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The Waterright philosophy

Chapter 2  Irrigation scheduling

Poor scheduling is a major cause of excess water use and poor plant growth.

I have spent many years observing how people actually schedule and I see several common errors.  These errors can lead to double the amount of water being used often with loss of plant production.

Many growers simply rely on inspecting the plants and the checking to see if the surface soil is dry.  Experienced growers can readily detect when a plant is in stress then it is too late as t when the plant shows signs of stress there has been a loss of production and possible damage to the root system which will talk some time to regrow. 

Many plants have a dual root system, tough survival roots (which are not normally used when there is plenty of water) and fine hair roots which easily collapse due to the suction.  These have to regrow before there is good growth
 
One very common error is the failure to adjust water application to the weather conditions.  Oddly enough people will over irrigate in really hot weather.   They understand that the evaporation is very high but with many plants the stomata will close and the soil surface will dry out so the actual water use is less.
 
Exactly the opposite happens in cloudy conditions, particularly after a rainfall when people underestimate the plants water use and under irrigate.
 
I have monitored vineyards with both plant and soil moisture sensors.  In general the experienced irrigators will apply a reasonably accurate amount of water on average over the season.  Partly this is forced upon them by water availability.  It is very noticeable however that they fail to adjust the water applied with the day to day weather variations.  At times this can lead to serious stressing of the plants and at others over irrigation with loss of water past the root zone.
 
This is most obvious in automated irrigation systems which are set to a fixed schedule which fail to take into account local variations.  Fortunately some of these automated systems are now being equipped with soil moisture sensors which abort an irrigation if the soil moisture level is above a certain threshold.
 
The household irrigator is far more likely to adopt a standardized irrigation schedule regardless of water demand and I have so say that many of the regulations on water use from the water authorities encourage poor irrigation scheduling. 

The crop factor system based on estimating plant water usage based a crop factor for the specific plant and evaporation was developed to provide a more accurate method of scheduling. However there are many problems. 

The first is establishing what is the appropriate crop factor.  Usually this is obtained from the literature however crop factors vary widely for the same variety depending on local conditions. Plants water use (crop factor) will also vary throughout the season and this is often not fully appreciated leading to miss irrigation.

The crop factor -evaporation system has the big advantage that generally it lead to a much more consistent application of water avoiding he swing found in manual scheduling.  There is always a problem in allowing for rainfall.  The first part of any rainfall may be lost by evaporation however knowing how much is lost is never clear.

But her biggest limitation is that it is open loop system - in other words the crop factor can be wrong leading a consistent misapplication of water, particularly too much water.
 
But perhaps the most common cause of poor irrigation is applying small irrigation so the soil is not properly wetted out so more frequent irrigation is required.  Small but frequent irrigation is probably the largest cause of waste water, particularly in the urban households, who generally have a rather limited understanding of the complexities of irrigation scheduling.
 
Typically the first 10mm of above ground irrigation will be lost by subsequent evaporation from the surface.  It is only when the top 50 mm of the top layer of the soil has dried out that surface evaporation is reduced.
 
Crop factor schemes are usually use don conjunction with an estimate of water holding capacity of the soil based on soil type.  The water used is estimated from the crop factor and accumulated evaporation until the soil is estimated to have reached or at least approached the wilt point when it is time to re-irrigate.

This is the deep cycle method of scheduling, some irrigators use a shallow schedule so irrigating is frequent just sufficient to top up the moisture level.   This is wasteful of water as typically the first part of irrigation is lost by evaporation.  As water does not spread throughout the soil may results in the development of a small root system so the plant cannot take advantage of natural rainfall.
 
The crop factor based scheduling system , even though dated, may lead to better scheduling, but this is only because so many people are so bad at irrigating.  It is a complex way of achieving very small gains.  Far greater gains can be made using more up to date techniques.
 

Soil moisture probes

Soil moisture probes add a new dimension to scheduling and particularly with automatic recording systems providing a vast amount of information, the problem is to interpret this information into meaningful information.  For example the field capacity and minimum extraction level (wilt point) can be determined by inspecting the soil moisture traces over time and at different depths.

Data is easily misinterpreted, for example the data does not give the water holding capacity of the soil, although some commercial manufactures actually output this value.  This can only be determined by monitoring how much water must be applied to refill the profile.  With drip irrigation there is major variations in moisture level throughout the soil so the measurements will be grossly affected be the position of the probes.

One of the simplest method of using soil moisture probes is to position two probes in the wettest region.  The upper probe is used to indicate when to irrigate, the lower probe is used to indicate when to stop irrigation.

However the real strength of soil moisture (and plant moisture probes) is in adaptive schedule as described below.

There are four key technologies for improving the effectiveness of irrigation scheduling 1) adaptive scheduling, 2) anticipatory scheduling, 3) irrigation depth monitoring and the simplest and most effective of them all 4) the wicking bed technology.
 
These technologies were developed for the commercial rather than the domestic market, so I will describe the more sophisticated commercial technology then explain how these can be dramatically simplified for the home market.
 


Adaptive scheduling

 
Adaptive scheduling uses a self learning computer program which is continuously measuring and adapting the crop factor. The amount of water applied at each irrigation, the evaporation and rainfall, and the soil moisture (or irrigation depth) is measured. The computer starts with an estimated crop factor (from book values or from historic irrigation practice).
 
This initial or current crop factor is continuously adjusted throughout the season.
 
If the crop factor is correct the soil moisture content or irrigation depth is the same following each irrigation.  If the crop factor is incorrect the soil moisture level will increase or decrease. This leads to the generation of an error factor which is then used to adjust the crop factor.  (Mathematically a predictor corrector scheme).
 
The mathematics gives a very rapid convergence to the correct factor which is used for the next irrigation. The crop factor throughout the season is continuously adjusted.
 

Anticipatory irrigation

 
Anticipatory irrigation is a natural extension of adaptive scheduling.  Core weather details (temperature, wind speed etc) are fed into the computer which then predicts evaporation in the coming days.  The irrigation schedule is then based on future demand, if high temperatures are expected then extra water is applied so the plants have access to adequate water.  If rain is expected the minimum water is applied to satisfy plants needs until the expected rainfall.
 
Such technology proved highly effective in managing grape production in the Murray Valley.  Growers were able to maintain quality grapes while other vineyard suffered the severe grape damage due to the skins splitting.
 
This can lead to some practices which are counter intuitive.  For example irrigation may be recommended immediately after a small rainfall.  A small rainfall of say 10 mm is typically totally wasted as only the surface is wetted with no water entering the root zone.  The water in this surface layer is soon totally lost by evaporation.
 
Irrigating straight after a small rainfall until the profile is fully filled will avoid this loss of the first part of the irrigation and save significant water.
 

Modification of principles for the domestic market

 

a) Irrigate on accumulate evaporation

 
Adaptive scheduling is a sophisticated technology which is well beyond the means or interest of the average household.  However the basic principles can be used in the simplest of way with zero cost.
 
Let me give some examples of simple but highly effective irrigation scheduling techniques which can save significant water.
 
Simply place an ice cream or yogurt container place near the plants and fill with water after each irrigation.  Watch the plants and when they appear to be going into stress note the drop in water level in the container (or accumulated evaporation) then apply a full irrigation.
 
Now watch the water level and when it has dropped to about 80% of the level when stress first appeared. It is time to irrigate.  Applying irrigation before the water level drops to the stress mark ensures the plants are fully productive.
 
This principle of irrigating after accumulated evaporation is an incredibly simple and cheap approach which is highly effective and if widely adopted would save large amounts of water.
 
It avoids the common site of people irrigating on a strict time based schedule when many times no irrigation is required.  It naturally extends the time between irrigations and so avoids the loss of water which occurs with the first part of the irrigation.
 
It tells when to irrigate but does not indicate how much water must be applied.  This is done using the principle of irrigation depth.
 

b) Irrigation depth measurement

 
Irrigation depth is the most critical factor in irrigation scheduling.  If too little water is applied so the water only wets the top layer then most of the water applied will be lost by evaporation from the surface.
 
If too much water is applied so it penetrates beyond the active root zone then both water and nutrients will be lost, (and possibly causing environmental damage).
 
It takes some time for the water (or flow front) to stabilize; it is also requires quite sophisticated measuring equipment to measure the position of the flow front in real time.  The most practical way is to measure the irrigation depth some time after irrigation and adjust the amount of water applied at the next irrigation to give the required irrigation depth.
 
This does require that the accumulated evaporation between irrigation is maintained.
 
Irrigation depth is easily measured; the easiest is using a simple augur which only cost a few dollars.  In some soils a simple wire or screw driver poked into the soil is adequate.
 
However the ideal way is to use an electronic irrigation depth sensor.  Commercial versions are expensive but a simplified domestic version has been designed and could be manufactured for a few dollars in volume production.
 

Benefits over web based scheduling

 
Web based scheduling is based on standardized book values for crop factors for specific plants and water holding capacity for various types of soil.  The actual values for crop factor and water holding capacity may vary widely from these standard values depending on local conditions e.g. position on a slope, degree of shade, proximity to other plants, the effect of the condition of the soil (has it recently been worked) and the addition of mulches etc which can raise water holding capacity.
 
This inevitably means that irrigation scheduling based on standardized values is invariably inaccurate.  There are ways of compensating for some of the errors but this inevitably leads to more complex and difficult to use systems.
 
The aim of all modern thinking on irrigation scheduling is to get away from the restrictions of using standardized or book values and measure the actual local values.  Of course they are still using the basic concept of crop factor and water holding capacity but they aim to use real life local values.
 
To this end a significant number of measurement technologies have been developed and are widely available from a range of commercial manufactures.  These include soil moisture and depth sensors, sensors for measuring the water content or consumption of the plants directly, automated weather stations for measuring evaporation and rainfall etc.
 
These are typically expensive and sophisticated pieces of equipment which required a level of sophistication for there effective use.  They are therefore generally only used by commercial growers and then only those with financial and technical resources, (which represent a minority of even commercial growers).
 
I have tried to show that households and many commercial growers can readily apply these basic principles using low or zero cost items, providing they have a basic understanding of the principles involved.
 

Irrigation scheduling is the cheapest and easiest method of water saving, I now move onto the more challenging problem of how to apply the water.
 

subsurface and improved flood irrigation

 

  


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27 March 2009