Tropical Deforestation: Hydrology Implications


Paul Collar

    Paul is a hydrogeologist and environmental engineer and the owner/operator of Osa Water Works.  You may reach him at



The most important process that native forests play in the water cycle is evapotranspiration, a process in which water is taken from land and introduced into the atmosphere through the agency of photosynthesis in trees.  Large amounts of water respond predictably to convective pressures of the tropical sun to spawn native rains that are important contributors to yearly totals.  Storm born precipitation blown in on weather fronts is also a large component of annual rainfall.  But without the in situ rain factory of a tropical rain forest, it would cease to be one, and water would migrate elsewhere.  Evaporation is responsible for transferring as much as 70% of the rain that falls in tropical rain forest to the atmosphere, much of which is returned as local rain, to be cycled again.  The role that forests play in the equilibrium of water between the atmosphere and lithosphere, something that we cannot see but can feel and measure, cannot be overstated.  Evapotranspiration is the 800 megaton gorilla in the tropical rain forest.

Forests introduce other variables that play critically into the water cycle—beyond the vastness of evapotranspiration—that are the driving forces of terrestrial hydrology, what is left behind for riparian admirers everywhere, after the atmosphere sucks up its aliquot.


The percolation of rainfall runoff into soils and the recharging of aquifers are facilitated by forest root systems, which provide openings in soil layers that allow infiltrating waters to search out the water table.  But the forest itself is furthermore a giant repository of water.  Humans are 60% water, trees, just 50%.  Tropical rain forest soils are notoriously poor, and all the ecosystem’s nutrients are held in the biomass, which is why slash and burn is an effective pioneering approach in soils not natively apt for agriculture.  Just as the forests are a repository of living nutrients, so too are they a repository of living water; 50% of their mass.  Forests hold soils in check, and the streams draining standing forest are of an extraordinarily high quality as a result.

Only scientists get to dabble in a perfect world, however, and in the civil engineer’s mission to intrude benevolently upon nature for favorable community outcomes, sometimes mistakes get made.  Indeed initial assumptions in tropical forestry that forests increased downstream water flow and provided checks against floods turned out wrong.  The societal implications of decades of forestry research provide a rough blueprint of the cause and effect relationships that exist between tropical hydrology and deforestation, at both the micro and macro scales.  A few things we have learned along the way:

  • Up to 70% of useable water resources exist as a result of forest capture of rain water.
  • Forests hold soils in place and sustain high water quality in streams and rivers. The cutting of forests exposes soils and increases erosion, promoting the decline in downstream water quality and siltation in low-lying areas, plus the clogging of nearshore environments, including sensitive ecosystems like coral reefs.
  • Formerly considered a guardian against devastating floods, contemporary science has found that forests really do not protect against downstream flooding at all.
  • Forests hold water and release it slowly. Deforestation results in greater downstream water flow of lower quality.  Preservation of forests results in increased infiltration and higher ground water levels and a more gradual release of high-quality water to downstream areas through springs and runoff.
  • Water is neither created nor destroyed in the surface environment. It is simply moved around.  Once fresh water is discharged to the ocean, its role in the water cycle becomes restricted to evaporation and weather systems and is forever lost to engineering aspirations for its productive use.

Landslides present a scientific microcosm for understanding the relationship between water, soils and forests.  The fresh soils exposed by slope failures are readily eroded by rains pounding their denuded surface.  Runoff from these naturally-marred land surfaces is clogged with clay and sediment, and runoff bleeds chocolate rivulets into receiving streams.  But landslides are needles in the haystacks of natural forests.  Fire fires and anthropogenic deforestation produce circumstances analogous to landslides, however, and put soil erosion on steroids, swelling chocolate rivers with high turbid flows that in coastal settings sweep quickly to sea and thereby displace terrestrial water to the ocean, where it is no longer available for conventional human use and for good measure also contaminates coastal environments with suspended clay and silt.


For better or for worse, Humanity requires vast amounts of fresh water to sustain not just its potable demand but also agriculture and food production.  As a result, mankind shall always face the challenge of how best to manage its water and forestry resources, not just for the protection of wild areas that warrant protection for the biodiversity they harbor, but also for the protection of human needs, both for potable needs and for the vastly greater demand that comes from agricultural requirements to sustain the human appetite for three squares a day.

Setting aside the effects on purely technocratic considerations like water supply, however, the sediment that is eroded from deforested lands has dramatic downstream consequences.  Changes in sedimentation patterns affect river channels and increase the frequency and severity of devastating floods.  While flood waters historically introduce organic nutrients that increase agricultural productivity in later years, the sediment eroded from denuded forests is organic poor and depresses agricultural productivity, leading to greater reliance on agrochemicals to sustain production.

And in places like the Osa Peninsula, with nearby fertile coastlines teeming with life, the sediment eroded from deforested plots overwhelms some sea life, particularly among corals and other benthic invertebrates which, unlike fish, cannot swim away from the threat.  Corals comprise a foundation of a vital ecosystem upon which a large range of marine organisms depend for a living.  As coral productivity is pounded from unnaturally high sediment loads from land disturbances and from agrochemical pollution resulting from farmers’ responses to declined soil fertility, the anastomosing effects boomerang up the food chain to quickly curtail the fisheries upon which coastal communities depend as a source of both food and commerce.

In the Golfo Dulce sediment loading from the combined effects of deforestation and gold mining has wiped out framework corals, and researchers have written off corals for this remarkable body of water, their state of decline irreversible.  Researchers point out that increased fresh water flows have also affected the corals adversely.  Whether this is due to increased runoff as a result of deforestation or changes in precipitation patterns is a question for further investigation.


Absent a single further penny in research, however, decades of forest hydrology research and human experience has shown that forests are integral to the water cycle and key to environmental sustainability of the places they exist.  Forests capture water and recharge aquifers, purify and discharge high quality water, are themselves repositories of water, and through photosynthesis exchange vast amounts of water with the atmosphere to drive weather patterns.  Forests are enormous attenuators that keep our surface environment in a semblance of balance in a Koyaanisqatsi universe.

The take-away?

It can never hurt to plant a tree.

See the original article here

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