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  • From: Mark <mpludwig@facstaff.wisc.edu>
  • To: permaculture@lists.ibiblio.org
  • Subject: Re: [permaculture] "thirsty " trees....
  • Date: Mon, 02 Sep 2002 11:00:12 -0500

At 08:04 AM 9/2/02 +0930, you wrote:

In my area the public water system is supplied from underground aquafiers.
However the draw down is exceeding the intake and levels are down to
dangerous point. If they get too low sea water will contaminate them and
destroy them. One sloution suggested is to clear 200 square kilometers of
trees from the catchment area. It saddens me that we have to contemplate
this sort of action. The loss of animal and bird habitat and at this scale
it could even reduce the rainfall.

Sounds like a poor idea. Too drastic, scales all wrong. Perhaps there is some need to open things up in smaller plots.

In most other places in Oz removal of trees has increased to level of
aquafiers, they have risen up through layers of salt to be close to the
surface. Ground evaporation then concerntrates these salts in the top soil.
It has been estimated that we will loose one third of productive land in the
next twent years unless drastic action is taken to lower the water tables.
Just to explain further, trees absorb perhaps 80percent of rain fall,
drainage into aquifiers could be as low as 5 percent,
Grass land only absorbs 15 percent of rainfall with aquifier recharge of
perhaps 70 percent, the figures will vary depending upon climate and species
and topography etc but I guess you can get the picture.

Wes


Im curious how these numbers were derived and what they really mean. I understand that trees can hold lots of water in the canopy, but I also know that well vegetated grasslands can absorb huge amounts of rainfall without runoff (perhaps reflected in the 70% above). Grass also shows a significant advantage in riparian areas as it holds soil much more reliably than a tree canopy will alone. Of course if you can archive both some tree cover with grass sod underneath thats probably the best.


Mark P. Ludwig
Poultry Research Lab
University of Wisconsin -Madison
608-262-1730 WK
608-846-7125 HM





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