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Groundwater pumping poses worldwide threat to riverine ecosystems

Groundwater pumping poses worldwide threat to riverine ecosystems

Post by Inge de Graaf, Assistant Professor of Hydrological Environmental Systems at the University of Freiburg.


With the climate strikes happening all over the world, I sometimes wish I had a crystal ball that would allow me to look into the future. Or even better, a crystal ball that could show me different scenarios of what will happen if we change, or not.

Well, I do not have a crystal ball, but I do have a global scale hydrological model. I use this model to glimpse into the future and see what will happen to our rivers and streams if we keep pumping groundwater like we do now. Me and my co-authors recently published a paper on this in Nature.

Over the last 50-years strong population growth and economic development have led to a large increase in freshwater demand, especially for the irrigation of food crops. About half of the water used for irrigation is pumped from groundwater. In many dry regions around the world, more groundwater is pumped than recharged from rain, causing water levels to drop. When the water levels drop, the flow of groundwater to rivers and streams will reduce. As a consequence, river flows will decline (or even completely dry up) and water temperatures will rise, forming a major threat to fish and water plants.

In our study, we used a new global-scale hydrological model to investigate how freshwater ecosystems have been, and will be, affected by groundwater pumping. Using the model, I am able to calculate the flow of groundwater to rivers all over the world. This allows me to study how a reduction of this groundwater flow, when it is pumped, impacts river flow.

Our calculations show that almost 20% of the regions where groundwater is pumped currently suffer from a reduction of river flow, putting ecosystems at risk. We expect that by 2050 more than half of the regions with groundwater abstractions will not be able to maintain healthy ecosystems. Our estimates of when and where critical river flows are first reached are presented in Figure 1.

Figure 1. First year critical river flow is reached and aquatic ecosystems are threatened due to groundwater pumping.

The most striking insight is that only a small drop of groundwater level will already cause these critical river flows (Figure 2). Moreover, the impact of groundwater pumping will often become noticeable after years, or decades. This means that we cannot detect the future impact of groundwater pumping on rivers from the current levels of groundwater decline. It really behaves like a ticking time bomb.

Figure 2. Groundwater level drop that causes critical river flows to be reached.

We already see the negative impact of groundwater pumping on river flows in Central and Western United States and in the Indus river basin in Asia. If groundwater pumping continues as it is now, we expect negative impacts to occur in Southern and Eastern Europe, North Africa, and Australia in the coming decades. Climate change will accelerate this process.

Although seeing the consequences of groundwater pumping on the environment globally is rather shocking, I am still optimistic about the future. I hope we can raise awareness on a slowly evolving crisis. A reduction of groundwater pumping will be the only way to prevent negative impacts, while at the same time, global food security should be maintained. Groundwater should be used more sustainably. It is important to develop more efficient irrigation techniques worldwide and experiment with crops that use less freshwater or can live in salty water.

If you would like to read about this research of mine in more detail, read my publication Environmental flow limits to global groundwater pumping.

I conducted this work with my co-authors Tom Gleeson (University of Victoria), L.P.H. (Rens) van Beek, Edwin H. Sutanudjaja, and Marc F.P. Bierkens (all form Utrecht University).

de Graaf, I. E. M., Gleeson, T., (Rens) van Beek, L. P. H., Sutanudjaja, E. H. & Bierkens, M. F. P. Environmental flow limits to global groundwater pumping. Nature 574, 90–94 (2019).

Groundwater and a ‘green drought’

Groundwater and a ‘green drought’

Post by Andy Baker, professor in the Connected Waters Initiative Centre at UNSW Sydney, Andreas Hartmann, assistant professor in Hydrological Modeling and Water Resources at the University of Freiburg, and Romane BerthelinPhD student in Hydrological Modeling and Water Resources at the University of Freiburg.


Here in New South Wales (NSW) in southeastern Australia, a long-running drought continues. The government’s water minister Melinda Pavey noted recently that “This drought is more severe than NSW has ever experienced” and some of the worst in living memory. Almost all the state of NSW is suffering from drought, according the NSW Dept of Primary Industries, with some regions predicted to run out of water by November.

Almost a year ago, one of us (AB) wrote about the relationship between groundwater and drought, concluding that a groundwater drought had commenced across much of New South Wales. We noted that flooding rains were needed to generate some river recharge, to replenish our groundwater resource. It hasn’t happened. But, in many places, the landscape is green, and the term ‘green drought’ is now being used. What is it?

Wellington, NSW. August 2019. This is the UNSW Research Station, normally stocked and cropped, but not in 2018 or 2019.

Before answering that, we will quickly explain some groundwater terminology. Firstly, recharge is the process by which water reaches, or ‘tops up’, an aquifer. The two main types of recharge are ‘river recharge’, where water leaks from the base of a river, lake or reservoir and into the groundwater system. The other is ‘rainfall recharge’ which occurs when the water holding capacity of a soil is exceeded, allowing the downward flux of water. Quantifying the relative contributions of river recharge and rainfall recharge is not easy. For example, groundwater levels measured in bores or wells would be determined by one or both of these processes, plus of course any other non-climate factors such as groundwater abstraction.

So how does this relate to a ‘green drought’? Well, the term is being used because in some regions there has been enough rainfall for some shallow rooted vegetation to grow. In farming regions, a ‘green drought’ can specifically refer to the fact that fields look green, but the colour comes from weeds and other undesirable plants. When you see extensive green weeds across the landscape they reflect that there is another environmental problem – a paucity of recharge heading towards the aquifer. This rainfall can penetrate the soil profile, but only to a limited extent before the water is taken up by plants or evaporates back into the atmosphere (both process together are called evapotranspiration). In a ‘green drought’, none of this water penetrates deeper parts of the soil profile; there is not enough rain to saturate the soil and generate either a downward flux of water from the soil profile (often called deep drainage) or surface runoff to rivers. The groundwater recharge drought continues.

Here in the Central West NSW we have some excellent data to show exactly what is going on. At a karstified limestone site we have been simultaneously measuring rainfall, soil water storage and recharge events expressed by cave drip waters since April 2018. All part of an international network to characterize karstic recharge and evapotranspiration.

On the surface, Andreas Hartmann downloads data from the network of soil moisture probes at Wellington, New South Wales. 25 meters underground, loggers record when water reaches the cave.

Since April 2018 we have had only one recharge event recorded in the cave. This occurred in October 2018 and was after 50 mm of rainfall on one day (5 October 2018). No other rainfall events have generated recharge at the monitoring site since April 2018, despite many rain days with much lower rainfall amounts. The frequency histogram of daily rainfall shows this:

Since the start of 2018, there have been just over 100 days where 10 mm or less of rain has fallen. Some of this rain will have contributed to vegetation growth and the greening of the landscape. But only one day had enough rain to generate rainfall recharge.

What about the soil moisture? The probe network shows that the highest water content of the soil occurred after the October 2018 rainfall event, the same one that led to the only recharge event. However, there are also many other rainfall events that increased the soil water content, but did not lead to recharge. This increased soil water can be utilized by vegetation, leading to a greening of the landscape.

Hence the term ‘green drought’. However, as our monitoring network shows, this green drought is also a groundwater drought. The date when water will run out, or ‘Day 0’, is approaching for some towns. The drilling of new ‘emergency bores’ to tap groundwater are planned to help maintain water supplies in drought affected regions. Let’s hope for groundwater recharging rainfall events soon. Our monitoring at Wellington, NSW, shows how much rain is needed and how rarely such events occur.


Andy Baker is a Professor in the Connected Waters Initiative Research Centre at UNSW Sydney. His research interests include the study of unsaturated zone hydrological and geochemical processes in karst. Find out more at his personal webpage www.bakerlabgroup.org
Andreas Hartmann is an Assistant Professor in Hydrological Modeling and Water Resources at the University of Freiburg. His primary field of interest is karst hydrology and hydrological modelling. Find out more at his personal webpage www.subsurface-heterogeneity.com
Romane Berthelin is a PhD student in the Hydrological Modeling and Water Resources at the University of Freiburg. Her primary field of interest is karst hydrogeology. In her PhD, Romane is collecting soil moisture data and soil water isotopic profiles at five representative sites across the globe.

Urban water underground: How green infrastructure makes it visible

Urban water underground: How green infrastructure makes it visible

Post by Theodore Lim, assistant professor of Urban Affairs and Planning at Virginia Tech. He researches the socio-hydrology of green infrastructure planning and implementation.


In order for people to care about something, to value it, they have to be able to see it and experience it. This point should not be taken lightly. So much about decision-making and policy-making depends on how much public support can be organized around any given issue. So, when it comes to protecting the water resources that sustain society and the natural environment, it is perhaps unsurprising that groundwater is the part of water cycle that most folks tend to ignore.

Historically, urbanization has made it difficult for residents to experience key parts of the water cycle. In Philadelphia, for example, in order to improve sanitation, reduce flooding, and make way for the regular street grid, dozens of streams were channeled into brick sewers that were then buried under roads to make transportation routes more efficient and the city easier to navigate. Today, these historical streams are literally buried underground, and still carry a mix of raw sewage and stormwater runoff, that overflows into neighboring creeks and streams when it rains.

Sewering Mill Creek in West Philadelphia, 1883. Image Source: http://www.phillyh2o.org.

Green infrastructure is an approach to urban water management that reintroduces key parts of the hydrological cycle to the visible urban environment. Green infrastructure can involve surface water, through daylighting streams, but it can also means removing concrete and asphalt, to allow water to trickle slowly into the ground, be soaked up by plant roots, and evaporated back into the air, or to recharge deeper groundwater tables. An example of this is a rain garden, or bioinfiltration facility, which intercepts rainfall with vegetated land, as an aesthetic and low-maintenance alternative to conventional drainage systems.

Bioinfiltration facility installed in a residential neighborhood in Washington, D.C. (Photo by author)

How do urban residents “see” these important elements of the hydrological cycle? One answer is: by their recognizing them as “infrastructures” for modern cities — elements that provide critical services, in addition to environmental amenities. Plants and restored soils are not just “nice to have,” they help moderate surface temperatures, protect water quality, and support biological function. Public investment in street tree planting has been shown to increase pedestrian activity, public health, and property values. Green infrastructure programs in cities can also be integrated with climate action plans, sustainability initiatives, and parks and recreation programs; such programs often attract businesses and residents with their progressive attitudes.

Urban residents can learn about the “ecosystem services” associated with hydrological cycle improvements thanks to scientific signage, infographics, partnerships between  environmental non-profits and community outreach, programs run by municipal water managers (water/wastewater utilities, departments of environmental protection, or stormwater districts). Most of all, residents learn from each other and are more likely to adopt environmentally friendly behaviors when they are surrounded to neighbors who do the same.

Stormdrain art in Philadelphia, designed by children in Philadelphia public schools (Photo by author)

Green infrastructure monitoring and signage in Chicago. Image: https://nextcity.org/daily/entry/chicago-sensors-green-infrastructure-study

Restoring hydrological function within urban areas can be seen as a microcosm of larger-scale environmental policy-making. On the regional scale, “green infrastructure” can refer to the large swaths of undeveloped land in a natural state, or to working lands, such as those used for timber production or agriculture, which all provide critical ecosystem services to society. Environmental planners who work at the state or regional level or with land trust organizations, might use policies, economic incentives, and land regulations to protect these landscapes from low density suburban sprawl or urbanization. However, on both urban and regional scales, the decision-making about land use, management, and development is heavily dependent on co-operation between diverse stakeholders, and relies on a mutual understanding of the value of natural environments for various communities..

Urban and environmental planners tell stories that bring together multiple voices in collaboration.  These stories also give the historical and social context to decision-making around environmental systems, which is vital to ensure equitable outcomes. Unfortunately, despite advances in integrated modeling and the scientific knowledge of complex interrelations between water and society, decision-making still falls back on heuristics and rules-of-thumb. A highly relevant question therefore is: how can we integrate groundwater science into more robust city and regional-scale participatory planning, that is equitable and implementable? The answer will hopefully lead us to a strategy where urban and environmental infrastructures visibly advance the well-being of communities.

Quest for Sustainability of Heavily Stressed Aquifers at Regional to Global Scales: Upcoming Chapman Conference

Quest for Sustainability of Heavily Stressed Aquifers at Regional to Global Scales: Upcoming Chapman Conference

Abstracts are due soon (July 10th) for the upcoming Chapman conference on groundwater sustainability on Oct 21-24, 2019 in Valencia, Spain. Hopefully this will be a rare opportunity where many of the leading people on groundwater sustainability will gather with a shared intention to share, discuss and debate scientific advances and encourage a pivot towards groundwater sustainability.

A range of prestigious invited speakers will provide diverse perspectives on groundwater sustainability. We have limited travel funding from the NSF – priority will be given to US-based students and early career researchers (pre-tenure faculty and postdoctoral fellows). Please pencil this in the conference date and submit an abstract here, and pass this along to anyone who might be interested!

WaterUnderground founder Tom Gleeson is part of the Chapman conference organizing committee and is leading an effort to draft ‘The Valencia Statement and Action Agenda on Global Groundwater Sustainability’. Please get in touch with Tom  if you are interested in contributing!

 

Data sharing: an update on new and existing initiatives

Data sharing: an update on new and existing initiatives

Post by Anne Van Loon, Gemma Coxon, and Bentje Brauns.


Last year, Anne Van Loon wrote about data sharing initiatives in hydrology (“Data drought or data flood? 28 May 2018). This post gives an update on existing and new initiatives.

CAMELS (Catchment Attributes and MEteorology for Large-sample Studies) 

The CAMELS datasets are expanding: from the United States and Chile to Great Britain and Australia.  The CAMELS-GB dataset will consist of hydro-meteorological timeseries and catchment attributes for 671 catchments across Great Britain and is expected to be released on the Environmental Information Data Centre later this year.

The Groundwater Drought Initiative

The Groundwater Drought Initiative is collecting more and more groundwater level data and groundwater drought impacts. The Initiative is very happy to welcome new partners and supporters from as far East as Ukraine and as far South as Albania, increasing the number of participating countries and countries currently considering to participate to 23 (see map). Additionally, a first getting-to-know-each-other & info meeting was held at EGU19 with participants from Austria, Belgium, Canada, Estonia, Germany, Latvia, Luxembourg, Netherlands, Norway, UK, Ukraine, and Switzerland. If you are from Bulgaria, Greece, Hungary, Italy, Romania, Slovakia or any of the other yellow countries on the map below and you have groundwater data (or contacts in organisations who could help) or you are interested in groundwater drought, please contact Bentje Brauns (benaun@bgs.ac.uk).

The IAHS Panta Rhei Working Group on Large Sample Hydrology

The IAHS Panta Rhei focus on efforts to facilitate the production and exchange of datasets worldwide.  This year at EGU, the group organised a splinter meeting to discuss the generation of large sample catchment datasets in the cloud and a session (HS2.5.2 Large-sample hydrology: characterising and understanding hydrological diversity) that showcased several recent data- and model-based efforts on large-sample hydrology from new global datasets to large multi-model ensembles.  If you are interested in being updated on the activities of the group then please contact Gemma Coxon (gemma.coxon@bristol.ac.uk) to be added to the mailing list.

There seems to be a lot going on in the world of hydrological data sharing! To share your own story or initiative, please leave a reply below.



Anne Van Loon (website | @AnneVanLoon) is a Senior Lecturer in Physical Geography  in the School of Geography, Earth and Environmental Sciences at the University of Birmingham.

Gemma Coxon (website) is a Postdoctoral Research Associate and Lecturer in Hydrology in the School of Geographical Sciences at the University of Bristol.

Bentje Brauns (website) is a Hydrogeologist at the British Geological Survey.

Video: Linking water planetary boundaries and UN Sustainable Development Goals

Video: Linking water planetary boundaries and UN Sustainable Development Goals

Water Underground creator Tom Gleeson prepared this quick research video (with no more than a toothbrush, a file holder, and a doughnut, in one take!) for the Ripples project meeting at the Stockholm Resilience Centre, that was held in April. In this video, he talks about using doughnut economics for linking water planetary boundaries and UN Sustainable Development Goals.

 


Curious about why a toothbrush features in the video? For the answer, you’ll need to watch Tom’s previous research video from last summer (see below), on “Revisiting the planetary boundary for water”.

Have you ever wondered if groundwater is connected to climate?

Have you ever wondered if groundwater is connected to climate?

Post by Tom Gleeson Assistant Professor in Civil Engineering at the University of Victoria.


‘Groundwater-surface water interactions’ has become standard hydrologic lexicon and a perennial favorite session title at various conferences… but how often do you hear the phrase ‘groundwater-climate interactions’?

A group of hydrologists, hydrogeologists, atmospheric scientists and geodesists that met in Taiwan this week would say ‘not enough!’ We met to discuss how groundwater, the slow-moving grandparent of the hydrologic cycle interacts with the atmosphere, the fast-moving toddler. The 2nd international workshop on Impacts of Groundwater in Earth system Models (IGEM), was a follow-up of a 2016 workshop in Paris in 2016 (and part of a the bilateral French-Taiwanese IGEM project).

Sessions were focused around a few themes:

  • Groundwater use and its impacts
  • Groundwater representation, assimilation and evaluation in climate models
  • Remote Sensing and in-situ observations on groundwater
  • Groundwater-climate interactions with a special focus on Nebraska

 

And in the afternoons we convened discussion groups focused on ‘groundwater representation in continental to global hydrologic models’ and ‘groundwater-climate interactions’ and arguably just as importantly we ate lots of great food including an awesome fusion dinner and dumplings at the famous Din Tai Fung.

I would love to say that we could provide you with a simple, robust answer to the leading question of how and where groundwater is connected to climate – a holy grail of Earth System science. But like all good questions, the answer at least right now is ‘a little bit in some places, and it depends how you look at it’. We discussed the first enticing but preliminary results of potential hotspots of groundwater-climate interactions, expounded on the importance to water sustainability and dissected vadose zone parameterizations in land surface models but the quest for this holy grail goes on… We plan to meet again in a few years in Saskatchewan and maybe have a few more answers. Do you want to join us on this holy grail quest, and maybe end up making ‘groundwater-climate interactions’ more standard lexicon?

P.S. Thanks to Min-Hui Lo and his group at National Taiwan University for the excellent hospitality and organization!

P.S.S. Just in case it goes viral, the term ‘baddest-ass model’ was first used by Jay Famiglietti (see below).

Celestial groundwater – the subsurface plumbing for extraterrestrial life support

Celestial groundwater – the subsurface plumbing for extraterrestrial life support

Post by Kevin Befus Assistant Professor in Civil and Architectural Engineering at the University of Wyoming.


Have you ever taken a walk on the beach during a lowering (ebbing) tide and see mini-rivers grow and create beautiful drainage patterns before your eyes? These short-lived groundwater seepage features (Fig. 1A) are tiny (and fast) analogs of how groundwater has shaped some parts of Mars! It appears that groundwater loosening sediments can lead to all sorts of scales of erosion on both Earth and Mars.

Figure 1. A) Beach drainage pattern on the order of 1 meter (Source: https://epod.usra.edu/blog/2017/01/beach-drainage.html), B) Martian “alcoves” suggesting groundwater seepage [1].

Mars is not currently a friendly place for water to exist at the surface or even the subsurface, but an abundance of photographic and topographic evidence point to there having been the right conditions for active groundwater flow on Mars.

But isn’t Mars too cold for liquid water? The answer is generally a strong yes for the past few billion years, but amazingly enough, there appears to have been some local places where groundwater discharged to the Martian surface and left behind telltale signs.

Because Mars is cold at its land surface (mean surface temperature of -50 C with daily swings from 0 C to -100 C) with a thinner atmosphere than Earth’s, water on the Martian surface can exist as ice (as in the polar ice cap), but sublimation and evaporation would quickly wick any water near the surface. So, liquid water on Mars needs both more pressure and a good bit of heat for mobile groundwater based on the phase diagram below (circle with M shows the present day Martian surface conditions).

Figure 2. Phase diagram showing average conditions at the planetary surface for Earth (E) near the triple point, and atmospheric conditions for the frozen Mars (M) and vapor-rich Venus (V). source: http://www1.lsbu.ac.uk/water/water_phase_diagram.html#intr2; License: https://creativecommons.org/licenses/by-nc-nd/2.0/uk/)

It turns out that the most expansive evidence of liquid groundwater on Mars comes from deep at the bottom of craters (…deeper than 5 km!), where the Martian geothermal gradient (~10 C/km [Michalski et al.2013]) heats up to the point where groundwater systems, probably made up of brines, can seep across the crater walls. Without the craters, the groundwater wouldn’t have anywhere to discharge, but extraterrestrial hydrogeologists (really based on the geomorphology, but using E.T. hydrogeology principles) have identified numerous craters with groundwater seepage erosional patterns (Figure 1). The question remains open on how connected the Martian “aquifers” could be, or if the craters represent only local flow systems.

With liquid groundwater transporting the chemical-rich waters from deeper geothermal areas, the conditions could be right for supporting a deep Martian biosphere. Buried in under the Martian ice, soil, and rock microbial life could have evolved in the subterranean shelter from cosmic radiation. Groundwater flow, potentially related to geothermal conditions, could then have served as the conveyor belt for energy-rich molecules to feed microbial life in the subsurface (and still could?).

So far, Earth is the only celestial body in our solar system with an active water-hydrologic cycle, making us the lucky green planet. But, there could be a methane-based hydrologic cycle on Titan with “methanifers” as methane aquifers! For more information on extraterrestrial hydrogeology, Baker et al. (2005) provides a great overview of the planetary, lunar, and exo-planetary potential for water and groundwater, loosely summarized in this table.

At the moment, Earthlings don’t know that much yet about the paleo-hydrologic processes on Mars. But with new boots…I mean wheels…on the ground in two water-focused locations, new clues could start rolling in on Martian groundwater. The recently-arrived InSight lander will probe the Martian subsurface by drilling 5 m deep and listen for acoustic signals for even more information on the interior of Mars. The next Mars Rover is scheduled to take flight in 2020 for the Jezero Crater, where a river delta could help unravel the water-life story of Mars. And could have some groundwater surprises! At only about 1 km deep, the focus in mainly on tracking down signs of life and unravelling surface hydrologic and erosional processes on Mars, but a long list of expected outcomes does show the mission will keep an eye out for evidence of groundwater activities. Keep your feet grounded, eyes in the sky, and visions of Martian groundwater flying high and drilling low!

References
[1] Malin, M. C., and K. S. Edgett (2000), Evidence for Recent Groundwater Seepage and Surface Runoff on Mars, Science, 288(5475), 2330–2335, doi:10.1126/science.288.5475.2330.
[2] Michalski, J. R., J. Cuadros, P. B. Niles, J. Parnell, A. Deanne Rogers, and S. P. Wright (2013), Groundwater activity on Mars and implications for a deep biosphere, Nat. Geosci., 6(2), 133–138, doi:10.1038/ngeo1706.
[3] Stofan, E. R. et al. (2007), The lakes of Titan, Nature, 445(7123), 61–64, doi:10.1038/nature05438.
[4] Baker, V. R., J. M. Dohm, A. G. Fairén, T. P. A. Ferré, J. C. Ferris, H. Miyamoto, and D. Schulze-Makuch (2005), Extraterrestrial hydrogeology, Hydrogeol. J., 13(1), 51–68, doi:10.1007/s10040-004-0433-2.
[5] Robinson, K. L., and G. J. Taylor (2014), Heterogeneous distribution of water in the Moon, Nat. Geosci., 7(6), 401–408, doi:10.1038/ngeo2173.
[6] Jurac, S., M. A. McGrath, R. E. Johnson, J. D. Richardson, V. M. Vasyliunas, and A. Eviatar (2002), Saturn: Search for a missing water source, Geophys. Res. Lett., 29(24), 25-1-25–4, doi:10.1029/2002GL015855.

Kevin Befus leads the groundwater hydrology group in the Civil and Architectural Engineering Department at the University of Wyoming. With his research group, he studies how groundwater systems respond to hydrologic conditions over glacial timescales and in mountainous and coastal environments.  You can follow along with Kevin’s research through any of the links below:

Personal Webpage | Twitter Research Group Page | UW Faculty Page

 

 

 

 

 


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Water: underground source for billions could take more than a century to respond fully to climate change

Water: underground source for billions could take more than a century to respond fully to climate change

WaterUnderground post by Mark O. Cuthbert, Cardiff University; Kevin M. Befus, University of Wyoming, and Tom Gleeson, University of Victoria


Groundwater is the biggest store of accessible freshwater in the world, providing billions of people with water for drinking and crop irrigation. That’s all despite the fact that most will never see groundwater at its source – it’s stored naturally below ground within the Earth’s pores and cracks.

While climate change makes dramatic changes to weather and ecosystems on the surface, the impact on the world’s groundwater is likely to be delayed, representing a challenge for future generations.

Groundwater stores are replenished by rainfall at the surface in a process known as “recharge”. Unless intercepted by human-made pumps, this water eventually flows by gravity to “discharge” in streams, lakes, springs, wetlands and the ocean. A balance is naturally maintained between rates of groundwater recharge and discharge, and the amount of water stored underground.

Groundwater discharge provides consistent flows of freshwater to ecosystems, providing a reliable water source which helped early human societies survive and evolve.

When changes in climate or land use affect the rate of groundwater recharge, the depths of water tables and rates of groundwater discharge must also change to find a new balance.

Groundwater is critical to agriculture worldwide. Rungroj Youbang/Shutterstock

The time it takes for this new equilibrium to be found – known as the groundwater response time – ranges from months to tens of thousands of years, depending on the hydraulic properties of the subsurface and how connected groundwater is to changes at the land surface.

Estimates of response times for individual aquifers – the valuable stores of groundwater which humans exploit with pumps – have been made previously, but the global picture of how quickly or directly Earth’s groundwater will respond to climate change in the coming years and decades has been uncertain. To investigate this, we mapped the connection between groundwater and the land surface and how groundwater response time varies across the world.

The long memory of groundwater

We found that below approximately three quarters of the Earth’s surface, groundwater response times last over 100 years. Recharge happens unevenly around the world so this actually represents around half of the active groundwater flow on Earth.

This means that in these areas, any changes to recharge currently occurring due to climate change will only be fully realised in changes to groundwater levels and discharge to surface ecosystems more than 100 years in the future.

We also found that, in general, the driest places on Earth have longer groundwater response times than more humid areas, meaning that groundwater stores beneath deserts take longer to fully respond to changes in recharge.

Groundwater stores are ‘recharged’ by rainfall and ‘discharge’ into surface water bodies such as lakes. Studio BKK/Shutterstock. Edited by author.

In wetter areas where the water table is closer to the surface, groundwater tends to intersect the land surface more frequently, discharging to streams or lakes.

This means there are shorter distances between recharge and discharge areas helping groundwater stores come to equilibrium more quickly in wetter landscapes.

Hence, some groundwater systems in desert regions like the Sahara have response times of more than 10,000 years. Groundwater there is still responding to changes in the climate which occurred at the end of the last glacial period, when that region was much wetter.


Read MoreThe global race for groundwater speeds up to feed agriculture’s growing needs


In contrast, many low lying equatorial regions, such as the Amazon and Congo basins, have very short response times and will re-equilibrate on timescales of less than a decade, largely keeping pace with climate changes to the water cycle.

Geology also plays an important role in governing groundwater responses to climate variability. For example, the two most economically important aquifers in the UK are the limestone chalk and the Permo-Triassic sandstone.

Despite both being in the UK and existing in the same climate, they have distinctly different hydraulic properties and, therefore, groundwater response times. Chalk responds in months to years while the sandstone aquifers take years to centuries.

Global map of groundwater response times. Cuthbert et al. (2019)/Nature Climate Change, Author provided.

In comparison to surface water bodies such as rivers and lakes which respond very quickly and visibly to changes in climate, the hidden nature of groundwater means that these vast lag times are easily forgotten. Nevertheless, the slow pace of groundwater is very important for managing freshwater supplies.

The long response time of the UK’s Permo-Triassic sandstone aquifers means that they may provide excellent buffers during drought in the short term. Relying on groundwater from these aquifers may seem to have little impact on their associated streams and wetlands, but diminishing flows and less water could become more prevalent as time goes on.

This is important to remember when making decisions about what rates of groundwater abstraction are sustainable. Groundwater response times may be much longer than human lifetimes, let alone political and electoral cycles.The Conversation


Post written by:

Mark O. Cuthbert, Research Fellow & Lecturer in Groundwater Science, Cardiff University;

Mark Cuthbert is a Research Fellow and Lecturer in the School of Earth and Ocean Sciences, at Cardiff University in the United Kingdom. Mark’s work currently focuses on coupled hydrological-climate process dynamics in order to: understand groundwater sustainability; improve interpretations of terrestrial paleoclimate proxy archives;  and understand how Quaternary paleoenvironments influenced human evolution.

 

Kevin M. Befus, Assistant professor, University of Wyoming; 

Kevin Befus leads the groundwater hydrology group in the Civil and Architectural Engineering Department at the University of Wyoming. With his research group, he studies how groundwater systems respond to hydrologic conditions over glacial timescales and in mountainous and coastal environments.

 

 

Tom Gleeson, Associate professor, University of Victoria

Tom Gleeson leads the Groundwater Science and Sustainability group in the Civil Engineering Department at the University of Victoria.  His research interests include groundwater sustainability, mega-scale groundwater systems, groundwater recharge and discharge and fluid flow around geologic structures. Tom is also the founder of this blog, WaterUnderground.

 

 


This article is republished from The Conversation under a Creative Commons license. Read the original article.

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Groundwater and drought

Groundwater and drought

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

__________________________________________________

Drought is in the news here in New South Wales, Australia. But how are rainfall, drought and groundwater related?

First, we need to understand what drought is. Is it a water shortage? Or a lack of rainfall? Or something else? In the USA, the National Climatic Data Center define drought as the ‘absence of water’. They identify four types of drought: 1) meteorological drought (a lack of rainfall), 2) hydrological drought (a loss of surface water or groundwater supply), 3) agricultural drought (a water shortage leading to crop failure), and 4) socioeconomic drought (where demand for water exceeds availability).

Here in Australia, the Bureau of Meteorology defines drought as ‘a prolonged, abnormally dry period when the amount of available water is insufficient to meet our normal use’.  They add that ‘drought is not simply low rainfall; if it was, much of inland Australia would be in almost perpetual drought’. Much of inland Australia depends on surface and groundwater for their economy. If those regions experienced a groundwater drought, it would therefore be bad news.

Let’s look at New South Wales again. It covers both coastal regions, such as Sydney (where I am writing this), as well as a vast interior (where most of my research is based). The Bureau of Meteorology produces meteorological drought maps based on rainfall amounts over recent months. The current map shows large areas of New South Wales are experiencing rainfall totals that are in the lowest 10 percentile (‘serious’), lowest 5 percentile (‘severe’) and the lowest on record.

How does this deficiency in rainfall affect groundwater? And is there a groundwater drought? Long-term measurement of groundwater levels in boreholes (also called wells, depending on your country) can tell you whether water levels are rising or falling. Wells integrate groundwater recharge that comes from both surface water (e.g. rivers that lose water through their base) and from rainfall (also called diffuse recharge).

Real-time data of water levels from telemetered boreholes can provide timely information on groundwater drought (for example, here for NSW). Satellite products such as GRACE, which can infer groundwater levels from small changes in gravity over time, can provide large scale spatial coverage. Modelling products can calculate water balance from meteorological, soil and land use data.

The current Bureau of Meteorology map shows that deep soil moisture is very much below average across New South Wales. If we assume that deep soil moisture levels are only determined by rainfall recharge, then from this we would expect no rainfall recharge of groundwater to be occurring over large parts of New South Wales. From one location, Wellington, close to the middle of the drought region, we have the measured evidence from inside a cave that shows that rainfall recharge hasn’t occurred for 18 months (and counting).

Since 2011, forty loggers have been measuring the water percolating through the unsaturated zone of the limestone at a depth of 25 m at Wellington Caves. This winter, I did the latest download of the data. Or rather, the lack of data, as only four drip water sources were still active. Conditions in the cave are the driest since we started collecting data in 2011.

Drip rates have been on the decline since the winter of 2016. But note the decline temporarily slowed in 2017, starting in early April. That is the response to the last time there was rainfall recharge there – owing to almost 70 mm of rain falling over three days in late March 2017. Eighteen months ago.

In the inland of New South Wales, it is clear that in dryland farming regions, the lack of rainfall has now led to an agricultural drought. In contrast, latest available data from our groundwater monitoring networks shows that there is currently no decline in groundwater levels in the major irrigation districts, which is where river recharge occurs. But for our dryland farmers, and ecosystems that rely on rainfall recharge, the karst drip data show that the groundwater drought has hit. Australia is often called a country of drought and flooding rains. Flooding rains are what we need next so that we also have some river recharge to replenish our groundwater resource.

 

Wellington, NSW. July 2018. This is the UNSW Research Station, normally stocked and cropped, but not this year.