The unsung linchpin : groundwater helps stabilize the climate

Aquifer rain and the earth's sweat glands

Humanity has depleted groundwater around the world. This has consequences for global rainfall and temperatures. Groundwater, brought up by trees, and then transpired into the atmosphere, helps create rain. I call this process aquifer rain. When humans deplete the groundwater to levels that tree roots can no longer reach, then rainfall lessens.

There is a general public awareness that global warming is causing more droughts. There is growing awareness, small yet significant, that land degradation also leads to droughts. But there is barely any awareness yet that groundwater depletion can also lead to more droughts.

Groundwater is hidden out of sight, and so its easy not to think about it. There is surface earth layer between the groundwater and the atmosphere, so the causality chain of groundwater creating rain is somewhat concealed.

Oceans have a big effect on our rain. If the ocean currents are hotter then you have more rain, because there is more evaporation. like in. If the ocean currents are colder then you have less rain, because there is less evaporation. North Carolina and South Carolina will get more rain than California and Arizona, which are at same latitude, because the east coast of the US has much warmer ocean waters than the west coast has. In the 1970s and 1980s there was a reduction of 30% in rain in Sahel, Africa, in part because the temperatures of the sea surfaces, adjacent to the Sahel, dropped during those decades.

Land also has a significant effect on our rain, because if it can absorb more of the rain, there is then more water available to transpire back up. If the land has more trees and wetlands, then more water can evapotranspire.

The ocean and land contributions to rain are studied in the scientific research community quite a lot. But there are currently only a handful of scientists studying how groundwater contributes to precipitation.

Michael Barlage of NOAA (National Ocean and Atmospheric Administration) and his colleagues are part of that handful are studying it. They studied the effect of adding groundwater into their models, and found it increased the rain by 150mm (5.9 inches) in the Central US. The groundwater also helped cool the land by 3 degrees. When they put groundwater into their climate models, they also found it simulated actual rainfall and surface temperatures better than other climate models without groundwater. [Barlage 2021]

Evapotranspiration is made up of evaporation, which is water evaporating from the land, and transpiration, which is plants releasing water vapor. In simulations, when groundwater is added, the transpiration part of evapotranspiration increases from 47% to 62% worldwide, an increase of 15%. [Maxwell 2016]. The researchers Reed Maxwell and Laura Condon state that the “results suggest that lateral groundwater flow processes, currently overlooked in global approaches, may play an important role in characterizing evapotranspiration at large scales.”

In places with more pronounced dry seasons in subtropical climates, like Arizona, North Carolina, Queensland, Guanxi (China), Sierre (Switzerland), the plants get 49% of their water from aquifers in dry seasons, and 29% of their water from aquifers in wet seasons [Barbeta 2017]. Those plants then transpire that water into the air, increasing humidity, and rain through precipitation-recycling.

If ocean temperature drops for multiple years, there will be much less ocean moisture blowing inland. In those times, the transpiration of groundwater by trees becomes even more crucial. The groundwater will make up an even larger portion of the humidity and rain during those periods.

One of the central problems an ecosystem has to figure out is how to get water from the wet times to the dry times. In addition to creating rain, trees can also bring up the aquifer water and spread it around the neighboring soil, in a process called hydraulic redistribution. This process helps keep a lot of the vegetation alive during extended droughts.

Another related effect, is that if groundwater drops in an area, then there will drier and hotter winds, which will then lead to more wildfires.

Groundwater exists in much larger supply than surface water and atmospheric water. Groundwater couples to surface water and atmospheric water via vegetation. The aquifer water acts like a bank - water/money can be withdrawn when needed. Groundwater dampens the fluctuations in surface and atmospheric water.

Heres a map of the water tables in aquifers around the world -

[Fan 2013]

Here is a picture of groundwater levels in Iberia -

[first figure shows groundwater levels in meters above sea level, second figure is meters below ground level Ben Salem 2023]

If we want to bring back the summer rain in Iberia, it would be helpful to bring the groundwater back up to levels that the trees and marshes can reach. That way they can then transpire more water into the air. To increase groundwater levels, we can improve the soil content of the land. Bare land repels the rain, and leads to more runoff. We can depave the concrete which repels rain. We can create more earthworks like ponds and swales to catch the rain. We can restore floodplains, so that during big winter storms the rivers can overflow into the floodplains, and then seep into aquifers below.

Here’s a picture of how much groundwater went down from 2002 to 2023 in the Western US.

Replenishing the groundwater in the Western US can lead to less wildfires and more rain. It can lead to more rain falling into the Colorado River, which would help with its dwindling supplies. Seven states depend on water from the river. Those seven states are engaged in serious discussions because of the ramifications of the reservoirs having less and less water. None of them though, are looking at increasing the rain as a solution, and none of them are looking at replenishing groundwater as a solution to increasing the reservoirs supplies. For such a water shortage situation a more holistic understanding of the hydrological cycle is necessary.

The Great Plains is one of the breadbaskets of the US, producing a lot of its food. The area has unfortunately though depleted the gigantic Ogallala aquifer to produce this food. Modern agricultural practices there are also degrading the soil, so that it no longer absorbs as much of the rain, which means less of it then goes to recharging the aquifers. Authorities are awakening to the potential crisis they have on hand - aquifer depletion means food production drops drastically. While bringing up groundwater has temporarily created an ‘artificial’ rain effect, because some of that water then evapotranspired upwards, this taking of the groundwater cannot continue forever. When the pumping stops there will also be a drop in rain. Thus the problem in the Great Plains is even more dour. Its not just that they may run out of aquifer water, but that if it runs out, it will be accompanied by even less rain.

Groundwater also act as the sweat glands of the earth. When tree roots bring up the groundwater, then transpire it, it creates an evaporative cooling effect similar to the sweat cooling effect in humans. If our sweat glands do not work in our body, all sorts of other physiological factors can start to malfunction. In a similar way if we drain our lands of groundwater, then the earth physiology can start to malfunction in all sorts of ways, e.g. having more intense wildfires.

There is a framework for the planetary boundaries of the earth, developed by a group of 28 scientists, led by Johan Rockstrom of the Stockholm Resilience Center. There are nine boundaries in their framework. If the earth crosses any of these nine boundaries - climate change, change in biosphere integrity (biodiversity loss and species extinction), stratospheric ozone depletion, ocean acidification, biogeochemical flows (phosphorus and nitrogen cycles), land-system change (for example deforestation), freshwater use, atmospheric aerosol loading (microscopic particles in the atmosphere that affect climate and living organisms), and introduction of novel entities, then the earth tips into a more precarious state. Freshwater use includes green water use, which is water associated with soil, plants and rain, and blue water use which is water in lakes, rivers, and aquifers. The people who drafted the planetary boundaries idea were probably not thinking about groundwater’s effect on the climate, I propose we include that in the science behind the freshwater use boundary.

There are many ways we can recharge our aquifers. The sink it part of the slow it, sink it, spread it slogan refers to guiding rainfall to go back into the land. Agroforestry, permaculture, Natural Sequence Farming, and regenerative agriculture have many techniques to help this. There are also managed aquifer recharge methods, like flooding farms with rains from the winter storms.

With a dedicated cooperative effort we can bring groundwater levels back up to where tree roots can reach them, restore the aquifer-rain, and get back inside this planetary boundary.

……..

Related previous articles in this newsletter “The plan to replenish our groundwater : Helen Dahlke interview”, “The secret life of groundwater”, “Taming the hot dry winds that cause wildfires; sponging up freak storms”,“The missing link: groundwater creates rain”, and “Groundwater lessens wildfires”

References

Barlage, Michael, Fei Chen, Roy Rasmussen, Zhe Zhang, and Gonzalo Miguez‐Macho. "The importance of scale‐dependent groundwater processes in land‐atmosphere interactions over the central United States." Geophysical Research Letters 48, no. 5 (2021): e2020GL092171.

Barbeta, Adrià, and Josep Peñuelas. "Relative contribution of groundwater to plant transpiration estimated with stable isotopes." Scientific reports 7, no. 1 (2017): 10580

Fan, Ying, H. Li, and Gonzalo Miguez-Macho. "Global patterns of groundwater table depth." Science 339, no. 6122 (2013): 940-943.

Lubczynski, Maciek W. "Groundwater evapotranspiration–underestimated role of tree transpiration and bare soil evaporation in groundwater balances of dry lands." In Climate Change and its Effects on Water Resources: Issues of National and Global Security, pp. 183-190. Springer Netherlands, 2011

Maxwell, Reed M., and Laura E. Condon. "Connections between groundwater flow and transpiration partitioning." Science 353, no. 6297 (2016): 377-380

Nahed Ben-Salem, Robert Reinecke, Nadim K. Copty, J. Jaime Gómez-Hernández, Emmanouil A. Varouchakis, George P. Karatzas, Michael Rode, Seifeddine Jomaa, “Mapping steady-state groundwater levels in the Mediterranean region: The Iberian Peninsula as a benchmark” Journal of Hydrology, Volume 626, Part A, 2023 https://doi.org/10.1016/j.jhydrol.2023.130207

Taylor, Richard G., Bridget Scanlon, Petra Döll, Matt Rodell, Rens Van Beek, Yoshihide Wada, Laurent Longuevergne et al. "Ground water and climate change." Nature climate change 3, no. 4 (2013): 322-329