From: From: warwick@bettong.EEPO.DIALix.oz.au
One of the major problems we are suffering from in this area of
the world - Australia - is rising salt levels in the topsoil, forcing
farmers to take millions of hectares of land out of production.
The major cause of this is overclearing, bringing water tables
closer to the surface, and then capillarity bringing moisture so high
in the soil profile that high solar radiation, and high velocity low
humidity winds evaporate "surface" water, and so increase salt levels in
the root zones.
But this principle - up to a max of about 100 cms of capillarity above
the water table level - can be used in reverse.
I recently constructed a very small (12m long) variation on the
vectored situation you describe. I selected a site in the local community
garden that received the most sunshine, mainly for the winter months.
I then constructed windbreaks and sun traps to ensure maximum temperatures
above the surface.
To simulate the heavy rainfall in the root zone, I laid out a plastic sheet
in a wide shallow U; \__________________/ about 90 cms wide, and 10 cms
high. This had a slight fall (5 cms) from the ingress point of the water
to the end of the plastic. That the water eventually drained off the end
was checked by using a hose, and "levelling" by trial and error..
This whole channel was then filled with a fine mulch - a mix of compost,
wood chips, grass clippings, and worm castings. The idea was that the high
organic/humus level would hold water, and wick it up off the plastic by
capillary action, and hold it for the plants' use. This mulch was put on 80
- 100 cms deep. It settled by 20-30 cms, and has been topped up twice in
two years to maintain this depth.
It doesn't just hold rainfall or sprinkler water. It is the focal point for
a drainage system, picking up rainfall from 600 sm of carpark and laneway
uphill from the entry point. Some very rough calculations give a yield into
this area of 1 cu m for each 2mm of rainfall (once flow starts). There is an
underground pipe with three front end filters, leading to a sump at the head
of the water storage system.
It is producing sugar cane, sweeet potatoes, cinnamon, ginger, and bananas
are growing but yet to yield.
Another observation that may be of some use:
A trickle pipe going through a sawdust path had a profile going almost
horizontally out from the pipe drippers. It didn't start getting a curved,
and then horizontal boundary, for a surprising distance.
Again graphics are hard - but a simple table tells the story..
Horizontal distance Vertical distance
from pipe (cms) below pipe (cms)
From 10 cms down we were in classic sands of this area - grey, coarse,
non-wetting. But their behaviour was very seriously modified (for the
better) by being under the mulch of sawdust.
Permaculturists around the world have achieved similar "perched water table"
effects in the small scale with newspaper. Twenty or more sheets will sure
slow down the effects of gravity on water..
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