The quest to figure out the origin of rain : part I

A small water cycle and precipitation recycling journey

When I got into this water field, I found myself amongst folk that were proposing an alternative worldview of water. While the mainstream worldview was saying that rain only came from the oceans, and that global warming was only due to greenhouse emissions, the alternative worldview was centered around a concept called the small water cycle, and it proposed that i) rain comes from both the ocean and from evapotranspiration from the land, ii) trees were helping to create the rain, iii) evapotranspiration was helping cool the surface of the earth (although it didn’t clarify for the most part whether this was a local cooling or global cooling). It posited a different origin story for rain. A lot was at stake in which origin story was true, because if soil and trees were creating rain then this explained how humans were creating droughts, deserts, and water scarcity by paving over the land and chopping down trees. And if rain cycled around in a small water cycle, that explained the loss of a portion of the earths ability to evaporatively cool itself, when humans destroyed this moisture feedback loop.

This worldview was well encapsulated in the booklet “Water for the Recovery of the planet, the new water paradigm”, published in 2007 by Michal Kravcik, Jan Pokorny and colleagues. It was promoted by Judith Schwartz’s books “The Reindeer Chronicles” and “Water in Plain Sight”. Charles Eisenstein discussed it in his book “Climate”, Walter Jehne explained it in his videos, Zach Weiss spread it on his social platforms and in his water restoration trainings, David Maher got the word out to his watershed crowd, Ananda Fitzsimmon articulated it simply in “Hydrate the earth”, Erica Gies describes it in “Water always wins”, hydrologist Sieger Burger wrote about it in his blog, Stefan Schwarzer discussed it in his climate landscape essays, the ‘Water Man’ of India, Rajendra Singh, counselled the masses about it, Neal Spackman explained it while doing his desert regreening projects, and a handful of seemingly renegade climate scientists, Millan Millan, Antonio Nobre, Anastasia Makarieva, and Victor Gorshkov expounded on this to the public and the government alike.

The subculture of this ‘alternative’ water paradigm world, which included in their mix many permaculturists, ecosystem restorationists, regenerative agriculturists, maverick hydrologists, and nature-based solutions people, believed that climate scientists, with a few exceptions like the above mentioned, somehow had missed discovering this crucial truth about ecology, water and climate. Faced with impending ecological and climate chaos, this seemed like an exasperating situation.

I delved into the scientific literature to see what was going on, and discovered, excitedly, that were a number more climate scientists that were studying this ‘alternative’ worldview than the initial handful I had heard of. The scientists were calling the small water cycle by the names precipitation recycling, a quirk of historical nomenclature, that hindered the spread of the climate scientists research to the eco crowd. A while later I decided to map out more comprehensively the research that was being done in this field, and took an even deeper dive into world of meteorological journals.

I was surprised at what I discovered. There were hundreds and hundreds of atmospheric scientists studying this phenomena, and a plethora of papers researching precipitation recycling. I was especially shocked to find that the two of the most central figures in the carbon greenhouse movement, Syukuro Manabe, who had won the Nobel for his carbon climate model, and Jule Charney, who’s Charney report had exploded the carbon emissions issue to the world’s attention, had themselves done key groundbreaking work in this field. Manabe and Charney were amongst the first scientists to find in their climate models that soil moisture and vegetation affects the rain, and that the small water cycle emerges from the earth’s natural conditions.

What was going on? Why wasn’t this more widely known? I didn’t find any popular science account of this climate science precipitation recycling story. You won’t find a book about it on the shelf of your local bookseller. This water paradigm was there though, enmeshed in the core evolution of meteorology and atmospheric science, hidden, and sometimes not so hidden, in the ever more sophisticated global climate models. I poured through papers, going through probably more than a hundred, to figure out what had happened historically. I was to find that the climate scientists had fleshed out the many dimensions of the small water cycle to a surprising depth, and unveiled unexpected connections of it to many other climate phenomena.

The story of the origin of rain and the story of the small water cycle is full of twists and turns - it begins with people believing that rain comes from the land, to switching to think that rain comes from the ocean, and to then, as the climate models become more sophisticated, to believing that it comes from both land and ocean. It metamorphizes from people believing in precipitation recycling, to being against it, and to then being for it again.

……

Up until the 1930s it was generally thought that rain came from evaporation in the local area. US senators were debating where to put a man-made lake, so that the evaporation from it could come back down to give water to Arizona cities.

At that time, weather stations had accumulated enough data so that it was known that two thirds of the rain that descended onto the land went back up again as evapotranspiration. The question that then arose was - does that evapotranspiration then become part of future rains that fall on the land, or would it blow out to the ocean? It was thought at the time, that most of that evapotranspiration would come back down over the land.

Benjamin Holzman, a US meteorologist, though, had a different opinion. He argued that most of that evapotranspiration would blow back out to sea. The rain we got, he said, was from the ocean, arriving on maritime air masses. Air masses could be classified as maritime when they came from the Pacific or Gulf of Mexico, or continental, when it came from Canada, Artic, or the southwest. Continental air masses were very dry, so as they blew across the US, they would absorb all the evapotranspiration without creating rain. It was only when they blew back out to the ocean, that they finally accumulated enough moisture to rain. [Holzman 1937]

Two Johns Hopkins meteorology professors, George Benton and Mariano Estoque would also be of the opinion that rain came from the ocean. They argue that just because there is a lot of evapotranspiration, that does not mean there is enough vertical motion to turn that water vapor into rain before the horizontal velocity carries it out to sea. They estimate that in the Mississippi Basin 10% of the rain comes from the land, and 90% from the ocean. [Benton, Estoque 1950]

Across the ocean, scientists were also analysing the origin of the rain.

Mikhail Budyko was confident, edgy, and not afraid to ruffle feathers. Possessing a wide scope of knowledge, the Russian meteorologist had the ability to sense what key questions a branch of science should tackle, and how answering those questions could birth new subdisciplines. He was particularly interested in foundational questions of energy and the earth like what was driving the circulation and the thermodynamics of the atmosphere. He wanted to understand how the sun radiated its energy onto the earth, how soil and water transformed that energy, and then how that energy heated up the lower atmosphere, propelling its circulation. Metaphorically speaking, he was interested in how the stove flame heated the metal of the pot, and how that metal then transferred its heat to the water in the pot.

In 1948 he is researching the thermodynamics of water evaporating from the earths surface and pens a monograph “Evaporation under natural conditions”. Then, along with his Leningrad University colleague O.A. Drozdov, he turns his attention, a couple of years later, to the small water cycle. To study this they create a simplified one dimensional mathematical model. Beginning at the sea’s edge and then moving inland, the air will pick up at each stage some amount of evapotranspiration and release some amount of rain. On the ground, some amount of the rain goes into the land, and some amount becomes runoff. The sun helps drive the small water cycle by driving evapotranspiration and convection. In wet climates, the more sun, the more evapotranspiration. In dry climates, most of, or all the water, evaporates. As the water evaporates it cools the earth. All this information will be packed into what will later be called Budyko’s equation and Budyko’s framework. [Budyko Drozdov 1953] [Budyko 1971]

The equation captures how the water moisture hops inland, how the rain amount changes as it progesses, and how it cools the land as it goes along. The percentage that the evapotranspiration makes up the rain is called the precipitation recycling ratio. Budyko, though he doesn’t have enough real world data to input into the equation at the time, makes an estimate of the amount of precipitation recycling in the Soviet Union, 4% in October, 18% in April, with an average of 10%. It is a somewhat small number, so Budyko professes the opinion that most of the rain comes from the ocean.

In 1958 Budyko writes a book Heat Balance at the Earths Surface that quantifies the many complexities of how the sun drives the atmosphere, and his work finds it way from the Soviet Union to the West despite the cold war. It helps birth the discipline of physical climatology. Mapping out this field of knowledge and its impact, his book observed that “Investigations of the heat balance at the earth’s surface are now occupying an important place in all hydrometeorological disciplines.” [1958], a heat balance that plays a role in the origin of rain, in the driving of, and in the after effects of the small water cycle.

In the decades to come, meteorologists will accessorize and adapt the Budkyo equation so it can better model the real world complexities of land and air, they will integrate it in different ways into global climate models, and they will have more accurate data to feed into it. In so doing the precipitation recycling ratio that the equation outputs will increase, and climate scientists will evolve, over time, to the opinion that rain also comes from the land, not just from the ocean. Budkyo and Drozdov’s work laid the framework for future scientists to discover how the small water cycle affects the heat distribution on earth, how it affects mesoscale and large scale atmospheric circulations, and how it can change weather nonlocally through teleconnections.

Observations of human impact on rain

Humans have been destroying forests, restoring land, and moving water around to irrigate to a large degree the last century. The question arises of how this land use changes affects the rain. The answer to this question will give clues to the origin of rain.

Numerous researchers have studied the impact of deforestration on rain. Rain loss followed deforestration in many places in India, in Chota Nagpur [Ranganathan 1949] - in Udhagamandalam [Legris & Blasco 1969], in the Nilgiri district [Padmavalli 1976], in the Ranchi plateau [Warren 1974], and in Kerala [Soman 1988]. Rain loss followed deforestration in Guanacaste, Costa Rica [Fleming 1986]. In Gambia and Senegal [anon 1974] deforestration lead to more erratic rainfall. Many cases like these are documented and reviewed by the Indian meteorologist V.M. Meher-Homji in his paper “Probable impact of deforestation on hydrological processes." [1991].

In 1964, Israelis moved water from Lake Tiberias in the north, with its Mediterranean climate, to grow agriculture in the dry arid southern desert region, where it is brown, rocky and dusty, with rainfall that is only 100mm (4 inches) a year. Researchers came into track what repercussions this had, and found a decrease in wind and temperature variations, and a large increase in October rains, which occur at the onset of the wet season [Otterman 1990].

These above examples give more credence to the theory that forests and human land use change is playing a role in the creation of rain, but single case observational studies are only a suggestion of what is going and not conclusive, because there are many processes around the world that could be affecting the rain in any given locale, from ocean surface temperature changes to jet stream blocking to greenhouse global warming.

It would be better if we can find two or more areas of land in the same location with differing land use and vegetation, and see if they have different rainfall.

One example is on Marajo Island, a flat island the size of Switzerland, off the coast of Brazil. The eastern half is savannah and wetlands where domesticated water buffalo roam. Because the soil is salty it makes it hard for trees to grow. The eastern has a severe and long dry season. The western half is a jungle where night monkeys, savannah foxes, bats, armadillos, and chalk browed mocking birds thread their way through acai palm, kapok tree, and ant trees. Thickets of shrub manatee bush line the rivers, wetlands there are home to a variety of sea birds. On the western half, there is daily rain, punctuated by thunderstorms that drench the forests. [Friedman 1977]

Another example is in the Western Ghats in India, which runs down the south west side of India. Nestled in the series of undalting mountains are montane rainforests, lush and multi-storied, covered with climbers, epiphytic orchids and ferns, and housing a treasure trove of wildlife - lion-tailed macques, sloth bears, nilgiri tahrs, and tigers. Logging and conversion of land for agricultural uses has led to the loss of some of these forests. Meher-Homji (1980a, 1980b) researched there, and found that the land that had been deforestrated had declining rainfall, and that the land that had not stable rainfall.

A third example is in Yunan, where the hilly topography houses tropical forests full of banyan trees, jackfruit, and the ‘Sky Tree’, which can grow to 80 meters high, a tree people come from all round China to see. The rainforest is so thick some of the plants will flower on trunks and stems. In Yunan forests have been cut down for rubber plantations and other tropical agricultural cash crops. One area that was deforested was compared by researchers with two other areas that were within 100km of it. The untouched forests did not show rainfall changes, while the deforested area had rainfall decrease by about 4%, while also experiencing more extreme rainfall that subsequently lead to more floods. [Zhang 1986]. Its possible that forests may bring more rain in general, while also lessening larger rains, thus acting as a buffer against extreme weather.

If you listen to older folk in the Colombian Andes, in African towns, in the Australian outback, in Indian villages, in locales throughout the world, many of them will tell stories and anecdotes of rainfall disappearing as forests turn to farms, as urban sprawl takes over rural land. Meanwhile, researchers have also been documenting this loss, working to make these anecdotes more rigorous. The voices of the people, and the data sets of scientists, are a clue to the origin of rain.

Part II of this post is here

……….

There are a lot of parts and drama in this story. This is the beginning of the story. Its taken me quite a lot of time to dig through the history, understand the papers, write a cogent summary of them, and report back.

Do you think the origin of rain and the small water cycle would make a good book?

If you would like to read some of the original papers you can go to google scholar to find the doi number, then input the doi number to the website sci-hub.ru to get the original paper to read.

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