How much land do we have to restore to bring back the rain?

The question everyone seems to want the answer to.

One of the most commonly asked questions when people learn that trees can bring rain is: how much land do we need to restore to bring back the rain?

First, it’s important to understand that rainfall depends on many factors, including how atmospheric moisture is transported into and out of a region. We also need to consider local humidity levels and whether evapotranspiration can raise humidity enough to cross the threshold for condensation. Additionally, it’s crucial to clarify whether the goal is to increase rainfall locally or farther downwind. Increasing rainfall hundreds of miles away can still benefit the original area, since rivers fed by that rain may flow back, replenishing local water sources.

Meteorologist Millán Millán spent many years studying how the destruction of forests and wetlands in Spain was causing the country to lose its rainfall. By gathering first observations from community meetings and local knowledge, and then using more sophisticated observations, sensors, and climate models, he tracked where water vapor was rising, where clouds were forming, and where rain was—or wasn’t—falling. In this way, he developed a detailed understanding of the micro-meteorological processes that produce rainfall. Millán also examined similar patterns in other regions around the world. Based on his research, Millán estimated that in relatively flat terrain, around 10 km × 10 km (6 miles × 6 miles, or roughly 25,000 acres) of restored forests and wetlands would be needed to regenerate local rainfall. In valleys, he noted, a smaller area of restoration could be sufficient.

More precise studies have since added weight to Millán’s estimate. Atmospheric scientist Ronny Meier led a team of atmospheric scientists in 2021 to study how land use affects rainfall across Europe, with its largely temperate climate. They looked at cells roughly 11 km × 7 km in size and found that areas with 20% more forest cover experienced significantly more rain. Their 2021 paper, "Empirical estimate of forestation-induced precipitation changes in Europe," examined 1,512 sites across the continent and confirmed that forest cover has a tangible effect on local rainfall. Their study area was similar in scale to what Millán had proposed. Their findings suggest that restoring even 20% of a 10 km × 10 km area could be enough to increase rainfall. It's possible that even interventions at a smaller area could have an effect, but their research did not study those smaller scales.

In 2019, climate scientists Oliver Branch and Volker Wulfmeyer from the University of Hohenheim developed a climate model to study the effects of large-scale Jojoba tree plantations on regional rainfall. They simulated the impact of planting a 100 km × 100 km area of jojoba trees in the deserts of Israel and Oman. The simulation included movement of water through the soil, and transpiration rates of the jojoba trees. Their findings showed that such plantations would indeed enhance rainfall, but not enough to fully sustain the trees themselves. The trees would induce a pressure change which caused the wind to converge on the plantation. The friction of the trees would create turbulence, and facilitate updrafts that helped to create the rain. Given the extreme aridity of these regions, significantly more evapotranspiration would be required to raise atmospheric humidity to the point where water vapor could condense into clouds. This suggests that a much larger area would likely need to be restored to trigger consistent rainfall in desert environments. The study did not explore whether smaller plantations could have a similar effect.

Anecdotal cases offer additional insight into the scale of local restoration needed to influence rainfall. In northern Mexico’s Chihuahuan Desert, decades of overgrazing had left large areas with compacted soils and sparse vegetation. In Water in Plain Sight, Judith Schwartz describes visiting farmers who used holistic grazing practices to restore a combined area of approximately 32 km × 32 km—or about 260,000 acres—across three properties. During the dry season, rain often fell on their restored lands while neighboring, still-degraded areas remained dry. This suggests that restoration efforts on scales smaller than 32 km × 32 km can affect local rainfall patterns, even in arid environments. Compared to the jojoba plantation simulation, one might also consider that regenerating grasslands could be more appropriate for such climates. Jojoba trees do not restore the soil between the trees that well, and the soil remains hardened. In arid climates, grasslands and savannahs may thus restore soil health more effectively, enhancing infiltration and potentially supporting more sustained rainfall through improved soil moisture dynamics. When there is more rain, trees become the better way to restore rain. One of the qualities of a tree is that it can slow the air above, creating a turbulent layer, that slows the water vapor molecules so they have a greater chance of nucleating into a cloud droplet.

A study of rainfall patterns in western Karnataka, India, which has a subtropic climate, by researcher P.S. Meher-Homji in 1979 gives some clues too about how much land use will affect rain. They analyzed the data from 29 rain gauge stations, spaced at 40 km or more apart. These stations were situated in forested and deforested areas. They found that deforestation led to a measurable reduction in rainfall, except for stations on the coast that still received moisture from the ocean. This suggests that there is a land scale less than 40 km × 40 km that affects rain. [Meher-Homji 1979]

Terrain matters to how much land one needs to restore. In a valley, the atmospheric moisture may be partially blocked from escaping by nearby mountains. In those cases, less land needs to be restored before rain comes back. Rajendra Singh, known as the "Waterman of India," has found that for valleys, restoring areas as little as just 1.5 km × 1.5 km to 2.8 km × 2.8 km (600 to 2,000 acres) can bring rainfall, thanks to enhanced water retention and terrain effects.

In India’s Maharashtra’s Paani Cup initiative, 8000 villages built swales and ponds to capture and infiltrate rainwater. After recharging their aquifers, they began replanting forests. The land size varied of these villages, but they were approximately 2.8 km × 2.8 km (2,000 acres). With that many villages doing restoration, villages downwind will get the benefit of restoration from villages upwind. Many of these villages were located in mountainous areas, and so may have benefited from the valley effect. The villages that kept rain data found an increase in their rain. However, lacking comparison to rain data from villages nearby that did not restore their lands, it cannot be determined for sure if it was global warming or other long-term climate fluctuations that caused the increase in rain. If neighboring villages had found less rain, or the same amount of rain on their lands, then we could more assuredly attribute the restoration work to the increase in rain.

An especially striking case involves Ernst Götsch, a Swiss agronomist who restored about 2.2 km × 2.2 km (1,185 acres) of degraded land in Bahia, Brazil. Originally logged and compacted into pasture, the land had become severely degraded. Götsch applied a method he developed based on ecological succession and plant diversity, a method which would become known as Syntropic Agriculture. Over time, his land regained fertility, refilled aquifers, and began to support year-round streamflow for seventeen streams that previously only ran for part of the year. During regional droughts, his land still received rainfall, while neighbors’ land did not. Visitors to his land talked about seeing clouds over his land while neighboring properties remained cloudless.

[Gotsch’s land in Bahia, Brazil]

A question arises: why was Götsch’s land, not being in a valley, able to restore the rain at these smaller scales, at about 1/20th the size of Millán’s estimate? The answer probably has to do with the rich biodiversity the method of Syntropic Agriculture managed to produce. (Syntropic Agriculture would later gain worldwide fame for its rich biomass, and people all over the world would start practicing its methods.) The biodiversity creates richer soil and more land cover, which means the land is better able to absorb rainfall and refill aquifers. This keeps streams running and keeps riparian vegetation hydrated into the dry season—an example of what is called slow water. Replenished aquifers could increase rain via trees bringing up the groundwater and evapotranspiring it. Millán’s 10-by-10 kilometer estimate may not have accounted for aquifer effects. The essence of the slow water concept is to capture wet-season water in aquifers to extend its availability into the dry season. Consider that a substantial portion of a wet-season storm could be evapotranspired back up in the dry season to create rain.

Another reason why Götsch’s land might have succeeded in creating rain at smaller scales than Millán’s estimate was that it may have had an abundance of fungi spores, tree terpenes, and bacteria, that could float into the sky, and then nucleate water vapor into clouds and rain.

Land restoration is one way to bring back rain. River and aquifer restoration can also bring back rain. Check dams, leaky weirs, and beaver dams can slow water in creeks, which then enables water to seep into neighboring floodplains and aquifers. Trees can then bring up and transpire that groundwater in the dry season to help create rain. Valer Clark and Josiah Austin placed 20,000 small rock barriers across the creek that flowed through their 1,800-acre land in Arizona. This caused the peak flow in winter to lower and increased river flow at other times by 28%. The creek flowed four weeks longer into the dry season. Vegetation became greener in the floodplains next to the creek. Aquifers refilled. How much of this kind of creek restoration is needed to bring back the rain has not been studied as much, but one could guess that placing enough slow water structures to impact the hydrology of 10 km × 10 km could bring back the rain.

This kind of restoration can also happen by releasing beavers into the wild, where they build dams that increase wetlands and restore aquifers. Beavers are now starting to be lauded by local governments and farmers for their land hydration restoration abilities. A question for researchers to study more is how much beavers could bring back the rain.

In summary: how much land do we need to restore to bring back the rain? It depends on local conditions. But we do have some studies and stories that offer useful estimates. We have a 100km x 100km simulation of a monoculture plantation in the desert, where it is harder to generate rainfall. There is an actual example of a 32km x 32km place in the desert that had rainfall when its neighbors did not. A research study done in a subtropical climate, looking at differences in rainfall at ranges of 40 km × 40 km found forests affected rainfall. A meteorologist's estimate puts the needed area closer to 10 km × 10 km. An atmospheric science research study shows that restoring just 20% of an 11 km × 7 km area in a temperate climate (Europe) can have an effect. In some cases, very effective restoration—rich in biodiversity and soil moisture, with aquifers refilled—might need as little as 2.2 km × 2.2 km in a tropical climate. In valley regions, where terrain helps trap moisture, even 1.5 km × 1.5 km may be enough, to restore rain.

2.

How do we go about restoring land and water systems? There are several important factors to consider. First, conserving existing old-growth ecosystems is essential. These areas provide irreplaceable ecological functions and biodiversity, and should be protected wherever they remain.

It’s also important to understand what the landscape used to be—whether it was originally forest, savannah, or wetland. This historical context offers guidance for what kind of ecosystem restoration is most appropriate. Forests are typically more effective at evapotranspiring water and helping to generate rainfall. However, in drier environments, savannahs or grasslands may be better suited. They can still absorb rainfall and return moisture to the atmosphere, but require less water to sustain.

Letting nature rewild itself can be one of the most effective strategies. Animals such as birds, squirrels, and monkeys naturally disperse seeds and help regenerate ecosystems over time. Where possible, allowing this kind of natural reforestation can be more sustainable than direct human intervention.

Often, the best place to begin is by restoring the water cycle. In Maharatra’s Paani Cup, the villages began by building swales and ponds. In Africa’s Great Green Wall project across the Sahel, some of the most effective reforestation efforts have used techniques like placing seeds in zai pits—small indentations that collect water—or half-moon-shaped swales that catch and retain rainfall. Practitioners like those from Water Stories—Zach Weiss, Nick Steiner, and others—typically begin restoration by first focusing on the water cycle, using terraces, ponds, and other features that slow rainwater and filter it into the ground to recharge aquifers.

[Half moon structures in Great Green Wall project, in Sahel, Africa. Picture from Andrew Millison’s Youtube video ]

A helpful framework for thinking about water management comes from Brock Dolman, who coined the phrase “Slow it, sink it, spread it” to describe effective hydrological restoration. For rain restoration, I’ve extended this phrase to:
“Slow it, sink it, spread it, lift it, hop it,” and added a second formula: “Lift ≤ Sink.”

The idea of “lifting” refers to how tree roots bring up groundwater, while “hopping” refers to how moisture is evapotranspired and later falls as rain—often nearby—through the small water cycle (precipitation recycling). Together, these steps describe a cycle: water is slowed, sunk into the ground, spread across the land, lifted by vegetation, and then transpired to become rain.

The phrase “Lift ≤ Sink” expresses a basic ecological balance: the amount of water lifted into the atmosphere must be less than or equal to the amount being sunk and stored—otherwise, the system becomes unsustainable. For example, in California’s Central Valley, it would be better to recharge aquifers, and grow trees or crops that lift water at a rate lower than the recharge rate, rather than planting high water-use crops like almonds and cotton, which have led to Lift > Sink conditions.

Another consideration is whether to use native or non-native species. Native plants are typically better adapted to local conditions, less likely to succumb to disease, and less likely to disrupt water cycles. Its a nuanced discussion though about which non-native species might be ok.

Closely related is the question of polyculture versus monoculture. Diverse, polycultural plantings improve soil health, absorb more water, and are more resilient to disease and fire. In Spain and Portugal, efforts to reforest with native species—such as those led by Rewild Spain—have proven far more sustainable than monoculture eucalyptus plantations, which have drained groundwater and increased wildfire risk. Similarly, in China’s Loess Plateau restoration project, areas where they planted native tree plantings were much more successful in maintaining groundwater levels, while areas where they planted non-native species often caused further drying, potentially leading to long-term problems.

Much of today’s farmland was once forest or wetland. In these cases, agroecological approaches such as agroforestry—where crops grow beneath tree cover—and wetland restoration can help recover both ecological and agricultural productivity. Techniques such as permaculture, syntropic agriculture, agroecology, and Natural Sequence Farming offer a wide range of tools for bringing degraded land back to life.

Finally, consider the surface itself. Impermeable surfaces like concrete and asphalt prevent rainwater from infiltrating the ground. Depaving urban areas can allow aquifers to recharge naturally. Where full depaving isn’t practical, replacing traditional pavement with permeable alternatives is a meaningful step toward restoring the water cycle.

Masanobu Fukuoka, the Japanese farmer and philosopher known for natural farming, said “It was in an American desert that I suddenly realized that rain does not fall from the heavens – it comes up from the ground. Desert formation is not due to the absence of rain, but the rain ceases to fall because the vegetation has disappeared.”

Let us restore the vegetation, and plant the rain.

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References:

Branch, Oliver, and Volker Wulfmeyer. “Deliberate enhancement of rainfall using desert plantations.” Proceedings of the National Academy of Sciences 116, no. 38 (2019): 18841-18847.

Keys PW, Wang-Erlandsson L, Gordon LJ (2018) Megacity precipitationsheds reveal tele-connected water security challenges. PLoS ONE 13(3): e0194311. https://doi.org/10.1371/journal.pone.0194311

Meher-Homji, V. M. "Repercussions of deforestation on precipitation in Western Karnataka, India." Theoretical and Applied Climatology 28, no. 4 (1980): 385-400

Meier, Ronny, Jonas Schwaab, Sonia I. Seneviratne, Michael Sprenger, Elizabeth Lewis, and Edouard L. Davin. "Empirical estimate of forestation-induced precipitation changes in Europe." Nature Geoscience 14, no. 7 (2021): 473-478

Millán, Millán M. "Extreme hydrometeorological events and climate change predictions in Europe." Journal of Hydrology 518 (2014): 206-224

Staal, Arie, Jolanda JE Theeuwen, Lan Wang‐Erlandsson, Nico Wunderling, and Stefan C. Dekker. "Targeted rainfall enhancement as an objective of forestation." Global Change Biology 30, no. 1 (2024): e17096