Climate and Water : Regenerative Water Alliance
The impact of the water cycle on our climate is significant. And the way we treat our vegetation, forests, wetlands, and soil has a large effect on the water cycle. We thus have an effect on the climate through out interactions with nature.
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We are shifting to a slightly different schedule for the Regenerative Water Alliance meetings. On the first Thursday of each month we will have a session of a certain topic (this months topic will be Climate and Water), followed by breakout groups of various types to network around certain topics. On the third Thursday of each month we will have a Watershed Wisdom Council meeting, where you can come learn how to set up a neighborhood watershed group to activate and do water projects in your local area. You will also network with other neighborhood watershed groups to support, learn, and share with each other.
This month’s Regenerative Water Alliance topical session will be on “Climate and Water”, with the discussion being led by Marcel Berg and Pieter-Paul de Kluiver. The session is at 9am Pacific Time on Thursday Aug 4th.
This month’s Watershed Wisdom Council meeting will be at 9am Pacific Time on the Thursday Aug 18th.
Both meetings will use the zoom link https://us02web.zoom.us/j/4387884267?pwd=Qk1Ob0RIbjhEQzBCcWpyazFXVG1QUT09
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Here is some reading on Climate And Water in preparation for our upcoming rwa (regenerative water alliance) meeting.
UNEP, the United Nations Environment Programme, which works to promote a coherent environmental agenda within the United Nations framework has noted the connection of water and climate.
From their UNEP Foresight Brief of July 2021:
”The continued destruction of forests, the deterioration of soils, the subsequent loss of terrestrial soil water storage and the reduction of water retention in the landscape are disrupting the movement of water in and through the atmosphere. This disruption causes major shifts in precipitation that could lead to less rainfall and more droughts in many areas of the world, increases in regional temperatures and an exacerbation of climate change. These changes affect regional climate, but can also impact regions far away. Understanding the interwoven relationships and the subsequent fluxes of energy between plants, soils and water on the ground, as well as in the atmosphere, can help mitigate climate change and create more resilient ecosystems.
Vegetation plays an important – and often neglected role in regulating the climate. Think of the difference between standing on a hot summer afternoon on a ploughed and barren field or in a dense forest. Clearly, the conversion of, for example, forests to cropland or urban areas brings major changes that can influence the climate. From the solar radiation reaching a densely vegetated field surface only 1% is used for photosynthesis and 5-10% heats the air (“sensible heat”). Over 70% of the radiation is used for transpiration by the plants, by which liquid water is transformed to water vapor, a very energy demanding process (“latent heat”). Counting non-vegetated and water surfaces, around 50% of the solar energy reaching the ground is used for evaporation and transpiration of water (“evapotranspiration”). As these masses of air rise into the atmosphere, the water vapor will eventually condensate and release the same amount of energy as consumed on the ground, some of it dissipating into space. The newly created clouds will reflect incoming solar radiation and are the
source of new precipitation.
Of the approximately 120,000 km^3 of water that falls on terrestrial surfaces as precipitation each year, around 60% comes from the ocean while 40% derive from land. 60-80% of this land-derived atmospheric moisture comes from transpiration by plants demonstrating the important role vegetation plays in feeding the precipitation cycle, as well as in transferring energy from the ground into the upper atmosphere.
Until recently, human impact on water vapour in the atmosphere was assumed to be negligible, compared to evaporation from oceans. However, the impact humans have on atmospheric water vapour stems from major human-induced land cover changes, not only from industrial emissions, as previously argued. These land cover changes indeed have a major influence on the atmospheric water vapour cycles. Almost half of the world’s forests have been lost since the beginning of agriculture (with most of the deforestation happening since 1950 and converted into much less vegetated fields. What impacts do these vast human-induced land cover changes have on the earth’s water and energy fluxes?
Every tree in the forest is a water fountain, sucking water out of the ground by its roots, pumping it through the trunk, branches and leaves, releasing the water as water vapour through pores in its foliage into the atmosphere. On a normal sunny day, a single tree can transpire several hundred litres of water, cooling its environment with a 70kWh of power output per 100 litres, which represents a cooling effect equivalent of two domestic air conditioners running for 24 hours. In their billions, the trees create giant rivers of water in the air (“flying rivers”) – rivers that form clouds and create rainfall hundreds or even thousands of kilometres away.
Globally, 40-60% of the rain falling over land comes from moisture generated through upwind, land evapotranspiration, mostly by transpiring trees. In some regions of the world, the share amounts to 70% of the rainfall. This recycling becomes more dominant further inland. Tropical evergreen broadleaf forests only occupy about
10% of the Earth’s land surface, but contribute 22% of global evapotranspiration22, highlighting their importance for the supra-regional water cycle. The typical distances that moisture evaporated from land travels in the atmosphere before it falls back to the land are on the order of 500–5000 km; the typical time scale ranges from 8-10 days. For example, moisture evaporating from the Eurasian continent is responsible for 80% of China’s water resources. The main source of rainfall in the Congo Basin is moisture evaporated over East Africa, while in its turn, it
is a major source of moisture for rainfall in the Sahel. The state of the West African rainforest is particularly important for the flow of the Nile. This explains why even in major river basins, including the Amazon, Congo and Yangtze, precipitation is more strongly influenced by land-use change occurring outside than inside the basin. Even in several river basins that do not span multiple countries, flows were considerably affected by land use in other countries.
Models show that local changes from forests or grasslands to croplands reduce their annual terrestrial evapotranspiration by 30-40%. On a global scale, land-cover change between 1950-2000 reduced annual terrestrial evapotranspiration by 4-5% or 3,000-3,500 km3, and increased surface water runoff by 6.8%. Scientists found, on the other hand, that increased vegetation has a cooling effect that comes from an increased efficiency in the vertical movement of heat and water vapor between
the land surface and atmosphere.
Satellite observations suggest that forests have a major influence on cloud formation, not only in the tropics, but also in temperate zones: disappearing forests can lead to significant decreases in local cloud cover and thus rainfall. Modelling has shown that the extensive global deforestation between the 1700s and 1850s resulted in a decrease in monsoon rainfall over the Indian subcontinent and southeastern China and an associated weakening of the Asian summer monsoon circulation. In the tropics, deep cumulus convection has been considerably altered as a result of landscape changes (mostly the conversion of forest to crop land). This not only affects local precipitation, but also has an impact over long distances through processes known as “teleconnections”. These teleconnections can have impacts at higher latitudes, which significantly alters the weather in those regions. Even relatively small land-cover perturbations in the tropics can lead to impacts at higher latitudes as for example
connections between the Amazon and northwest United States. Vanishing forests can also lead to less rainfall and longer dry seasons locally as reported for example from Rondônia in Brazil or Borneo, where it was found that the watersheds with the greatest forest loss have seen a 15% reduction in rainfall. In India, patterns of declining rainfall during the Indian monsoon matched changing forest cover in India, due to reduced evapotranspiration and subsequent decreases in the recycled component of precipitation. This demonstrates the large patterns of water vapour and precipitation flows.
Normally, more than 50% of the sun’s solar radiation reaching the earth’s surface will be converted by evapotranspiration into latent heat, which in turn gets transferred into the atmosphere, feeding the precipitation cycle, and partially radiating back into space. On bare surfaces, for example fallow fields, dry meadows (in the summer season and after hay harvest), and on concrete or asphalt surfaces, the soil will absorb more incident solar radiation, heat up, create sensible heat and emit, proportional to the fourth power of its absolute emperature (Stefan-Boltzman Law), heat power into the atmosphere. Surface temperature differences between these bare surfaces and forested areas can, based on a central European example, be as much as 20°C on summer afternoons. In the Indonesian island of Sumatra, temperature differences between forest and clear-cut land of up to 10°C were found, explained, again, by an evaporative cooling effect of forests, which outweighs the albedo warming effect generated by the darker forested surfaces. This highlights the fact that local biophysical processes triggered by forest losses can effectively increase summer temperatures in all world regions. Historical deforestation has indeed reduced the latent heat flux on land and increased sensible heat on the ground. Deforestation has caused significant warming in the decade from 2003-2013, of up to 0.28°C on average temperature trends in tropical regions, and a strong warming of up to 0.32°C in the
southern temperate regions. At the current rate of deforestation, tropical forest loss could add 1.5°C to global temperatures by 2100, not accounting for other human-induced temperature increases. Between 1950 and 2000, surface temperature increased globally by 0.3°C due to land cover changes. Perturbations in the surface energy balance generated by vegetation change from 2000 to 2015 have led to an average increase of 0.23°C in local surface temperature where those vegetation changes occurred. Mean warming due to land cover change may explain 18-40%
of current global warming trends through the reduction of evapotranspiration and in spite of the increase in surface albedo”
For full UNEP document see https://wedocs.unep.org/bitstream/handle/20.500.11822/36619/FB025.pdf
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We are accepting submissions for articles about regenerative water for upcoming editions of this newsletter.
Also if you have upcoming events related to water, let us know, and we can print them in upcoming editions of this newsletter. This newsletter will be coming out again a month from now.