The missing link: groundwater creates rain

Groundwater creates rain.

This is a process that science has begun to show is real in recent years.

Its an important process to highlight, because while the world has been blaming extreme droughts solely on global warming, the worldwide practice of draining our aquifers is likely also a contributing factor to the severity of droughts.

We have been depleting aquifers around the world. The Great Plains in the US has been draining the massive Ogallala aquifer. Central Valley in California and Mexico City are sinking from the draining of the aquifers. Europe has been losing 84 gigatonnes of groundwater a year. The Indo-Gangetic groundwater basin in India has reached tipping point depletion levels. The draining of our aquifers is probably also depleting our rain, and thus contributing to water scarcity.

If the water level in the aquifers are too low, then the tree roots cannot bring up the water to create precipitation recycling/small-water-cycle. We need a base level of groundwater in our continents in order to have these feedback loops.

Groundwater tends to be out of sight, out of mind. Most people have no idea of its importance. And the topic of groundwater’s influence on climate has so far been an underground research topic. Its time to transpire its nascent insights to the world.

Harold Mooney was the first scientist to postulate - (some indigenous peoples probably had already figured this out) that tree roots were bringing up water from the aquifers to hydrate the surrounding soil in a process called hydraulic redistribution. This was then experimentally observed by the ecologists James Richards and Martyn Caldwell. (See previous article “Secrets of groundwater” )

By tracking the flow of water through the ecosystem and atmosphere through its isotopic chemical signatures, Inez Fung, and her Hydrowatch group at Univ. of California Berkeley, found that trees were bringing a lot more water up during the drought season than had been previously thought. Using hydroclimatological models Fung and colleagues found that the groundwater lifted upwards by the trees increased transpiration by about 40% in the Amazon. [Lee, Fung 2005]

The hydrologist Laura Condon at the University of Arizona, notes “Groundwater depth .. is an important controlling factor for soil moisture, which in turn controls evapotranspiration.” [Condon 2013]

Using numerical simulations, Xiang Yu Luo, and colleagues at the University of Pittsburgh, studied how groundwaterwater was brought up in arid climates around the world. In their paper on the topic they wrote “We have found that hydraulic redistribution is particularly important in water-limited climates under which plants have high transpiration demand. At the beginning stage of a dry period, hydraulic redistribution modulates the severe impacts that climate has on plant transpiration. Only after a prolonged dry period, impacts of hydraulic redistribution are lessened when the groundwater table drops below the depth of water uptake by roots and are diminished when plant transpiration is decoupled from groundwater dynamics.” [Luo 2016]

Francina Dominguez, from the University of Illinois, and her colleagues studied the groundwater in the La Plata basin in South America, which is home to the Amazon. They looked at climate models that did and didn’t include groundwater, and found that “Including groundwater leads to an increase in precipitation over parts of the central and southern La Plata basin during the early rainy season (October–December). The overall increase in evapotranspiration and precipitation over the southern La Plata basin during the early rainy season is 13% and 10%” [Martinez, Dominguez 2016]. Francina Dominguez notes that these are theoretical climate models, we don’t actually know the actual percentage that the groundwater contributes to the rainfall, and still need to do isotope studies to track chemically as the water moves from aquifer to precipitation.

Michael Barlage, at the National Center for Atmospheric Research, wrote “Groundwater can play an important role in the water balance of the land/atmosphere interface. However, its role in weather and climate models has often been ignored due to the complexity of including a reasonable representation in the land-surface model employed.” Barlage and colleagues figured how to include groundwater in their climate models. When they turned the models loose on the Central US, they found the inclusion of groundwater would increase by 6 inches the amount of evapotranspiration, especially in the summer months of July and August. They also found interestingly enough, that the groundwater would also reduce by about 3 degrees Celsius the summer temperatures in the central US. When they included groundwater in their simulations, it led to a better predictions of the meteorological data for the Central US. [Barlage 2021]

Its important to increase the amount of groundwater. There are ways to recharge our aquifers, for instance by flooding farmland during particularly wet seasons. ( See previous article “The plan to replenish our groundwater”). Or by digging earthworks, reforestation and restoring wetlands to capture rainwater. (See previous article “India’s regenerative water movment”). Its key to be able to get the rainwater from large storms and atmospheric river dumps to infiltrate downwards, instead of so much becoming runoff (see “California flooding: a water principles perspective”. This requires many concepts from the slow water movement and the sponge city movement, restoring soil, restoring floodplains, depaving, unchannelizing rivers so water can overflow into neighboring wetlands etc. It requires being a lot more thrifty in some places with water for awhile to get groundwater back up. The more useful places to increase groundwater are those where it can still impact how much tree roots can reach the water. In those places it may take months, years, or in some places decades, to get groundwater levels back up. As we do this, we lessen wildfires, we make our water supplies healthier, and we increase food security.

References

Barlage, M., Chen, F., Rasmussen, R., Zhang, Z., & Miguez-Macho, G. (2021). The importance of scale-dependent groundwater processes in land-atmosphere interactions over the central United States. Geophysical Research Letters, 48, e2020GL092171. https://doi.org/10.1029/2020GL092171

Condon, L. E., R. M. Maxwell, and S. Gangopadhyay (2013), The impact of subsurface conceptualization on land energy fluxes, Adv. Water Resour., 60, 188–203, doi:10.1016/j.advwatres.2013.08.001

Luo, Xiangyu, Xu Liang, and Jeen‐Shang Lin. "Plant transpiration and groundwater dynamics in water‐limited climates: Impacts of hydraulic redistribution." Water Resources Research 52, no. 6 (2016): 4416-4437

Jung-Eun Lee, Rafael Oliveira, Todd E Dawson, Inez Fung “Root functioning modifies seasonal climate” November 21, 2005 102 (49) 17576-17581 https://doi.org/10.1073/pnas.0508785102

Martinez, J. Alejandro, Francina Dominguez, and Gonzalo Miguez-Macho. "Impacts of a groundwater scheme on hydroclimatological conditions over southern South America." Journal of Hydrometeorology 17, no. 11 (2016): 2959-2978