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Harvesting smaller rains
Droughts are not new, we have known about the flood and drought cycle for years and the storage capacity of our dams per person far exceeds most other countries. Global warming, the increase in our population and the expansion of our irrigation, which put extra pressure on our water resources, are not abrupt changes, they are trends which have been going on for years and will continue.
In community action on water we saw that we are only harvesting the larger rains but the volume of water falling in smaller rains is more than adequate to meet our needs. However, the problem is to harvest these smaller rains and to avoid loosing them again by evaporation.
When these smaller rains fall on soil they are absorbed and quickly evaporate.
There are three solutions - capture water that falls on hard surfaces, working the soil to funnel the water into percolation holes and anticipatory scheduling.
Catching the water from the roofs of our houses using small water tanks is the simplest way. Over the years water tanks have not met with much enthusiasm from authorities, in fact, in many areas they were banned. (Coming shortly water and social values).
Even today there are spurious augments from the authorities on the cost and purity of the water. The fact is that water tanks are highly cost effective and the quality of the water can be very high if a few simple precautions are taken. The water authorities that deride water tanks are at pains not to disclose that the water they supply has originated from run off and is initially contaminated by an array of animal droppings. It is basically processed cow shit. May be well processed cow shit but still cow shit.
Rain water falling on the roofs in most areas has a far higher standard of purity. Contaminations comes mainly from leaves in the guttering and some bird droppings. With a simple bit of care and using a simple first flush system, these are resolved. For those wanting a second layer of protection, household filters are available which provide a far higher level of filtration than used in commercial water systems.
There is no doubt that tank water can be made safe and is generally of a far higher quality than the processed dam water we are accustomed to drink.
The only problem with tank water is that there is not enough of it for all our needs. It is fine for normal household use, drinking, cooking, washing, the toilet etc but there is no where near enough water for food production or to create the green environment that most people aspire to.
Roads and hard surfaces
The area of hard surfaces particularly in an urban area, is simply huge and enough water lands on these surfaces to meet all our needs.
Road water is the complete opposite of roof water, which is high quality but limited in volume. There is lot of road water but it is probably about the dirtiest source of water available. It is not just the animal dropping, dogs, birds cats etc there is all the street rubbish and on top of that there is all the oil, exhaust particulates, brake lining dust etc from the traffic. It wins at least the bronze gong for yukky water. But it is available in large quantities.
And this is where one of the holy cows of the water supply industry comes in to play - and that is the resistance to segregation. Traditionally all water delivered through our public system has to be potable water. The concept of a twin pipe system is regularly thrown around and rejected largely because of cost. It is thought wrongly that this second pipe system has to be a duplication of our existing premium water reticulation system.
The arguments for segregation are just overwhelming. We use a very small volume of high quality water to drinking standards, and very large volumes of water where some level of contamination is acceptable or in some cases desirable - for example adding nutrients to irrigation water. So why the resistance?
The point is that to segregate water supplies we do not have to duplicate the existing infrastructure.
Instead, we capture roadside run off using percolation holes. In the simplest case, this is just a question of boring holes through the existing drainage pipes to allow the water to enter the sub soil. Where it goes to after that will depend on the local hydrology but the water will either enter a local aquifer or will emerge lower down the slope as a wet spot.
The water can then be captured by building a local dam or lake, which can become a community asset or a bore hole can be sunk to fill a local header tank on demand.
Water can then be fed from these dams or tanks under gravity to provide utility water to the local residences. The water will be cleaned up to a large extent by filtering through the ground. It will certainly not be drinking standard, but will be perfectly adequate for irrigation and general yard duties such as cleaning cars etc. and can be distributed locally using low cost poly pipes.
It is still possible to use percolation holes in vegetated areas. This applies not just to catchment areas but to any land area. The section on water in arid landscontrasts Australia with water supplies in other arid regions and shows that most other areas rely on water harvested in mountain regions, while Australia has few serious mountains but a lot of land. We have to accept that some land will have to be used for water catchment, not just by conventional dams which only harvest large rains, but by catching and storing the water in the ground.
Throughout the world the volume of water stored in the soil far exceeds all the fresh water stored in dams and natural lakes. While maybe only 10% or so of the soil volume will be available for storing water the shear volume of soil gives a total water storage capacity many multiples of our dam infrastructures.
The problem is that most of the rain that falls on this land is simply captured by the surface layer and evaporates away without either running off or penetrating beyond the evaporation and transpiration layers (root zones).
But just take a walk through the bush after rain. If it is a small rain of just a few mm (say under 10mm) all the rain is absorbed in the soil and there is no water on the surface. There seems no way of harvesting these rains other than creating an impervious seal such as a road or a plastic film as used in water harvesting.
With a medium rain (say 10 – 50mm) there will be water lying on the surface in numerous small puddles and depressions. At the lower end of this rainfall band isolated patches of surface water form, but they do not converge together giving any surface flow.
With a bit more rain these isolated patches of water or puddles tend to converge and form not exactly a stream more a tribulette which will fill a local hollow which does not overflow, so there is still no contribution to run off.
It is quite common to see some water running off the sloping areas forming a tribulette or baby stream which simply stops flowing when it reaches the flatter areas.
They can be seem just driving along our typical bush roads. Water will run off the hard road into the roadside drain if there is one. Typically the water will flow along the roadside until there is some small depression in the ground when it will veer of, flow for some distance, then simply stop as the water is progressively absorbed by the soil.
Australia is just full of these little ephemeral creeks which start and stop. There is no run off and shortly after the rain stops the water evaporates.
Dividing the total rain that falls on Australia by the total population shows shows we receive a million litres per person per day - a staggering figure. Most of this rain falls as these smaller rains and is simply lost by evaporation. Of course the rainfall distribution or spread of small and larger rains varies greatly across the continent and in the extreme North and South the larger rains make the highest contribution. In the dry centre there is not much rain anyway and it tends to result from large freak storms.
But in the coastal fringe, where the majority of the population lives, the bulk of the rain falls in this medium rainfall band. These coastal rain are reliable even falling in drought. They are expected to remain reliable even with global warming. Some people argue that with the higher sea temperatures these coastal rains may actually increase, yet we largely waste this valuable asset. Why? This will be discussed in water and social values coming shortly..
Micro dams and percolation holes
We can capture a useful amount of this water by using micro dams and percolation holes. The idea of a micro dam is not to store water, they will spend most of the time being dry, they simple catch and hold the water while giving it an opportunity to soak deep into the ground.
There are arguments that using percolation holes and storing water in the soil is stealing water from other users eg that it is using water that would otherwise flow into the rivers for others to use. There is little substance in this argument. Percolating water into the soil is a slow process so in a major rainfall there will still be run off. Percolation holes capture the water from small rains which would otherwise evaporate away.
Negative arguments, such as depopulating catchments areas increase run off are totally false. (See depopulation of catchments coming shortly) .
The top layer of soil in arid areas (most of Australia) forms an insulating crust which protects deeper water from evaporation. This is one mechanism which allows desert plant to survive. This crust has to be wetted out before there is any run off or capture of the water deeper in the soil. This water to wet out the surface is simply lost by evaporation. Percolation holes are simply making use of water that would otherwise be lost.
The wicking bed technology started off in a very modest way and it is only now that the full implications are being appreciated.
I was involved with a project in Ethiopia working out how to provide water to grow enough food in that country racked by starvation. Before I left Australia I had impressions that turned out to be totally wrong. Ethiopia is not some dried up desert with people trying to scrape a living out of sand; the climate is not that different to Australia with quite reasonable rainfall and agricultural production adequate on average to support a significant population. It is just like Australia in that it suffers a highly erratic rainfall.
Crops can be growing well and looking fertile then a break in the rainfall at the critical time when the seed head are filling can leave the population without food.
Just a couple of weeks without rainfall at the critical time can cause untold suffering.
The problem is one of short term local storage of water. The original solution was simply to increase the water holding capacity of the soil by creating what is essentially an underground pond. In its earliest version a channel was dug, lined with a plastic film and the soil replaced.
It was soon realised that this could be extended to a highly efficient form of subsurface irrigation by laying a pipe along the base of the channel. In Ethiopia flood irrigation is often the only way of irrigating crops, but the available flow rates are very low so often much of the water is lost soaking into the ground before the water reaches the end of the channel.
With a wicking bed it is just like filling an underground bath tub, the water will just trickle along the pipe without soaking away. The high flow rates necessary for efficient flood irrigation are just not needed.
Wicking beds with rain harvesting
Wicking beds can catch water locally by extending the plastic sheet so that any rainfall is funnelled into the base of the bed – water amplification.
This is more than simply amplifying the rainfall by the increased area. The plastic film will catch even small rainfalls and the water will naturally flow to the bottom of the bed protected from evaporation by the insulating crust. Just as in conventional agriculture, any small rains that fall on the bed area itself just wets the surface and are soon lost by evaporation. The amplifying wings of the bed will therefore increase the effect of the rainfall many fold.
At the other extreme a heavy rain in conventional agriculture can simply pass through the soil beyond the root zone and again be lost. A wicking bed will capture much of these larger rains providing water for much longer than would by simply storing water in the soil. With both small and large rains wicking beds make for more effective use of the water.
Obviously any row crop in Australia could use a wicking bed with amplifying wings to grow crops where there is no irrigation or more likely limited irrigation water.
Wicking beds with grey water
It must be said that virtually all the wicking beds constructed so far in Australia have still used some form of external water sources, even if it is only for an occasional top up.
In the simplest version they may be connecting the down pipe from a roof directly into the wicking bed eliminating any intermediate storage such as a dam or tank. This obviously has advantages in cases where the space is limited so it is difficult to find room for a tank. The disadvantage of course is there is no control of the irrigation scheduling. Most wicking beds are currently used to grew high value horticultural crops which typically have shallow roots so most wicking beds have some access to an external water source.
The major exception is in the use of grey water which provides a fairly constant supply of water. However the problem with any grey water system is that while the supply of water may be fairly constant the demand from the plants is not. Just relying in grey water would result in an over supply of water in the rainy times.
Avoiding grey water escaping to the general environment is a critical factor in grey water use.
It is therefore much better to have a large enough area of wicking beds with grey water providing a small proportion of the water and additional water being added from an external source as needed. This also leads to a continuous dilution of grey water which is generally alkaline.
Wicking beds with external water source
One of the fascinating features of wicking beds is that generally they give a much higher productivity than conventional beds. This is essentially an experimental observation rather than based on some theory understood before hand. The theoretical challenge is to work out why.
There are a number of possible explanations.
The first is the very nature of the mechanics of wetting. The plant roots system are totally wetted from underneath and then as the water is used air from above is sucked in, giving natural breathing action to the soil. It is essentially a flood and drain system which is widely regarded as the most efficient way of watering.
Another explanation is that each plant needs an ideal ratio of water to air around its roots. The soil at the surface of a wicking bed is dry (apart from the seed germination period) while deeper in the ground the soil is saturated giving a moisture gradient form top to bottom so there is always one region with that ideal moisture to air ratio.
Yet another explanation comes from the mechanics of successfully operating a wicking bed. If for example the bed is constructed in heavy clay and this clay is simply loaded back into the bed the chances are that the soil will become water logged and the plants simply die from the anaerobic action. For this reason, the wicking beds are usually partially filled with a layer of waste organic material and preferably an inoculant or starter containing worm capsules with food and a microbe mix which will ensure the soil is maintained open and healthy. This obviously provides excellent growing conditions.
It is probably not one single reason but a combination of these that give the increased productivity.
Large scale use
Originally wicking beds were developed as a small scale way of storing extra water; the improvement in irrigation efficiency was just a bonus. At first it was though that their use in Australia would be limited to small scale application, more home and hobby farm use.
It is now becoming clearer that the improvement in irrigation efficiency by this very simple and cheap way of providing subsurface irrigation may lead to much wider application in commercial agriculture. Although individual beds are usually irrigated separately they can just as easily be linked together. This can either be done by cascading them so water flows from one bed to the next or even better fit each bed with a simple cut off valve so when the current bed being watered is filled that the water is diverted to the next bed, so every bed is watered in sequence.
In hilly country beds have to be aligned along a contour line as they need to be level, but irrigating in cascade by cut off valve makes them particularly suited for regions with some slope. They could well be the way of replacing the huge areas of flood irrigation in Australia.
Anticipatory irrigation is a simple way of achieving the twin aims of making use of the smaller rainfalls and minimising evaporation losses.
The aim is to get water deep into the soil protected from evaporation.
Just as there is a threshold for run off in our dams there is a threshold of irrigation water which must be applied before the water penetrates into the deeper soil.
There is always an insulating crust which must be wetted out first, and all the water used to wet out this crust will be lost by evaporation in a few hours.
Irrigators know that they have to apply enough water to fill the profile which will extend the time between irrigations and hence reduce these threshold losses.
It is less obvious that the best time to irrigate is just after a rainfall. The surface is then already wet so a smaller volume of water is needed to fill the profile.
There are times when rain is expected but the plants need some water now. The aim then is just to apply enough water to satisfy the immediate plant needs in the short term.
At other times extreme heat may be forecast when it is better to irrigate ahead of time rather than irrigate under high evaporation conditions.
All of this is just common sense, but to apply means knowing how much water is needed to fill the profile.
Probes are widely used to measure soil moisture. But there are two intrinsic problems. They only measure the moisture content very close to the probe. There is a wide variation in moisture levels throughout the root zone so the readings vary widely depending on where the probe is positioned. Experts try and position the probe in an average position but this is much more difficult than it appears.
An ever bigger problem is knowing the wetted volume. Irrigation systems never apply water uniformly and only wet out part of the roots zone.
This leaves us with what may appear to be an insurmountable problem, how to calculate the total water in the soil from a few sample points.
But the answer is just so stunningly simple. The best way of explaining this is by considering the problem of working out how much water is needed to fill a jar of stones which is already partially filled with water. This is exactly the problem we face in the soil.
A water expert may be tempted to try use a soil moisture probe to find out the amount of water currently in the soil, work out the empty spaces in between the stones and eventually calculate out the amount of water needed to fill the profile.
The solution is almost child like, simply measure how much water is needed to fill the jar. This tells exactly us how much water was needed to fill it up.
How do we apply the simple idea to irrigation scheduling?
We fill the soil with water and we use our soil moisture probes to measure when the soil is full, more specifically we measure how much water must be applied for the water to reach the bottom of the root zone. We do not care how much water is in the soil, we are just taking this as full.
Now we let the plant use up some water. Again we have little idea how much water the plant has used but we can measure this by filling the soil up again,.
Sounds simple but there is a snag. It can take a long time for the water to soak down to the base of the root zone so we cannot just keep on pouring on water until the profile is filled, that would give us big errors.
But there is an easy way of overcoming this snag. Make a guess of how much water has been used, (which we can do by guessing a crop factor and multiplying by the evaporation) and apply that amount of water.
We do not even have to start with the profile full, just guess a crop factor, apply the estimated water and measure the irrigation depth. All we have to do is keep on adjusting the crop factor until after we have applied the estimated amount of water the profile is full. Then at any point in time we know, just by looking at the evaporation how much water is needed to fill (or partially fill) the profile.
Guessing is a bit hit and miss, but we can make the whole process very efficient using a mathematical technique called predictor corrector which is build into a simple software program. So let us see how this works.
We need to know the amount of water the plants are using, and the maximum allowable deficit in the soil.
These are site specific so we have to measure them.
We cannot measure them directly but we can learn them by monitoring the site.
We make the best estimate of the crop factor and allowable deficit, erring on the side of caution.
We measure the evaporation and make a best estimate of current deficit from evaporation and the current crop factor, compare with allowable deficit and decide whether to irrigate or not.
After irrigating we measure either soil moisture or irrigation depth and use this data to adjust the current crop factor.
When the crop factor is stable we can measure onset of plant stress to determine the allowable deficit.
This is the home menu and let us imagine we are in mid cycle, we irrigated some time ago and entered all the irrigation, crop factor data etc and are now just watching for the system to tell us when to irrigate next.
We click the weather and irrigation data tab, this upper part of the column is the actual recorded evaporation data while the lower part is the predicted evaporation (which is used to give the total water content anticipated over the next period). The system is waiting for us to enter the measured evaporation for the date shown. Normally this would be yesterday, but some people measure today’s evaporation in the evening ready for an overnight irrigation.
The figure in the record evaporation tab is the predicted evaporation, so this has to be over written with the measured evaporation.
If you are working on yesterdays evaporation make sure you do not click the record button for today’s date.
If you make a mistake you can right click any value and correct.
If there has been any rainfall you need to enter this now.
You may also want to check that the last irrigation has been entered. Normally this is done automatically but if you missed this step you can enter manually now.
Click block water usage - this form is in two parts, the top form gives details of a specific block while the lower gives a summary of all blocks.
A full profile is taken as zero. The negative numbers show the water needed to refill the profile. The yellow indicates that that block has now reached a threshold, which we have set, indicating that the block is now ready to be irrigated and with the number indicating how much water is needed to refill the profile.
The chart indicates the predicted date when each block needs irrigating. We then consider the weather forecast and take the decision which blocks to irrigate and when.
Click on the irrigation planner button and drag the forms so they are all visible and the transfer can be checked. Double click on the blocks needing irrigation which transfers the data to the irrigation planner. Print a copy of the planner which shows the irrigation time.
After irrigating the soil moisture or irrigation depth should be measured allowing time for the water level to stabilise. Record the irrigations by right clicking each irrigation and then clicking record. This can be viewed by clicking the weather and irrigation button. You should also enter the irrigation depth.
Check that all the weather and irrigation details are entered. It is now time to get to the heart of the program and adjust the crop factor.
This can be based on either soil moisture or irrigation depth, but we recommend using irrigation depth.
Click the crop factor and crop factor data buttons and arrange the screens so they can both be viewed.
Click the calculate revised crop factor button and review the revised crop factor. It is very useful to view the changes in the crop factor graph to see how it is trending. If you are satisfied then click the record crop factor button.
This is the core routine in using the program. If the irrigation water is saline you may also include the soil salinity calculation button. With this method of only applying sufficient water to fill the profile salt will accumulate in the soil - so flushing may be needed.
You may also want to check the annual water use. Unfortunately, irrigation is not just about applying the right amount of water for the plants. It is a question of juggling the amount of water available.
This is an overview of the basic routine now we have to do this in the real world learning the correct crop factor and water holding capacity of the soil without damaging the plants or wasting water in the process. This is shown in the video on anticipatory irrigation
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