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  Graphical Irrigation Scheduling
In This Advisory
Purpose - The purpose of this advisory is to...
  • Explain one of the two main families of irrigation scheduling, "graphical" (or sometimes called "sensor-based" or "bottom-line")
  • Summarize data requirements for performing graphical irrigation scheduling
  • Provide examples of graphical irrigation scheduling
A modern irrigation system, whether furrow, border strip, sprinkle, or trickle, is an important and large investment. Whether you spend the money on precision land grading for surface irrigation, or for pump, pipes, and sprinklers/tricklers for sprinkle/trickle, you should want to make the best use of that investment. The best hardware in the world is no good if it is not used correctly.

Again, the keys to effective, efficient irrigations are knowing when, how much, and how to irrigate. Modern irrigation scheduling techniques help you to know when and how much to irrigate.   

"Irrigation Scheduling" is a generic term applied to any technique/practice that is intended to aid the farmer in determining when and how much to irrigate.  There are a number of ways in which the different techniques could be categorized.  A common way is to group them as either "water budget" and "graphical/sensor-based" methods.  This advisory covers graphical/sensor-based (also called "bottom-line") scheduling.  

Water budget irrigation scheduling can be complex, time-consuming, and ultimately expensive.  However, there are some situations where it may be a better fit.  Visit the advisory on water budget irrigation scheduling.

Again, the keys to effective, efficient irrigations are knowing when, how much, and how to irrigate. Modern irrigation scheduling techniques help you to know when and how much to irrigate.

Soil moisture levels can be measured in two ways, 1) as volumetric moisture, and 2) as soil moisture tension.  Volumetric soil moisture is a measure of how much physical water is in the soil.  Soil moisture tension is a measure of how much effort is needed to extract moisture from soil.

The most common method of describing volumetric soil moisture is a depth of water per depth of soil.   With English units this is most commonly "inches of water held per foot of soil".   (Some will use a percentage of water by volume measure.)  The measure for soil moisture tension is pressure.  The most common unit of measurement is "centibars of pressure".

Water budget irrigation scheduling generally works with volumetric soil moisture.   Graphical methods will use either volumetric or soil moisture tension measurements.

Soil will hold water against the pull of gravity, keeping it available for plants to extract through their root zones.  There are limits to the amount of available water. The upper limit is the Field Capacity (FC) of the soil.  The lower limit is the Permanent Wilting Point (PWP).

Essentially, field capacity is the most water the soil will hold against the pull of gravity.  You can add more water to soil that is at field capacity but the excess water will just drain down through the soil, below the effective root zone and away from the plant. 

Note however, that it may take some substantial amount of time for this excess water to drain down through the soil profile.  The amount of time depends on the type of soil and the depth of the root zone.  While this water is draining through the root zone, it is available for the crop to use.  This is a key aspect in refining water budget irrigation scheduling models as is explained in the advisory for water budget scheduling .

Permanent wilting point is the level of moisture in the soil when the plant can no longer pull water from the soil.  Thus the plant permanently wilts.  The PWP will change with the plant and soil as some plants can extract more water from a soil, or any given soil,  than others.

4. Between field capacity and permanent wilting point is "available" water.  This is water that the plant can pull from the soil through its root system.

The Effective Root Zone as a System
The Effective Root Zone is that depth of soil, determined by the farmer, where the farmer wants to control soil moisture.
Definition - Effective Root Zone (ERZ) - the depth of soil where you want to control soil moisture.   It may or may not be the full depth of the plant's roots.  (For example, in many vegetable crops, the effective root zone is much shallower than the full root system because of quality concerns.)  The ERZ is chosen by you.
The Effective Root Zone can hold up to some maximum amount of water that is available for the crop to use.  This is called the Available Water Holding Capacity.
Definition - Available Water Holding Capacity (AWHC) - the total of available water in the effective root zone, when the ERZ is at field capacity.  Thus:
    AWHC = field capacity - permanent wilting point

AWHC's are a volumetric soil measurement.  For irrigation scheduling purposes, we usually talk about AWHCs as the total inches of water available in the effective root zone.  Thus we might say, "the AWHC for that field is 5.5 inches total in the 4 foot root zone", or AWHC = 5.5 inches.
Normally you do not let the plant use up all the water available in the Effective Root Zone.  By definition this would put the plant at Permanent Wilting Point. A common name used to describe the amount of water allowed to be used by the crop between irrigations is the Management Allowed Depletion, MAD.
Definition - MANAGEMENT ALLOWED DEPLETION (MAD) - the amount of water that is allowed to be used by the plant between irrigations.   MADs can be described as a percentage of the AWHC or as "inches of water".  Thus we might say that "the MAD for the field is 45% of AWHC".  We are saying that we will allow the crop to use 45% of  the available water in the effective root zone between irrigations.  Or, we might say that "the MAD for the field is 3.5 inches".  We are now saying that we will allow the crop to use 3.5 inches between irrigations.

The MAD in inches is equal to the MAD percentage times the AWHC. For example...
   AWHC = 5.5 inches
   MAD = 45%
   MAD in inches is .45 x 5.5 = about 2.5 inches

Letting the plant use up the majority of  available water in the effective root zone between irrigations would put excessive stress on the crop, affecting yields and quality.  Also, if the plant was to use water all the way down to the permanent wilting point, it would be dead (the definition of PWP is just that, permanently wilted). 

As will be explained later in this advisory however, farmers desire to place more or less stress on a crop at any one time according to how they wish the crop to develop.   It is very common for winegrape growers to put stress on their vineyards purposely in order to increase quality.  A key objective of irrigation scheduling is to control the amount of stress on a crop.
Modern farmers look at the Effective Root Zone as a system.   Among other things, they want to control the amount of water in this system.

They attempt to measure or predict the amount of water in the ERZ at any one time.   As well, they try to measure or predict the amounts of water going into and out of the ERZ.

Water going into the ERZ is normally rain and irrigation.  Upflux from a high water table may be a significant factor in some areas.

Water going out of the ERZ is primarily deep percolation (from excess irrigation or infiltrated rain) and crop evapotranspiration (crop water use).
Modern water management looks at the Effective Root Zone as a system.  Managers try to:
  1. Identify water going into and out of the system,
  2. Measure these amounts, and
  3. Control these amounts.

A schematic of the effective root zone is seen below.  The primary sources of water going into the ERZ are rainfall and irrigation.  In areas with a high water table there may be a constant upflux of water into the root zone.  Also in these areas there may be significant amounts of water moving sideways into and out of the rootzone.  The primary losses of water are crop evapotranspiration  and deep percolation from excess irrigation or rainfall.
Figure 1 - Schematic of the Effective Root Zone showing the "types" of water that come into and go out of it

Graphical irrigation scheduling consists of two parts:
  1. Graphing/reporting frequent measurements of soil or plant moisture.
  2. Using additional data and calculations in conjunction with those measurements to guide irrigation system management.
Measurements are most commonly either of volumetric soil moisture, soil moisture tension, or plant moisture tension.

Additional data and calculations will usually involve an estimate of crop water use and how much water or set time is needed per irrigation.

The trend in measurements can be used to predict irrigation timing in low-frequency irrigation systems such as furrow or border strips.  If the measurement is calibrated to volumetric moisture it may also help in identifying required application depths.

Measurements are most often used to fine-tune run times with high-frequency, micro-irrigation systems.  Irrigations are managed so that the measurements stay within some pre-defined "bracket".
Water budget irrigation scheduling attempts to accurately model the physical processes involved with water moving into, through, and out of the effective root zone.   Actual measurements of soil moisture may be used to reconcile the model results with reality.

In contrast, graphical methods utilize actual measurements of soil/plant moisture as the main guidance.  Calculations of crop water use and/or required irrigations are commonly used in conjunction with the measurements.  However, there is no attempt to be EXACTLY accurate.  Rather, the calculations are used as a starting point.  The actual management is fine-tuned as a result of the field measurements.

Graphical/sensor-based scheduling does not care about accurately modeling   the "ins" and "outs" of water in the effective root zone.   All it worries about is the "bottom-line", how much water is in the root zone, or the plant, at any one time.  It is certainly simpler than water-budget scheduling.  It could be more labor-intensive or costly depending on how often you want to take measurements and what you use as an instrument.

However, there may still be many of the same information requirements as for water-budget scheduling.  It is relatively easy just to take soil or plant-based moisture measurements.  But somehow, you must be able to translate these measurements into the effect on your crop and make management decisions based on them.

Assume you are using a neutron probe to take soil moisture measurements.  The probe reports soil moisture on a "volumetric" basis.  That is, it measures actual quantities of water in the soil.  Assume that a reading over a four foot depth gave you 3.5 inches of water total.  The question is what does that 3.5 inches of water mean to the crop?  If it was a sandy soil it might mean there was plenty of water.  But it might mean stress if in a heavy clay.  Also, was a four foot depth the right depth to measure? And was the measurement taken in the right part of the field?  If you thought the 3.5 inch measurement, in conjunction with the trend of the measurements, indicated that an irrigation is needed, how do you decide how much water to apply?

Figures 2 and 3 below are reports for graphical irrigation scheduling systems.   You can see the type of data that is obtained.  The trend of soil moisture addition from irrigations and the extraction from crop water use is clearly evident.   The key is interpreting this data correctly, in conjunction with other data, in order to make correct management decisions.
Click for larger image
Figure 2 - Example report for a graphical irrigation scheduling system - approximately weekly readings of a neutron probe; irrigations are indicated by the "I"s just above the graph

Figure 3 - Example report for a graphical irrigation scheduling system - readings logged about every 15 minutes using electromagnetic sensors

The basis for graphical irrigation scheduling is frequent measurements of actual moisture conditions.  Decisions that need to be made regarding this measurement include:

  • What type of measurement to take?
  • Where in the field do you take these measurements?
  • When do you take these measurements- that is, how often?
  • How do you take these measurements- that is, what instrument is used?
What to measure - You need to decide first whether you want to measure soil or plant moisture.  Then, if you do decide to measure soil moisture, you need to decide whether to measure volumetric moisture or soil moisture tension. 

Volumetric soil moisture is a measure of how much physical water is in the soil.  With low frequency irrigation systems such as furrow,  measuring the volumetric soil moisture can provide direct estimates of net required application depths per irrigation. 

Soil moisture tension is a measure of how much effort is needed to extract moisture from soil.  Whether you measure soil moisture tension or not may depend on your soil type.  Tension measurements in lighter soils may not provide enough resolution as tensions tend to stay low in these soils until a certain moisture level at which point they increase dramatically.  This is due to the preponderance of larger soil pores which tend to give up water readily to the plant.   On the other hand, some agronomic systems depend on intentional stress as an important factor in crop quality, such as in some varieties of winegrape.  It may be easier for managers to repeat successful water regimes year-to-year and field-to-field if they are reading tensions instead of volumetric water.

Plant moisture tension is a direct reading on the plant's condition.  Although an advantage of this type of measurement is that you can go anywhere in the field, there are at least two problems.  First, the plant's condition also depends on fertility, disease, and pest factors.  Second, measuring plant moisture usually involves restrictions on the time of day. 

Where to measure - The point here is that the measurement must be representative of the field conditions- or at least at that point where you want to manage.  For example, some farmers will manage to the wettest part of a field, some to the driest, depending on the situation.   Some may key on the most vigorous parts of the field, others the weakest.

Often, farmers will say that they don't like soil moisture measurements since they only schedule to one place in the field.  That may well be true (except with the "feel" method).   However, the alternative is no scheduling- is that any better?  There are several alternatives in the situation where conditions in a field are that varied:

  • You could consider breaking the field down into smaller management blocks
  • You could consider taking measurements at more than one point in the field and integrating the results somehow
  • You could consider a point in the field that reflects the trade-offs that you must be making
  • Consider using a plant-based measurement that allows you to measure anywhere in the field
How (what instrument) to measure - There are many different instruments available for measuring plant and soil moisture.    These include:
Neutron probes - these instruments utilize a source of fast neutrons.  When the instrument probe containing this source is lowered into an access tube inserted in the soil, the neutrons are active in the soil around the probe.  When these neutrons strike hydrogen atoms, they slow down.  A counter in the probe counts the slowed neutrons.  The amount of hydrogen is basically due to the amount of water in the soil, thus the instrument measures volumetric moisture.  These instruments are expensive and require special handling, licensing, and training.   However, they are very accurate when calibrated and are somewhat of a research standard.  The neutron source will decay with time and thus, the instrument requires periodic re-calibration.  The need for an installed access tube limits the number of places that will be measured in a field and they haven't been adapted to continuous-reading systems yet.

Figure 4 (above right) - A Neutron Probe- the actual source is in a probe that is dropped down into the access tube that the instrument housing is sitting on

Electromagnetic probes - these instruments are much like a neutron probe except that they use electromagnetic waves to measure volumetric moisture instead of a nuclear source.  Some instruments measure the capacitance of the soil (frequency domain response).  Figure 5 is a picture of one of these types installed in an orchard.  This is a permanent installation in a continuous-reading system.  Other types measure the time it takes to propagate a wave through a certain distance, such as the Gro-Point pictured in Figure 6.  These types of instruments may be portable, in which case they are used much like a neutron probe (except they are certainly easier to transport and store).  However, the great advantage of these types of instruments is that they can be integrated into systems for continuous readings.   However, the access tubes must be installed correctly and they can be expensive.

Figure 5 - Frequency Domain Response type instrument in an orchard- the C-Probe

Figure 6 (right) - Time Domain Transmissometry type instrument - the Gro-Point

Tensiometers - these consist of a sealed tube, filled with water, capped at one end with a porous ceramic tip.   The ceramic-tipped end is embedded in the soil at the depth to be measured.   As soil dries out, it will draw water from the tube through the ceramic tip.   A vacuum gauge will indicate soil moisture tension.  These instruments are inexpensive but must be maintained and have a limited measuring range (usually on the order of 0 to 85 centibars).  Electronic tensiometers are available so that they can be used in a continuous-reading system although this is not the norm.  They are most useful in fields that are kept consistently moist.

Granular matric blocks (commonly called "gypsum" blocks) - these consist of a porous material with two wires embedded.  They are usually plug-shaped and measure in the range of  2 inches long and an inch in diameter. A current is passed through the wires.  The resulting resistance to current flow from one wire to the other is related to the amount of moisture in the block.  The amount of moisture in the block is related to the tension in the soil versus the tension in the block, since they are both porous materials.  Thus, the output from blocks is calibrated to soil moisture tension.  These have a much-wider measuring range than tensiometers but are generally considered better in the drier ranges and on finer soils.  Some materials used will deteriorate with time.  They are amenable to being integrated in continuous-reading systems.

Leaf-pressure chambers - these consist of a sealed chamber.  One end is sealed with an elastic material with a small hole in it.  A freshly-cut leaf petiole is placed in this hole and the sealed chamber is pressurized.  When the pressure is high enough you can observe sap start to flow from the petiole.  A pressure gauge is read to give a direct measurement on plant moisture tension.  These are relatively expensive and slower to use.    Readings should be taken at the same time, usually at solar noon.    However, anywhere in the field can be sampled with this device.

(loamy sand)
(sandy loam)
(clay loam)
(field capacity) (field capacity) (field capacity) (field capacity)
.0 Leaves wet outline on hand when squeezed. Appears very dark, leaves wet outline on hand, makes a short ribbon. Appears very dark, leaves wet outline on hand, will ribbon out about one inch. Appears very dark, leaves slight moisture on hand when squeezed, will ribbon out about two inches.
.2 Appears moist, makes a weak ball. Quite dark color, makes a hard ball. Dark color, forma a plastic ball, slicks when rubbed.
.4 Appears slightly moist, sticks together slightly. Dark color, will slick and ribbons easily.
.6 Fairly dark color, makes a good ball. Quite dark, forms a hard ball. Quite dark, will make a thick ribbon, may slick when rubbed.
.8 Dry, loose, flows through fingers. (wilting point) Slightly dark color, makes a weak ball. Fairly dark, forms a good ball.
1.0 Lightly colored by moisture, will not ball. Fairly dark, makes a good ball.
1.2 Very slight color due to moisture. (wilting point) Slightly dark, forms a weak ball. Will ball, small clods will flatten out rather than crumble.
1.4 Lightly colored, small clods crumble fairly easily. Slightly dark, clods crumble.
1.6 Slight color due to moisture, small clods are hard. (wilting point)
1.8 Some darkness due to unavailable moisture, hard & cracked clods (wilting point)
2.0 --- --- --- ---
Table 1 - Guidelines for estimating soil moisture deficits from the look and feel of a soil sample

When (how often) to measure - How often you take the moisture measurement depends on the instrument and how much resolution you feel you need to be accurate with your scheduling.  Commercial consulting services that perform moisture measuring will generally do so on a weekly basis.  Figure 2 seen above is an example of the data that can be obtained with weekly measurements.  The newest development in the field is the continuous-reading system.   These utilize electric-powered sensors of some type (tensiometers, granular blocks, or FDR/TDR types) connected to datalogger.  Commonly, a solar panel with a battery-backup system is used for power.  The datalogger can be set to poll the sensor at many different frequencies, sometimes integrating continuous readings over time into one stored reading.  For example, a sensor may be read every 5 minutes, with the readings averaged each hour.  The hourly average is stored for later downloading.   Figure 3 above is an example of the type of data obtained from this system.

Micro-irrigation systems (drip, spitter, micro-sprinkler) are designed to be run frequently, sometimes daily. Many times we compare the gallons per hour per tree/vine of the system design to the daily water use of the crop (evapotranspiration, ETc) to determine required set times on a daily basis.

The equation to convert ETc to hours of system operation is...

    HR = ETc * AREA / (GPH * AE * 1.605)

HR = daily hours of system operation
ETc = daily crop water use in inches/day
GPH = total flow to each plant in gallons per hour
AE = system efficiency as a decimal 0 - 1.0

For example, there is a grape vineyard with two 1-gallon per hour emitters per vine. The vines are spaced 8 by 12. The estimated system efficiency is 80% (.8 as a decimal) and the current daily crop water use is estimated at .25 inches/day. Then...
HR = ETc * AREA / (GPH * AE * 1.605)
HR = .25 * (8 * 12) / (2 * .8 * 1.605)
HR = 9 hours of operation per day

You can use the same equation with "drip-tape" systems. Assume you have a drip-tape system on fresh tomatoes with a bed spacing of 60 inches (five feet). The tape is rated at .2 gallons/minute per 100 feet of tape (or 12 gallons/hour per 100 feet of tape). The crop is using water at the rate of .25 inches/day (ETc = .25 in/day). With an 80% efficiency...
HR = ETc * AREA / (GPH * AE * 1.605)
HR = .25 * (5 * 100) / (12 * .8 * 1.605)
HR = 8 hours of operation per day

A classic example of a graphical scheduling system would be:.

  1. Make an estimate of daily set times as above.  The estimate may be revised daily or weekly.
  2. Install tensiometers (or whatever device is appropriate or preferred) and read them every 2-3 days.
  3. Plot the readings, if they start to rise, indicating more tension, increase the set times. If they start to lower, indicating less tension, decrease the set times.
  4. Whenever the plot takes large swings up or down, determine whether your estimates of crop ET or wrong (indicating an incorrect crop coefficient curve or estimates of reference ET) or check the irrigation system.
Figure 2 is reproduced below.  You can see that around the end of May either a hot spell hit unexpectedly, the irrigation system malfunctioned, management stressed the crop intentionally, or management "dropped the ball".   Whatever the reason, irrigations increased in June until soil moisture was brought up to near field capacity.  Soil moisture was maintained very high through July and August, then allowed to dry off through September and October.  Note how soil moisture dipped briefly at the start of August as irrigations ceased for some reason.
Click for larger image
Figure 7 - Example data set from a graphical irrigation scheduling system