carbon cycle. This has led to rising atmospheric carbon dioxide levels and climate change.

Not all this extra carbon accumulates in the atmosphere as carbon dioxide. Carbon sequestration is also occurring, for example in the oceans and terrestrial biosphere. All the carbon fluxes and stores on the planet must balance. In recent years there has been a hunt within the terrestrial system to quantify some missing carbon, such as the particulate organic carbon in river systems and dissolved organic carbon in glaciers.

So, what about groundwater? Could this be a previously unrecognised source or sink of carbon? We already know that the global volume of groundwater of 1.05 x 1019 litres is the world’s biggest source of freshwater. But groundwater natural organic carbon concentrations are low: typically, 1 part per million (ppm). This means that the global groundwater organic carbon store is just 10.5 x1015 g. For comparison, rivers are estimated to sequester this amount in just four years. Basically, there’s no significant store of organic carbon in groundwater.

But hold, on, this raises another puzzle, which is: where has all the organic carbon gone? Groundwater is recharged from rivers and from rainfall. Rivers have much more dissolved organic carbon than the 1 ppm found in groundwater. And the recharge from rainfall passes through the soil. And soil leachates also have much higher dissolved organic carbon concentrations than groundwater. So, despite the high concentrations of organic matter in the soil and rivers, most of this organic matter is ‘lost’ before reaching the groundwater. Is it biologically processed (and therefore a potential source of carbon dioxide)? Or is it sorbed to mineral surfaces (and therefore a potential sink of carbon)?  Most likely, both processes occur in competition.

Groundwater organic matter: a carbon source or sink? We don’t know. But a few groups are working on the puzzle. For example, our group at UNSW Sydney is collecting groundwater samples and measuring organic carbon sorption to minerals, and microbial use. In the USA, groundwater data has been mined to understand the rate of loss of organic carbon in groundwater. This December, river and groundwater experts come together at the AGU Fall Meeting to share our understanding. Not least because surface and groundwater are interconnected systems.

Collecting groundwater samples to understand whether organic matter is a carbon source or sink. Long field days at the UNSW Wellington Research Station mean the final sample is often collected at dusk.

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Andy Baker is the Director of Research and UNSW’s School of Biological, Earth and Environmental Sciences. His research interests include hydrology, hydrogeology, cave and karst research, paleoclimatology, and isotope and organic and inorganic geochemistry. You can find out more information about Andy at any of the links below:

Research profile | Twitter | Facebook

Western water wells are going dry

Western water wells are going dry

Post by Scott Jasechko, Assistant Professor of Water Resources at the University of Calgary, in Canada, and by Debra PerronePostdoctoral Research Scholar at Stanford University, in the United States of America.

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Wells are excavated structures, dug, drilled or driven into the ground to access groundwater for drinking, cleaning, irrigating, and cooling. We recently mapped groundwater wells across the 17 western states [1], where half of US groundwater pumping takes place. The western states contain aquifers key to United States food production, including the Central Valley of California and the central High Plains.

Millions of water wells exist in the western US, alone. About three-quarters of these wells have been constructed to supply water for household uses. Nearly one-quarter are used to irrigate crops or support livestock. A smaller fraction (<5 %) supports industry [1].

Western US water well depths vary widely (Fig. 1). The great majority (90%) of western US well depths range between 12m and 186m. The median western US well depth is 55m. Wells with depths exceeding 200m tap deep aquifers bearing fresh groundwater, such as the basal formations in the Denver Basin aquifer system, and the deeper alluvium in the California Central Valley. Shallow wells are common along perennial rivers, such as the Yellowstone, Platte, and Willamette Rivers.

Fig. 1. Western USA wells depths. Each point represents the location of a domestic, industrial or agricultural well. Blue colors indicate well depths of less than the median (55m), and red-black colors indicate well depths exceeding the median.

The wide variability of well depths across the west (Fig. 1) emphasizes the value of incorporating well depth data when assessing the likelihood that a groundwater well may go dry.

We know wells are going dry in the western US: journalists have identified numerous communities whose well-water supplies have been impacted by declining water tables [2-4]. While several studies have assessed adverse impacts of groundwater storage declines—such as streamflow depletion [5], coastal aquifer salinization [6], eustatic sea level rise [7], land subsidence [8]—few studies address the question: where have wells have gone dry?

Here we put forth a first estimate of the number of western US wells that have dried up (Fig. 2). We compared well depths to nearby well water level measurements made in recent years (2013-2015). We define wells that have likely gone dry as those with depths shallower than nearby measured well water levels (i.e., our estimate of the depth to groundwater).

Fig. 2. Schematic of a well that has gone dry (left) and a well with a bottom beneath the water table (blue) that may still produce groundwater (right). Even wells with submerged bottoms may be impacted by declines in groundwater storage because (i) pumps are situated above the well bottom, (ii) pumping induces a localized drawdown of the water table in unconfined portions of aquifer systems, (iii) well yields may decline if the hydrostatic pressure above the well base declines.

We estimate that between 0.5% and 6 % of western US wells have gone dry [1]. Dry wells are common in some areas where groundwater storage has declined, such as the California Central Valley [9] and parts of the central and southern High Plains aquifer [10,11]. We also identify lesser-studied regions where dry wells are abundant, such as regions surrounding the towns of Moriarty and Portales in central and eastern New Mexico.

Dry wells threaten the convenience of western US drinking water supplies and irrigated agriculture. Our findings emphasize that dry wells constitute yet another adverse impact of groundwater storage losses, in addition to streamflow depletion [5], seawater intrusion [6], sea level rise [7], and land subsidence [8].

Some wells are more resilient to drying (i.e., deeper) and others more vulnerable (i.e., shallower). We show that typical agricultural wells are deeper than typical domestic water wells in California’s Central Valley and Kansas’ west-central High Plains [1]. Our finding implies that reductions to groundwater storage will disproportionately dry domestic water wells compared to agricultural water wells, because domestic wells tend to be shallower in these areas. However, in other areas, such as the Denver Basin, typical domestic wells are deeper than typical agricultural wells. This comparison of different groundwater users’ well depths may help to identify water wells most vulnerable to groundwater depletion, should it occur.

So, what option does one have when a well goes dry?

Groundwater users whose wells have gone dry may consider a number of potential, short-term remedies, some of which may include (i) drilling a new well or deepening an existing well, (ii) connecting to alternative water sources (e.g., water conveyed by centralized infrastructure; water flowing in nearby streams), or (iii) receiving water delivered by truck.

Drilling new wells, deepening existing wells or connecting to alternate water supplies is often costly or unavailable, raising issues of inequality [12]. Receiving water deliveries via truck [13] is but a stopgap, one that may exist in parts of the western United States but not elsewhere, especially if high-use activities (e.g., irrigated agriculture) are intended [14]. In places where water table declines are caused primarily by unsustainable groundwater use, a long-term solution to drying wells may be managing groundwater to stabilize storage or create storage surpluses.

Realizing such sustainable groundwater futures where wells are drying up is a critical challenge. Doing so will be key to meeting household water needs and conserving irrigated agriculture practices for future generations [15]. We conclude that groundwater wells are going dry, highlighting that declining groundwater resources are impacting the usefulness of existing groundwater infrastructure (i.e., wells). The drying of groundwater wells could be considered more frequently when measuring the impacts of groundwater storage declines.

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Scott Jasechko is an assistant professor of water resources at the University of Calgary. In November 2017, Scott joins the faculty of the Bren School of Environmental Science & Management at the University of California, Santa Barbara.

Find out more about Scott’s research at : http://www.isohydro.ca

 

 

 

Debra Perrone is a postdoctoral research scholar at Stanford University with a duel appointment in the Department of Civil and Environmental Engineering and the Woods Institute for the Environment. In November 2017, Debra will join the Environmental Studies Program at the University of California, Santa Barbara as an assistant professor.

Find out more about Debra at: http://debraperrone.weebly.com

 

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References

[1] Perrone D and Jasechko S 2017 Dry groundwater wells in the western United States. Environmental Research Letters 12, 104002 doi: 10.1088/1748-9326/aa8ac0. http://iopscience.iop.org/article/10.1088/1748-9326/aa8ac0

[2] James I, Elfers S, Reilly S et al 2015 The global crisis of vanishing groundwaters. in: USA Today https://www.usatoday.com/pages/interactives/groundwater/

[3] Walton B 2015 In California’s Central Valley, Dry wells multiply in the summer heat. in: Circle of Blue http://www.circleofblue.org/2015/world/in-californias-central-valley-dry-wells-multiply-in-the-summer-heat/

[4] Fleck J 2013 When the well runs dry. in: Albuquerque Journal https://www.abqjournal.com/216274/when-the-well-runs-dry.html

[5] Barlow P M and Leake S A 2012 Streamflow depletion by wells—understanding and managing the effects of groundwater pumping on streamflow. US Geological Survey Circular 1376 (Reston, VA: United States Geological Survey)

[6] Barlow P M, Reichard E G 2010 Saltwater intrusion in coastal regions of North America. Hydrogeol. J. 18 247-260.

[7] Konikow L F 2011 Contribution of global groundwater depletion since 1900 to sea-level rise. Geophys. Res. Lett. 38 L17401

[8] Galloway D, Jones D R and Ingebritsen S E 1999 Land subsidence in the United States. US Geological Survey Circular 1182 (Reston, VA: United States Geological Survey)

[9] Famiglietti J S, Lo M, Ho S L, Bethune J, Anderson K J, Syed T H, Swenson S C, Linage C R D and Rodell M 2011 Satellites measure recent rates of groundwater depletion in California’s Central Valley. Geophys. Res. Lett. 38 L03403

[10] McGuire V L 2014 Water-level Changes and Change in Water in Storage in the High Plains Aquifer, Predevelopment to 2013 and 2011–13  (Reston, VA: United States Geological Survey)

[11] Scanlon B R, Faunt C C, Longuevergne L, Reedy R C, Alley W M, Mcguire V L and McMahon P B 2012 Groundwater depletion and sustainability of irrigation in the US High Plains and Central Valley. Proc. Natl Acad. Sci. 109 9320–5

[12] Famiglietti J S 2014 The global groundwater crisis. Nature Climate Change 4 945-948.

[13] The Times Editorial Board 2016 When it comes to water, do not keep on trucking. in: LA Times http://www.latimes.com/opinion/editorials/la-ed-water-hauling-20160729-snap-story.html

[14] James I 2015 Dry springs and dead orchards. in: Desert Sun http://www.desertsun.com/story/news/environment/2015/12/10/morocco-groundwater-depletion-africa/76788024/

[15] Bedford L 2017 Irrigation, innovation saving water in Kansas. in: agriculture.com http://www.agriculture.com/machinery/irrigation-equipment/irrigation-innovation-saving-water-in-kansas

Everything is connected

Everything is connected

Post by Anne Van Loon, Lecturer in Physical Geography (Water sciences) at the University of Birmingham, in the United Kingdom.

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In recent years the human dimension of hydrology has become increasingly important. Major flood and drought events have shown how strongly water and society are intertwined (see here and here). The hydro(geo)logical research community is increasingly including this human dimension, for example within the IAHS Panta Rhei decade (link), which focuses on the interface between environment and society and aims to “make predictions of water resources dynamics to support sustainable societal development”. Previous Water Underground blog posts have shown the importance of this topic and highlighted opportunities and methodologies for scientists to engage with socio-hydro(geo)logy and humanitarian projects. Viviana Re, for example, introduces the term socio-hydrogeology and promotes sustainable groundwater management in alliance with groundwater users (link). And Margaret Shanafield argues that humanitarian groundwater projects are “an opportunity for scientists to have an impact on the world by contributing to the collective understanding of water resources and hydrologic systems” (link).

In our interdisciplinary project CreativeDrought (link), which uses local knowledge and natural and social science methods to increase local preparedness for uncertain future drought, we are applying these ideas and we realise how important different types of connections are in our two-way learning process. We just completed our second fieldwork phase of the project that consisted of workshops in which groups of people from a rural community in South Africa experimented with potential future drought scenarios and created stories about how they would be impacted by the drought and what they could do to prepare for and adapt to it. Our scientific team consisted of hydrologists and social scientists from local and UK-based institutes and the groups in the community who participated were the village leaders, livestock farmers, irrigation farmers, young mothers, and elderly people.

Young women collecting water from communal standpipe (photo: Sally Rangecroft).

Both the scientific team and the community groups were interested to learn from each other’s knowledge and experience (or just curious, see photo below of our Zimbabwean colleague Eugine measuring irrigation canal discharge with an apple). During the time we spent in the community (four weeks in March/April and two weeks in July) we both learned about important connections. As hydrologists and hydrogeologists we know that different parts of the hydrological system are connected and that these connections are extremely important if you want to understand, predict, and manage the system. Knowledge about the connection between groundwater and surface water is what we as hydrologists could bring to the community. The community was getting their water from different sources: drinking water from a groundwater well, irrigation water from a reservoir that releases water into the river, and water for bathing, washing, brick making, and cleaning cars from the river. By showing how a drought would affect each of these water supplies and discussing amongst groups that would be affected differently by a drought, they learned about the connection between the water bodies and how abstraction in one would affect the other.

Researchers measuring discharge with help of schoolchildren and collecting stories about previous droughts and floods (photos: Anne Van Loon and Sally Rangecroft).

We scientists also learned some important connections from the community. For example, our project focuses on drought but when we asked the community to tell us about droughts they had experienced in the past, many also told us about flood events. For the community, both are water-related extreme events that often even impact them similarly, with crop loss, drinking water problems, diseases, etc. Even though floods and droughts are governed by different processes (floods by fast, mostly near-surface pathways and droughts by slower, sub-surface storage related pathways) and different tools and indices are used to characterise both extremes, people at local scale have to deal with both floods and droughts when the hydrological system goes from one into the other or when both occur simultaneously in different parts of the hydrological system. We realised that our academic world is so fragmented that we often forget about connecting floods and droughts in our scientific work. Furthermore, we forget that we may affect one hydrological extreme when trying to manage our resources for the opposite hydrological extreme.

The most important, but unintended connections we discovered, however, were the connections between people. During our stays in South Africa, we connected as hydrologists and social scientists and between the UK-based and local researchers, learning to communicate across different disciplines, languages and audiences. The project also helped the community rediscover some connections between generations (young mothers and elderly ladies) and between different sectors (livestock farmers and irrigation farmers). And finally, we as a scientific team connected with the community. As a token for our newly established connection, the children’s dance group performed traditional dances during our final visit with the chief and the village leaders (see below), only bestowed on very special guests. That is the best confirmation we could get that personal connections are important and that our water management and our science depend on them!

Everyone connected: researchers, village leaders, dancers (photo: Khathutshelo Muthala).

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Anne Van Loon is a catchment hydrologist and hydrogeologist working on drought. She studies the relationship between climate, landscape/ geology, and hydrological extremes and its variation around the world. She is especially interested in the influence of storage in groundwater, human activities, and cold conditions (snow and glaciers) on the development of drought.

Bio taken from Anne’s University of Birmingham page.

Crowdfunding Science: A personal journey toward a public campaign

Crowdfunding Science: A personal journey toward a public campaign

Post by Jared van Rooyen, MSc candidate in Earth Science at Stellenbosch University, in South Africa.

Part one of three in a Crowdfunding Science series by Jared.

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When my supervisor, Dr Jodie Miller, suggested to me last year that we should look at crowdfunding as a way to potentially to fund my master’s project, I had no idea of what I was about to get myself into. All through my honours year I was not really interested in doing further postgraduate study. She kept warning me that I might change my mind and that I should apply for funding “just in case”. But I was sure of my position.  And then, as I started the final five weeks of my honours year, I finally got to focus 100% on my research project. Suddenly, as I focused in on my data, all the possibilities started to leap out at me. I went from a BSc (Hons) student, who was not considering continuing my postgraduate studies at all, to someone who is passionate about water resource research and continuing my postgraduate career. This is apparently common amongst postgraduate students in science, who become exponentially more immersed in their field of study as they realise that their work isn’t just numbers and experiments, but has significant real world applications.

Once I had committed – there was no turning back. The learning curve for mounting a successful crowdfunding campaign is steep and slippery. As much as it is hard, stressful work it is also fulfilling, fun, and full of surprises. The biggest obstacle is one that most modern day scientists are confronted with already: How do I make my research attractive to people who don’t have years of passion invested in my work?

Well, the answer is not simple.

I have completed a wide variety of modules in my tertiary studies but none in any forms of multi-media marketing skills. So naturally, when I had this crowdfunding campaign in front of me, I was so far out of my comfort zone that I felt like a geologist at a slam poetry evening. After numerous conversations with my peers who had experiences in marketing and graphic design, I had gathered a basic understanding of the inner workings of the unfathomably enormous media machine.

From the very first day I arrived back at the University in Stellenbosch I was drowning in ideas and administration. Setting up the social media accounts alone was a mission. Little did I know that running a social media campaign takes days and even weeks of preparation and planning each public post, including the post’s time, target market, outcome goals, and context. Each post on each platform had to be vetted and boosted appropriately. I was genuinely missing the late nights combing through complicated scientific articles and pounding through textbooks.

Making the campaign video was by far the hardest but definitely the most fun part of the process. The hours and hours of footage I have of retakes and drone videos culminated in, what I believe, is the pinnacle of my creative career (which is minuscule).

About a week before the initial launch date, we ran into some red tape within the University. Naturally, as someone who has never done anything more than post a couple photos of rocks on Instagram, I had no idea that a project like this needed to go through a number of stages before being approved by the university (which included: legal, ethics, corporate, marketing, and the faculty itself). A couple of panic-ridden meetings and documents later, we were ready for lift off, although a week later than originally planned.

As a geologist, I am not afraid of hard work, so engulfing myself in learning as much as I could in the little time I had came more naturally. What was most intimidating though, was the thought of putting myself and what I am passionate about out there. Publicly declaring the fact that what I wanted to achieve was not funded was daunting at first, but in time became a revelation in self-awareness and that asking for help is more constructive than admitting defeat.

I believe that postgraduate crowdfunding may prove to be invaluable in the future of students that have all the potential but their projects remain unfunded. Not only does it allow for the financial security of your project, but it attracts people that are interested in your field to you and to your work. The most significant consequence of this crowdfunding approach is that when you graduate, you already have a network of people in the industry that know who you are and know of your potential.

The crowdfunding campaign was completed in early April of 2017. In the next blog I will talk about what worked and what didn’t work, who pledged funding and how did we reach them.

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Jared van Rooyen is an MSc student at the University of Stellenbosch in South Africa. His primary field of interest is in isotope hydrology with major applications in groundwater vulnerability and sustainability. Other research interests include postgraduate research funding solutions and outreach as well as scientific engagement with the use of modern media techniques.

 

Check out Jared’s (and research group’s) thundafund  page here.

Good groundwater management makes for good neighbors

Good groundwater management makes for good neighbors

Post by Samuel Zipper, postdoctoral fellow at both McGill University and the University of Victoria, in Canada. You can follow Sam on Twitter at @ZipperSam.

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Dedicated Water Underground readers know that this blog is not just about water science, but also some of the more cultural impacts of groundwater. Keeping in that tradition, today’s post begins with a joke*:

Knock, knock!

Who’s there?

Your neighbor

Your neighbor who?

Your neighbor’s groundwater, here to provide water for your plants!

Figure 1. Typical reaction to joke written by the author.

Ahem.

Perhaps this joke needs a little explanation. As we’ve covered before, groundwater is important not just as a supply of water for humans, rivers, and lakes, but also because it can increase the water available to plants, making ecosystems more drought resistant and productive. However, we also know that groundwater moves from place to place beneath the surface. This means that human actions which affect groundwater in one location, like increasing the amount of paved surface, might have an unexpected impact on ecosystems in nearby areas which depend on that groundwater.

Imagine, for example, two neighboring farmers. Farmer A decides retire and sells his land to a developer to put in a new, concrete-rich shopping center. Farmer B continues farming her land next door. How will the changes next door affect the groundwater beneath Farmer B’s land, and will this help or hurt crop production on her farm?

In a new study, my colleagues and I explored these questions using a series of computer simulations. We converted different percentages of a watershed from corn to concrete to see what would happen. Our results showed that the response of crops to urbanization depended on where the land use change occurred.

Figure 2. Conceptual diagram showing how urbanization might impact crop yield elsewhere in a watershed. From Zipper et al. (2017).

In upland areas where the water table was deep, replacing crops with concrete caused a reduction in groundwater recharge, lowering the water table everywhere in the watershed – not just beneath the places where urbanization occurred. This meant that places where the ecosystems used to be reliant on groundwater could no longer tap into this resources, making them more vulnerable to drought. However, places where the water table used to be too shallow saw boosts in productivity, as the lower water table was closer to the optimum water table depth.

In contrast, urbanization happening in lowland areas had a much more localized effect, with changes to the water table and yield occurring primarily only in the location where land use changed, because the changes in groundwater recharge were accounted for by increased inflows from the stream into the groundwater system.

So, what does this mean for the neighboring farmers we met earlier?

For Farmer A, it means the neighborly thing to do is work with the developers to minimize the effects of the land use change on groundwater recharge. This can include green infrastructure practices such as rain gardens or permeable pavement to try and mimic predevelopment groundwater recharge.

For Farmer B, the impacts depend on the groundwater depth beneath her farm. If the groundwater beneath her farm is shallow enough that her crops tap into that water supply, she should expect changes in the productivity of her crops, especially during dry periods, and plan accordingly.

*Joke written by scientist, rather than actual comedian.

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For More Information:

Zipper SC, ME Soylu, CJ Kucharik, SP Loheide II. Indirect groundwater-mediated effects of urbanization on agroecosystem productivity: Introducing MODFLOW-AgroIBIS (MAGI), a complete critical zone model. Ecological Modelling, 359: 201-219. DOI: 10.1016/j.ecolmodel.2017.06.002

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Sam Zipper is an ecohydrologist. His main research focuses broadly on interactions between vegetation and the water cycle, with a particular interest in unintended or indirect impacts of land use change on ecosystems resulting from altered surface and subsurface hydrological flowpaths. You can find out more about Sam by going to his webpage at: samzipper.weebly.com.

What is the difference between ‘water withdrawal’ and ‘water consumption’, and why do we need to know?

What is the difference between ‘water withdrawal’ and ‘water consumption’, and why do we need to know?

Post by Inge de Graaf, University of Freiburg, Environmental Hydrological Systems group

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Last week I had to teach my first class in global hydrology. When I showed the global trend on increasing demands and withdrawals (see Figure) I needed to explain the different terms as sometimes the term “water use” gets, well, misused.

The term “water use” often fails to adequately describe what happens to the water. So I told the students; if you see or hear to term ‘water use’ always ask yourself what’s actually being said. The term is often used for water withdrawals or water consumption, and it’s important to understand the difference.

Water withdrawal describes the total amount of water withdrawn from a surface water or groundwater source. Measurements of this withdrawn water help evaluate demands from domestic, industrial and agricultural users.

Water consumption is the portion of the withdrawn water permanently lost from its source. This water is no longer available because it evaporated, got transpired or used by plants, or was consumed by people or livestock. Irrigation is by far the largest water consumer. Globally irrigated agriculture accounts for 70% of the total water used and almost 50% is lost either by evaporation or transpiration.

Understanding both water withdrawal and consumption is critical to properly evaluate water stress. Measurements of water withdrawal indicate the level of competition and dependence on water resources. Water consumption estimates help to quantify the impact of water withdrawals on downstream availabilities and are essential to evaluate water shortage and scarcity. For example, most water used by households is not consumed and flows back as return flow and can be reused further downstream. However, water is rarely returned to watershed after being used by households or industry without changing the water quality, increasing water stress levels.

Already more than 1.4 billion people live in areas where the withdrawal of water exceeds recharge rates. In the coming decades global population is expected to increase from 7.3 billion now, to 9.7 billion by 2050 (UN estimate). This growth, along with rising incomes in developing countries, is driving up global food demands. With food production estimated to increase by at least 60% (FAO estimate), predicting water withdrawal and consumption is critically important for identifying areas that are at risk of water scarcity and where water use is unsustainable and competition amongst users exist.

Global trend I showed in my class, published in Wada et al (2016).

Ref:

Wada, Y., I. E. M. de Graaf, and L. P. H. van Beek (2016), High-resolution modeling of human and climate impacts on global water resources, J. Adv. Model. Earth Syst., 8, 735–763, doi:10.1002/2015MS000618.

 

How prehistoric water pit stops may have driven human evolution

How prehistoric water pit stops may have driven human evolution

Post by Matthew Robert Bennett, Bournemouth University and Mark O Cuthbert, Cardiff University

Our ancient ancestors seem to have survived some pretty harsh arid spells in East Africa’s Rift Valley over five million years. Quite how they kept going has long been a mystery, given the lack of water to drink. Now, new research shows that they may have been able to survive on a small networks of springs.

The study from our inter-disciplinary research team, published in Nature Communications, illustrates that groundwater springs may have been far more important as a driver of human evolution in Africa than previously thought.

Great rift valley.Redgeographics, CC BY-SA

The study focuses on water in the Rift Valley. This area – a continuous geographic trench that runs from Ethiopia to Mozambique – is also known as the “cradle of humanity”.

Here, our ancestors evolved over a period of about five million years. Throughout this time, rainfall was affected by the African monsoon, which strengthened and weakened on a 23,000-year cycle. During intense periods of aridity, monsoon rains would have been light and drinking water in short supply. So how did our ancestors survive such extremes?

Previously, scientists had assumed that the evolution and dispersal of our ancestors in the region was solely dependent on climate shifts changing patterns of vegetation (food) and water (rivers and lakes). However, the details are blurry – especially when it comes to the role of groundwater (springs).

We decided to find out just how important springs were. Our starting point was to identify springs in the region to map how groundwater distribution varies with climate. We are not talking about small, babbling springs here, but large outflows of groundwater. These are buffered against climate change as their distribution is controlled by geology – the underlying rocks can store rainwater and transfer it slowly to the springs.

The lakes of the African Rift Valley.SeaWiFS Project, NASA/Goddard Space Flight Center, and ORBIMAGE

We figured that our ancestors could have stayed close to such groundwater in dry times – playing a greater part in their survival than previously thought. When the climate got increasingly wet, groundwater levels would have risen and made springs more plentiful – feeding smaller rivers and leading to lakes becoming less saline. At this point, our ancestors would have roamed across the landscape free of concerns about water.

Life and death decisions

To test this idea, we embarked on a computer experiment. If the springs and water bodies are thought of as the rest stops, or service stations, then the linkages between can be modelled by computers. Our model was based on what decisions individuals would have taken to survive – and what collective behaviours could have emerged from thousands of such decisions.

Individuals were give a simple task: to find a new source of water within three days of travel. Three days is the time that a modern human and, by inference, our ancestors could go without drinking water. The harder and rougher the terrain, the shorter the distance one can travel in those vital three days.

We used the present landscape and existing water springs to map potential routes. The detailed location of springs may have changed over time but the principles hold. If our agent failed to find water within three days, he or she would die. In this way we could map out the migration pathways between different water sources as they varied through 23,000-year climate cycles. The map shows that there were indeed small networks of springs available even during the driest of intervals. These would have been vital for the survival or our ancestors.

The model also reveals movement patterns that are somewhat counter-intuitive. One would assume that the easiest route would be along the north to south axis of the rift valley. In this way, hominins could stay at the bottom of the valley rather than crossing the high rift walls. But the model suggests that in intermediate states between wet and dry, groups of people may have preferred to go from east to west across the rift valley. This is because springs on the rift floor and sides link to large rivers on the rift flanks. This is important as it helps explain how our ancestors spread away from the rift valley. Indeed, what we are beginning to see is a network of walking highways that develop as our ancestors moved across Africa.

Mapping human migration.

Human movement allows the flow of gossip, know-how and genes. Even in modern times, the water-cooler is often the fount of all knowledge and the start of many budding friendships. The same may have been true in ancient Africa and the patterns of mobility and their variability through a climate cycle will have had a profound impact on breeding and technology.

This suggests that population growth, genetics, implications for survival and dispersal of human life across Africa can all potentially be predicted and modelled using water as the key – helping us to uncover human history. The next step will be to compare our model of human movement with real archaeological evidence of how humans actually moved when the climate changed.

So next time you complain about not finding your favourite brand of bottled spring water in the shop, spare a thought for our ancestors who may died in their quest to find a rare, secluded spring in the arid African landscape.

The ConversationThis research was carried out in partnership with our colleagues Tom Gleeson, Sally Reynolds, Adrian Newton, Cormac McCormack and Gail Ashley.

Matthew Robert Bennett, Professor of Environmental and Geographical Sciences, Bournemouth University and Mark O Cuthbert, Research Fellow in Groundwater Science, Cardiff University

This article was originally published on The Conversation. Read the original article.

Is highway de-icing ‘a-salting’ our aquifers?

Is highway de-icing ‘a-salting’ our aquifers?

Post by Mark Cuthbert, Cardiff University, and Michael Rivett, GroundH20 plus Ltd; University of Strathclyde.

If you live in a cold climate, have you ever wondered where all the de-icing salt (or ‘grit’ as we call it in the UK) that gets spread on the roads in winter time ends up, aside from that accumulating salty grime that coats your car? As you might expect, most of the salt gets washed off the highways as the salt has the desired effect of melting the ice, or carried away by rain. This salty ‘runoff’ ends up in streams nearby via pipes which drain the highway. However, that is not the end of the story…

We studied a major highway intersection on the edge of UK’s second largest city, Birmingham (more than a million people), to see how much of the salt spread on the highway ends up in groundwater. Our interest stems from concerns of the national regulator to understand not only how much salt ends up in streams, but also the potential long-term build-up of salt in underlying groundwater resources that are pumped for public and private water supply. Although the origin of salt in streams and lakes is relatively well studied, the pathways by which salt moves from highways to groundwater are poorly understood and quantified.

The various ways salt could be getting in to the groundwater are shown in Figure 1 and a bit of detective work was required to find out what was going on.

Figure 1*

This involved dressing in silly (and warm) clothes, and pulling our equipment around on a sledge, which was a lot of fun (see photos below).

The authors, Mark (left) and Mike (right), enjoying some urban winter fieldwork.

Our main field activities were fortuitously timed to include the severe winters of 2009-10 and 2012-13. We collected information about the amount of salt spread on the roads and measured the salt concentrations in the local streams, as well as in the shallow and deep groundwater wells in the area around the highway network.

7 km of motorway drained to the studied stream. Over the winter of 2012-13 we estimated that around 510 tonnes of de-icing salt was applied to the highway and major road network across the catchment. Most of that washed off the road via drains when it next rained or as the snow and ice melted, and ended up in the stream which flows under the highways. Some of it ended up on the roadside verges – this was not quantified, but would likewise eventually leach into the underlying groundwater over time.

We estimated about 12% of the de-icing salt, that’s around 63 tonnes over the 2012-13 winter, leaked through the bottom of the stream channel into the groundwater in the sandstone aquifer beneath, and may pose a risk to groundwater supplies in the area (see “9” in Figure 1, and results in Figure 2). While increases in groundwater chloride concentrations observed to date have been modest, and fortunately remain far below the drinking water standards, the steady year-upon-year build-up of salt in groundwater remains a concern. This is especially the case as the UK’s de-icing salt applications reached record levels in the recent severe winters and add to a potential de-icing salt legacy in our aquifers that has accumulated now over some 50 plus years.

Figure 2*

So how concerned should we be that so much salt is flowing down these streams and into the groundwater around our highways? There isn’t yet a simple answer to that but, with a global trend towards increasing urbanisation, this is an important area of ongoing and future research.

 

Further details of our published research to date on the Birmingham case study may be found at:

Rivett, M.O.,  Cuthbert, M.O., Gamble R., Connon, L.E., Pearson, A., Shepley, M.G., Davis, J., 2016. Highway deicing salt dynamic runoff to surface water and subsequent infiltration to groundwater during severe UK winters.  Science of the Total Environment 565, 324-338. http://dx.doi.org/10.1016/j.scitotenv.2016.04.095 (*figures reproduced with permission) [Read More]

Fire and groundwater

Fire and groundwater

Post by Andy Baker, University of New South Wales

The effects of fire on the surface environment are clear to see. Landscapes are coated in ash. Intense fires can destroy all vegetation and alter soil properties. Less intense fires destroy just the surface leaf litter, grasses and shrubs.  Grass fires can be fast moving, destroying buildings and threatening lives. Intense fires can even form their own local weather systems.

But what about the effects of fire on water underground? Let’s think about what happens on the surface, and translate that to what is likely to happen to the subsurface.

Firstly, ash is burnt vegetation. It is rich in carbon, nutrients and trace metals, and can be mobilised after heavy rainfall. Heavy rainfall events are most likely to cause groundwater recharge as some of the rainfall makes its way down to the replenish the water table. As this happens we should also expect nutrients and carbon to be moved downwards towards the water table, as well as horizontally to rivers.

Secondly, where a fire destroys tree cover, then the canopy shading will be lost. We would predict hotter surface temperatures. This could alter the water balance, changing evaporation and also the amount or timing of groundwater recharge.

Finally, we would expect increased sediment movement. This could block flow routes to the subsurface, such as fractures and sinkholes. This would change the routes by which water moves from the surface to the water table.

Of course, the problem with water underground is that it is hard to see and measure. So how can we observe the effects of fire in the subsurface? We have been using caves in karstified limestone as a way to sample the water as it moves from the surface to the groundwater.

In caves, you can monitor the water chemistry and hydrology after a wildfire. If you know when a fire will occur, you can make measurements before and after. And you can investigate the chemical record of past fires preserved in cave stalagmites.

What do we see from the cave? We see increased evaporation and decreased recharge in the years immediately after  an intense wildfire. We see nutrient flushes from the surface to the subsurface, moderated by vegetation uptake as regrowth occurs. And we have started to compare the stalagmite geochemical signals of fire, cyclones and global warming.

We are starting to understand a little bit about the effects of fire on water as it moves through the unsaturated zone of limestone. What about other rock types? What about the groundwater aquifer itself? We need to know more about the effect of wildfires on our subsurface water resource.  The fire season is getting longer. A greater extent of the earth’s surface is being burnt. Both will continue to increase with global warming, but how will this affect the water underground?

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