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The Waterright philosophy
Chapter 3 Subsurface and improved flood irrigation
Are there better ways of applying water? This was the question I faced in the mid nineties.
Over the years there has been dramatic improvements in irrigation in horticulture drip, subsurface drip, micro sprinklers etc. However the reality is that horticulture only uses some 5% of irrigation water and is already reasonably efficient. A gain of 40% in efficiency, an ambitious target, would only net a saving of 2%.
The largest user of water by far is flood irrigation and that largely for the feeding of animals. Flood, even at its best, is highly inefficient, here was an opportunity to generate significant water saving. This was the logic, which I faced in the mid nineties.
The major challenge was cost and scale. Irrigated pasture is a lot less profitable than horticulture (on an area basis) so was there any way of developing a low cost alternative to flood.
Now I guess a that time I had become a little mesmerized by the benefits of subsurface irrigation. It avoids the big problem we face in Australia - excessive evaporation and leaves a clear dry surface for animals to graze. The challenge I decided to accept, was to se if I could develop an inexpensive system of subsurface irrigation suitable for pastures.
I knew this was going to be a monster challenge. I have to admit upfront that I failed in my primary objective, but failures lay the groundwork for ultimate success. Now readers who like to move to the crunch line may well like to move onto the next chapter. But if you believe, like I do, in the benefits of speculative research, and accept failure as the cost of innovation, then here is an example of a project, which while appearing to be a total disaster led to the development of technologies such as the wicking bed and soil carbon capture which may be of profound importance to all of us.
The inside outside pipe
Subsurface irrigation appeared to offer the most potential, but with the (then) current price of water and cost of installing conventional subsurface irrigation pipe this did not look at all feasible. Some major new innovation was required.
The first idea was the inside outside pipe - an idea which was certainly different and had the potential of being viable on a large scale at low cost.
The idea was simple, plough a length of lay flat pipe into the soil, inflate to form a tunnel through the ground - release the pressure and feed water into this tunnel to wet the ground from underneath.
The mechanics were very simple. A conventional water pump sucked water from a small reservoir at the bottom of the slope to pressurize the pipe and form the tunnel. The top end of the pipe exited the ground into a miniature dam which was filled by the water from the pump. No water could escape from the dam as the pipe, under pressure was pushing against the soil.
The pump was then turned off and the water in the pipe would run out backwards through the pump so the pipe would progressively collapse and as it did so it allowed water in the top dam to flow down the tunnel so wetting out the soil. A simple and ingenious system which worked well most times.
However there was a problem. If the ground was uneven with a variation in the slope the lower points would receive more water than they should giving a poor water distribution. No doubt this could have been solved by laser grading and controlling the flow back out of the pump - but another idea had emerged.
The froth machineOne of the major problems with all methods of irrigation is that the soil, at least locally becomes wetter than the ideal for plant production. With flood irrigation it is widely recognized that growth actually diminishes immediately after flooding, then raises to a maximum, the drops of again as the soil dries out. Proponents of more modern irrigation systems such as drip and micro sprinklers argue that the soil is wetted more uniformly. In reality water does not magically move through the soil to give a uniform water content. There has to be variation in moisture level to drive the water. After the initial movement when the soil is saturated the rate of movements rapidly drops off to leaving a wide variation in moisture content.
Plants give maximum growth when there is balance between water and air in the soil. This balance is never consistently achieved with conventional irrigation. So the idea came - why not inject a mixture of air and water directly into the soil. This would also reduce some of the engineering problems with irrigation. Both drip and micro sprinklers have very small orifices to regulate flow. In practice this gives a lot of problems with these orifices blocking up. Even with sophisticated filtration this is a real problem, growers are regularly inspecting their irrigation systems for blockages and this is one of the reason why subsurface irrigation is not as popular as expected from its theoretical benefits.
Using a froth, a mixture of air and water should give better growth from the improved air water ratio but result in a very simple and rugged system.
And so a froth machine was developed which would inject this mixture of air and water directly into the soil, letting the air distribute the water throughout the soil giving a uniformly moist, but not wet soil.
The world of innovation is filled with examples of systems which have what appears to be overwhelming theoretical benefits but with a wide gulf between theory and practice. Let it be said that the froth machine was such an innovation. My memory of the first trial in a farmers paddock was the farm dogs going wild as the thousands of emitters would whistle as the air rushed out. I would sooner forget about the divergence between theory and practice.
It was time for a major rethink.
Rethinking subsurface irrigation
In the world of innovation and technical development there is a universal trend called convergence. In the early stages here is a wide variations of systems but as the technology develops this wide variation gradually disappears until the various solutions are very similar.
This phenomena can be seen it the development of the automotive in which cars become almost indistinguishable until some major new development like front wheel drive or fuel injection comes on the scene with an initial variety of solution until the convergence process takes over.
Perhaps it was time to stop looking for way out ideas and look at what could be done to improve conventional subsurface irrigation. Commercial subsurface irrigation were little more than above ground dripper systems buried in the ground. So I asked the question, if we were developing subsurface form scratch, and the need was to produce a low cost system which was simple and rugged enough to replace flood - what would it look like.
One of the big problems (which is not widely understood) with drip irrigation is the movement of water through the soil. There are several mechanism for water to move through the soil but in conventional irrigation the relatively weak surface tension forces are typically dominated by gravity so there isonly little side ways movement and a lot of downward movement.
Applying the water at a faster rate results in the pressure flow becoming more dominant -with water literally being forced sideways. Test were done to confirm this and it was pleasing to find that water could be readily moved sideways by a couple of meters compared to some 300 mm relying of surface tension.
This meant that the number of lines needed could be reduced to a third, helping the cost target of making it cheap enough to replace flood irrigation.
However larger pipes would be needed and this had benefits as now the manufacturing process could be changed to a blown film line which is intrinsically cheap.
The next problem was the variation in pressure distribution along the line with the higher flow rates. This was solved by developing a computer program which drove a punch machines so the emitter could be placed at various spacing along the line to compensate for the pressure drop.
We were now ready for testing and trials were undertaken in a variety of farms with different soil types. The results were a mishmash, some systems worked well while others were hounded by a string of technical difficulties. Reviewing all these results led to the conclusion that while he system could be made to work with the care and sophistication in both the installation and management the system just did not seem to fit into the rugged farming scene.
Putting it bluntly farming is a pretty rugged business and products have to be simple and reliable. Technologies which may well work under controlled conditions are just not viable in the tough farming environment. If must also be said that a significant amount of money, measured in the millions of dollars had already been spent on the project, it was a time for a major rethink.
Reworking flood irrigation
My ambition has been to develop a system which would replace the inefficient flood irrigation, and so far we had failed miserably. There is the old saying 'if you cannot beat the join them' so the question was put if we cannot replace flood irrigation what can we do to make it more efficient.
As my background was in computer simulation it was natural that my first approach was to write up a simulation of flood irrigation. Soon the code was ready and I could start to play. The big advantage of a simulation is that you can play with all the variables very quickly and see what happens. In one afternoon of playing a learned more than could be learned in a life time or real world experiments.
Some variables were 'soft' for example the slope of the ground, the surface roughness had very little affect on the final results. Other variables, particularly the speed of application had a dominating effect. Basically the target was to achieve a uniform depth of infiltration along the paddock. A slow rate of water application would result in the water near the top of the paddock being saturated to a great depth while a very fast rate of application would result in a very shallow penetration.
However a two stage process could give very high efficiencies. The first stage required a very rapid flow rate which was cut off before the flow front reached the end of the paddock (but when there was just sufficient water in transit to reach the end of he bay) followed by a much slower rate of application so the entire paddock had a film of water on the surface which would then soak into the required depth.
Off course on a computer simulation you are not bothered by practical problems of how to achieve these flow rates, it is just a question of typing a number into the computer. However what the simulation showed was that it was possible to achieve very high efficiencies, well over 90% simply by managing flow rates.
The question was how to turn this understanding into practical system. It was never envisaged that the computer simulation would be widely distributed to farmers, this would be expecting farmers to learn a completely new skill set. While the most technically sophisticated may acquire this expertise the low tech farmers who would have no interest in this technology and they are the ones that waste most water.
The original layout of the flood irrigation system was based on large channels which can deliver water at high flow rates. However there is great pressure at the farm level to makes paddocks as big as possible, which leads to low efficiency and poor water distribution.
In the ideal world we would distribute water by pipes however to get the needed flow rates would require very large pipes which are expensive. Many attempts have been made to replace channels by large pipes but the economics rarely work out. A new approach was needed, - looking at the distribution system and the paddock as an integrated system.
A few simple calculation show the problem. Calculating the diameter of a pipe needed to feed a typical farm shows that a small pipe is perfectly adequate if it were to run continuously, however large pipes are needed to send the large slugs of water down the system needed for efficient irrigation.
It is a classic loose - loose situation. Big pipes or channels to increase efficiency but waste capital and encourage farmers to have even bigger paddocks while small paddocks to increase efficiency waste land and increase evaporation form the increased area of channels.
When these realities are understood and accepted the solution became obvious;- find a way of irrigating small area at a time in sequence. This allows efficient water distribution by small pipes and increases irrigation efficiency on farm.
This could be achieved at significant capital cost by using conventional irrigation valve and controllers. What was needed was a simple valve which would divert water from one area to the next once a certain volume of water had passed.
This led to the development of micro flood and the tilt valve. Very simply the title valve is a gravity operated valve in which water is bled into a hinged pipe which is tiled back to allow full flow of water. When sufficient water has filled the pipe it overbalances and shuts of flow into that section and automatically diverts flow into the next section.
Simple and effective but an idea which proved virtually impossible to sell.
But that was not a problem as by chance I was to find a system which was far more efficient virtually eliminating water lost past the root zone and evaporation from the surface so virtually all the water applied was used for plant growth. It also improved plant growth and productivity and the need for frequent irrigation.
It also had the ability to capture significant carbon in the soil, which could have major ramification in the battle against global warming.
I sometimes wonder if it is good things when ideas fail to catch on because they lead to the development of an even better idea.
This system was the wicking bed which I will talk about out in the next chapter.
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