[permaculture] How do trees make rain?

Rain Tenaqiya raincascadia at yahoo.com
Thu Aug 21 13:49:11 EDT 2008


The effects of trees on climate vary according to the region and are controversial.  In general, trees are believed to create a sponge-like effect in the water cycle, helping the land to absorb and then slowly release moisture.  Planting trees is generally credited with raising groundwater levels and creating or restoring springs and streams, while cloud seeding and mist gathering are known to occur only in specific climates.    
 
On the other hand, in some areas, trees are also planted to lower groundwater tables by increasing evapotranspiration (western Australia and South Africa come to mind, both arid climates).  It appears that thirsty trees, usually not native to the area, can actually lower groundwater tables.  The overall effect on the hydrology is not known, however.  In riparian areas, the effects are even more complicated.  Trees undoubtedly shade streams, reducing water temperatures and evaporation.  By providing more surface area and organic matter to slow the arrival of water to the streams, trees allow water to remain in the system for a longer time.  But I have personally witnessed stream levels rise (near Big Sur, California, another arid climate) after deciduous riparian trees lost their leaves.
 
Another example of the contradictory effects of trees:  soil moisture is retained longer underneath trees, but once dry, the soil takes longer to get moist again as most of the rain evaporates off the leaves before hitting the ground.  Some trees are shaped to funnel water to their roots, but most seem to merely dissipate the rain.
 
So, as you can see, when talking about trees, you need to take into account the actual species and site in question, a basic permaculture design principle.  Only by observing and testing designs in specific locales over time can we come up with sustainable systems.
 
Rain


      


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