Subject: vectored irrigation

From: [email-masked]
Date: Wed, 23 Nov 1994 01:24:45 -0500

From: From:  [email-masked]
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)
    
   10  0.5
   20  1.5
   25  2.5
   30  7.5
   35  15.0
  
 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. 
 
Another thought:

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..

Wx
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