The secret life of groundwater

How roots help groundwater to hydrate landscapes and decrease wildfire risk

Underneath the surface of the earth lives a world of roots, soil, bedrock and groundwater. How that groundwater interacts with, and hydrates nature’s ecosystems has been slow to be understood because of how hidden it is from normal eyesight. It is only in the last few decades that researchers have begun to see how deep groundwater can be brought up to hydrate the landscape during dry season - the hydration of which can help with the reduction of wildfire risk, and the cooling of the planet. A dynamical dance of water underneath the earth's surface is influencing what we see above.

In the Atamaca desert, in Chile, dry winds sweep across the sands. Standing resolutely in the stark landscape are the tamarugo, round-crowned trees that flower seasonally. Underneath these trees, one meter down, around the root mat of the tamarugo, is surprisingly moist soil, soil that is sandwiched between dry soil above and below. For many years researchers assumed that the soil moisture came from atmospheric water pulled down by the tree. But then in 1980, a Stanford ecologist, Harold Mooney postulated that it was not from the upward direction that the water came, but rather from the downward direction. The roots were bringing up the groundwater, and then spreading it forth into the soil. This was the hydraulic lift hypothesis. [1]

In the semi-arid climate of the Great Basin in Utah, sagebrush grows in loam-skeleton soil, soil that sits on beds of alluvial gravel. Two ecologists, Richards and Caldwell set out to measure experimentally the hydraulic lift hypothesis. [2] They found that the sagebrush's roots were indeed bringing up groundwater and spreading it around to the surrounding soil. And not only that, when they put a special isotope of water around the sagebrushes roots, they found that that isotope then spread to neighboring plants. It was curious tree behavior because one might think a tree would keep all its water to itself, and not pass any into the surrounding soil, or to other trees. Rather the ecosystem seemed to be in a state of cooperation, passing the water around to keep all species hydrated. If the whole ecosystem survives better then individual species do too. Its the concept of group selection.

Mooney, Richards, and Caldwell thus helped open a new field of scientific study, that of hydraulic redistribution. [3,4]

In the Amazon are broadleaf tropical rainforests nestled around rushing rivers in the South American continent. It experiences a dry season 3 months of the year. The trees bring the water from the surface down deep into the ground during wet season, and then bring the groundwater back up during dry season to hydrate the landscape. Hydraulic redistribution was thus a seasonal phenomena. [5]

{In the above diagram (i) the left picture shows water being drawn up and then passed out sideways (ii) the middle picture shows water being transported laterally across the soil (iii) water is sucked up from the soil and passed downwards}

Researchers have found over a hundred types of trees capable of hydraulic lift. Each one with its own specific rate of lift. In the North American Pacific Northwest Douglas Firs could bring up groundwater at the rate of around .07mm a day, New England sugar maples were shifting around .2 mm a day, East Africa Umbrella Thorns were bringing up .08mm a day, the Sierra Nevada forests in California were lifting .1 mm a day, the sagebrush of Utah about .8mm a day, and the Amazon rainforest about .2 mm a day. [3]

Some tree roots go quite deep, blue oak can go down to 80 feet. Others not so deep, Douglas Firs grow to 3 feet, sagebrush grow to about 5 feet, willow tree roots are just below the surface, preferring instead to grow laterally up to 100 feet. Others are of more medium depth, chaparral grow to 15 feet. Pines can vary from 3 feet to 75 feet. [6] 

Even if the groundwater table is quite low, the presence of a tree with deep roots can draw up that water, and then pass it to other trees and plants.

In areas where there is wildfire risk, and even if the groundwater is quite low, if you have tree roots that go deep as the water table, then the groundwater can help hydrate against fire. NASA research shows that soil moisture decreases fire risk [7] In addition, the vegetation transpires the groundwater into air, thus cooling the landscape, two to nine degrees Fahrenheit cooler, in a process similar to how sweating cools our bodies. Dry air can become somewhat more humid. Increasing groundwater is thus important for fire prevention even if the water tables are low.

.....

To understand more of what is going below ground, lets take a look at the historical process of landscape formation, and the dance of water with geology.

Over millions of years tectonic plates collide. Ground crumples upwards, and volcanoes spewed magma forth. Some magma cooled quickly forming volcanic rock and basalt. Some magma cooled slowly, allowing minerals to precipitate out, and then integrate with rock, creating granite. 

Weathering by rain and rivers eroded the rock to form sediment, and some of this sediment was compactified again to form sedimentary rock, eg limestone and sandstone. Different layers of rock bed would form over millions of years. Some of the layers were more pervious to water, some less.

Soil then began to form on top of the rock. Fungi and bacteria produced an enzyme that breaks down pebbles to mine for nutrients. Plants used sunlight, to help turn the minerals and air into complex organic molecules. Their roots exuded sugars, carbohydrates and a little protein. A kind of flour. It fed the fungi and bacteria. Protozoa, nematodes, micro-arthropods, and earthworms ate the fungi and bacteria, and then pooped out nutrient rich poop, which the plants absorbed for their growth. On the plants death, decomposers broke down complex organic material into simpler compounds containing nitrogen, calcium, and phosphorus. The organic material would mix the various sizes of decomposed rock - clay particles that were less than .002mm, silt particles that ranged from .002-.05mm, and sand particles from .05mm-.2mm.

The soil thus created, acted like a sponge, to absorb the rainfall. The more organic matter, the more sponge-like the topsoil becomes. Every extra bit of organic matter increases the number of pores in the soil, which can then fill with water.

The rainfall then gets passed down through the soil to the bedrock below, there the water can seep through fissures and cracks in the rock. It can flow though sedimentary rock like limestone and sandstone. Sandstone is made of sand particles with pores inside it, which can fill with water, as much as 20% water. The water moves slowly through these fissures, cracks and pores, taking days, months, years, millennia to go through different pathways. It then comes back out in the form of springs and rivers.

Groundwater in the bedrock may seem far away from the surface of the earth, and out of reach of the plants high above it. And it was thought for a long time that roots could not find its way down into bedrock. But in 2013 hydrologist Danielle Rempe looked into California landscape and found tree roots finding its way down into the bedrock through fissures and cracks. Her team calculated that a quarter of trees and shrubs in the US regularly tap into the bedrock layer for water, and in California and Texas over 50% of the groundwater used by trees comes from the bedrock layer. [8]

In Nevada county in California, which is at the foothills of the Sierra Nevada mountain range, the soil can go down to about 3 to 7 feet when it then turns into bedrock. Different wells in the area show groundwater depth from 6 feet to 100 feet. (You can use this United States Geological Survey database to see the water level in wells all over the USA) The blue oaks, white oaks and the chaparral are three plants in the counties ecosystem that can drill down into the bedrock to bring water up, and then pass that water around to hydrate the soil and environment. [9,10]

Groundwater can come from rain, and it can also come from rivers.

The pulsation of the rivers also helps to fill groundwater. River flow obeys a power law like earthquakes do. They are big flows that are orders of magnitude bigger than normal river flow. During those large flows, the banks overflow into the floodplains to create wetlands. The wetlands water then sinks slowly through the soil, and into the bedrock below.

The groundwater can keep rivers running during dry season by feeding it water. So there's a coupling of rivers and groundwater. During large rains and floods the rivers help fill groundwater, and during the dry season, groundwater can help rivers keep flowing. 

In North America beavers can create dams that help some of the river water flow laterally into the floodplains.

....

Our civilization is depleting its groundwater quickly. California has lost 18 million acre feet, or 6 trillion gallons of groundwater over the last century. The Ogallala aquifer in the US Midwest is one of the biggest aquifers in the world. In the last century farming and urban usage has drained it of around 89 trillion gallons. In the Czech Republic 80% of the wells are experiencing mild to extreme drought. The Nubian aquifer that underlies Egypt, Sudan, Chad, and Libya is having water withdrawn at unsustainable rates. In the Caning Basin in Western Australia the groundwater is being used at the third highest rate in the world.

Its important we recharge our groundwaters. It can be key step in reducing wildfire risk, and helping cool our planet. Its what many places around the world depend for their water. In Europe groundwater supplies 65% of its’ drinking water, and 25% of its’ agricultural irrigation. In Libya the Nubian aquifer supplies 95% its water usage. In the US, 43 million rely on domestic well water for their drinking water.

Groundwater recharge can happen by building up our soils again. However modern farming degrades the soil with its pesticides and chemical fertilizers. It is regenerative agriculture and agroecology practices instead, that can help increase the health of our soils. In the wilds and on our lands, we can grow a biodiversity of native plants and trees, help the microbiome and fungi grow, encourage insects and other decomposers, use compost teas. These all help increase the soil building engine. In doing so our soils become the sponge that guides the rainwater down.

We can build small check dams and swales to help catch the rainwater and guide it into the soil. Here in the photos below is an example of swales built on a property in the southern Sierra Nevada mountains of California, that over 3 years increased the groundwater enough so that it could bring back a dry stream bed back to flowing water life.

We also need to halt urbanization and the paving over of nature with concrete and asphalt, and instead depave and rewild, to create more soil once more.

We also want to restore our rivers, take out dams, levees that are too close to the river bank, so that rivers can overflow into floodplains once again, and recharge groundwater below.

We can form watershed wisdom councils in our neighborhoods which bring together neighbors to educate each other about our water. And these groups can work together to build swales, check dams, rain gardens, curb cuts, to help each other create healthier soil on their lands.

And we can tap into the momentum that is happening on the local, state, and national government levels. So for instance in Grass Valley, which is at the base of the Sierra Nevada mountains in California, the local government recently gave a grant to create a permeable road which would allow the rainwater to infiltrate into the landscape. And at the California state level the Sustainable Ground Water Management act, SGMA, is working to help local communities to recharge their groundwater.

Past issues of this newsletter can be seen at https://climatewaterproject.substack.com

You can subscribe for free, or for a donation of $5 a month which helps support this newsletter and various regenerative water projects.

References:

[1] Mooney, Harold A., S. L. Gulmon, Phil W. Rundel, and James Ehleringer. "Further observations on the water relations of Prosopis tamarugo of the northern Atacama desert." Oecologia 44, no. 2 (1980): 177-180. https://link.springer.com/article/10.1007/BF00572676

[2] Richards, James H., and Martyn M. Caldwell. "Hydraulic lift: substantial nocturnal water transport between soil layers by Artemisia tridentata roots." Oecologia 73, no. 4 (1987): 486-489. https://link.springer.com/article/10.1007/BF00379405

[3] Neumann, R.B. and Cardon, Z.G. (2012), The magnitude of hydraulic redistribution by plant roots: a review and synthesis of empirical and modeling studies. New Phytologist, 194: 337-352. https://doi.org/10.1111/j.1469-8137.2012.04088.x

[4] Horton, Jonathan L., and Stephen C. Hart. "Hydraulic lift: a potentially important ecosystem process." Trends in ecology & evolution 13, no. 6 (1998): 232-235. https://www.sciencedirect.com/science/article/abs/pii/S0169534798013287

[5] Oliveira, R.S., Dawson, T.E., Burgess, S.S.O. et al. Hydraulic redistribution in three Amazonian trees. Oecologia 145, 354–363 (2005). https://doi.org/10.1007/s00442-005-0108-2

[6] Stone, Earl L., and Paul J. Kalisz. "On the maximum extent of tree roots." Forest ecology and management 46, no. 1-2 (1991): 59-102. https://www.sciencedirect.com/science/article/abs/pii/037811279190245Q

[7] “NASA Tracks the Link between Soil Moisture and Fire Susceptibility in California” Sept 24, 2020 Nasa Earth Applied Sciences

[8] “Trees Drill into Deep Bedrock for Water Surprisingly Often” Scientific American Dec 1, 2021

[9] Hahm, W. J., Rempe, D. M., Dralle, D. N., Dawson, T. E., & Dietrich, W. E. (2020). Oak transpiration drawn from the weathered bedrock vadose zone in the summer dry season. Water Resources Research, 56, e2020WR027419. https://doi.org/10.1029/2020WR027419

[10] Ishikawa, C. Millikin, and C. S. Bledsoe. "Seasonal and diurnal patterns of soil water potential in the rhizosphere of blue oaks: evidence for hydraulic lift." Oecologia 125, no. 4 (2000): 459-465. https://link.springer.com/article/10.1007/s004420000470