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  Irrigation Performance Measurements - Distribution Uniformity and Irrigation Efficiency
In This Advisory
Purpose - The purpose of this advisory is to...
  • Provide a brief description of the two measures of irrigation performance- distribution uniformity and irrigation efficiency
  • Discuss the problems in identifying actual numbers for these measures
  • Identify the two important relationships between them
  • Explain why distribution uniformity is of primary importance
INTRODUCTION
Water managers are many times called into situations where they have to analyze a water system quickly.  A plan of attack is to break down the water system (assuming it's a farm) into the four basic components:
  1. Primary water supply
  2. Conveyance system
  3. The in-field irrigation system
  4. The drainage system
Then, start asking questions about each of these.   For example, considering the Primary Water Supply:
  • How much total water is available?
  • What quality is it?
  • How flexible is the delivery?
  • How much does it cost?
Finally, since the focus is usually on the in-field irrigation system, look for two measures of in-field irrigation system performance:
  1. How evenly is water being applied - that is, what is the distribution uniformity (DU)?
  2. How much control is there over the total application? - noting that this also implies that the grower knows how much he/she is trying to apply per irrigation.
This advisory explains these two measures and the important relationships between them
IMPORTANT!! Understanding the relationship between distribution uniformity and irrigation efficiency is crucial to improving in-field irrigation performance.

  MEASURES OF IRRIGATION PERFORMANCE
There are two measures of irrigation performance, distribution uniformity (DU) and irrigation efficiency (IE). 
Distribution Uniformity - Distribution uniformity is a measure of how evenly water soaks into the ground across a field during the irrigation.  If eight inches of water soaks into the ground in one part of the field and only four inches into another part of the field that is poor distribution uniformity.  Distribution uniformity is expressed as a percentage between 0 and 100%.  Although 100% distribution uniformity is theoretically possible, it is virtually impossible to attain in actual practice.  Good distribution uniformity is critical for reducing deep percolation.
Evaluating DU is a fairly straightforward, although a statistical sampling, process.  Questions concerning the actual number reported for DU involve:
  • The sampling procedure- that is, is the evaluation procedure giving a representative picture of the DU over the entire field?
  • The basis for the measurement- the most common measure of DU is to divide the average depth infiltrated in the 1/4 of the field with the lowest infiltrated depths by the average infiltrated depth in the field.  This is called the "low-1/4 DU".  However, some will hold to the lowest 1/8 of the field, and some to the absolute minimum depth infiltrated.  A very famous and widely-used measure of DU for sprinkle systems is the Christianson Uniformity (CU).  It is a measure of the average of the lowest 1/2 of the field.
While DU is a fairly simple concept (differences of opinion arising mainly from questions concerning the basis for the measurement), there are many measures of the efficiency of an irrigation system depending on the purpose of the efficiency measurement.   Many times, irrigation efficiency (or as used by some authors, "application efficiency") is used only to indicate how much of the applied water is stored in the root zone of the crop.  This stored water is then available for crop water use, evapotranspiration.  Crop water use is considered a beneficial use.  However, this narrow definition does not consider that some deep percolation may be required to maintain a salt balance.  This deep percolation, while not available for actual crop water use, is also a beneficial use.
Irrigation Efficiency - Irrigation efficiency was defined by the American Society of Civil Engineer's On-Farm Irrigation Committee in 1978 as the ratio of the volume of water which is beneficially used to the volume of irrigation water applied.  Beneficial uses may include crop evapotranspiration, deep percolation needed for leaching for salt control, crop cooling, frost control, and as an aid in certain cultural operations.  A review of irrigation performance "Irrigation Performance Measures: Efficiency and Uniformity" by Burt et al was published by the ASCE in their Journal of Irrigation and Drainage vol 123:6, November/December, 1997.   The reader is referred to this paper for a rather complete discussion, although controversial in some sections, of the current thinking concerning performance measurements and concepts.
There are many specific mathematical definitions of efficiency in use.   Differences in definitions are due primarily to:
  • Accounting for runoff and deep percolation.
  • Whether it is for an individual irrigation or an entire season.
  • Whether it is for an individual farm, irrigation project, or basin.
Many people will hold to a strict measure of efficiency considering only the beneficial use on the individual field.  Others will classify reuse of any surface or sub-surface drainage by other farms as beneficial use.  Some will ignore measurements of individual irrigations to focus on a seasonal efficiency.  The individual farmer should focus on individual, in-field irrigation efficiency because his/her crop development/yield and costs are dependent on this.  Basin and project-wide estimates of irrigation efficiency may be useful in political discussions but do not address the individual farm.
Irrigation efficiencies are also expressed as a percentage between 0 and 100%.   100% irrigation efficiency is not theoretically attainable due to immediate evaporation losses during an irrigation.  However, there could easily be close to 95% IE if a crop was under-watered.  In this case, assuming that there was no deep percolation, all water applied and not immediately evaporated would be used by the crop.
Under-watering a crop will theoretically result in a high irrigation efficiency.   However, it may not be a very effective way of farming and could actually lead to an inefficient use of resources.  This could be because of an inefficient use of fertilizer, a weak crop that is more susceptible to pest pressures and thus, requires additional chemical applications, or sub-par yields that would require additional cropped acreage to maintain farm income.
Note that the terms "irrigation efficiency" or "application efficiency" should not be confused with the term "water use efficiency" (WUE).  Water use efficiency is generally a measure of yield per unit water applied.  

  RELATIONSHIPS BETWEEN DISTRIBUTION UNIFORMITY AND IRRIGATION EFFICIENCY
Generally, the distribution uniformity of the irrigation system is the first concern.  The reason for this is explained by the four graphs below.  They are a profile view of two adjacent sprinklers in a field and the rootzone under them.  The spray patterns from the adjacent sprinklers must overlap to result in the same amount of water falling in all parts of the field. 
In the figures, the horizontal, dashed line depicts the depth of the actual soil water deficit at irrigation.  This is the amount of water that the grower would be trying to soak into the soil to satisfy crop water use requirements.  The dotted-dashed line depicts the actual depth of water infiltrated during the irrigation.   Deep percolation is indicated whenever the actual depth of irrigation (the dotted-dashed line) is below the soil water deficit line (the horizontal, dashed line).   Conversely, under-irrigation is indicated whenever the actual depth of irrigation line is above the soil water deficit line.  The depths multiplied by the area of a field indicate the volumes of water applied, stored, and percolated.
Figures 1 and 2 demonstrate the first relationship between DU and IE- There must be good distribution uniformity before there can be good irrigation efficiency, if the crop is to be sufficiently watered.

First relationship between DU and IE: There must be good distribution uniformity before there can be good irrigation efficiency, if the crop is to be sufficiently watered.

In Figure 1, the farmer has irrigated to sufficiently water the entire field.   The poor distribution uniformity, indicated by the uneven infiltrated water, has resulted in excessive deep percolation. That is, the deep percolation was much more than would be needed to maintain a salt balance.

FIGURE 1 -Depiction of irrigation resulting in poor distribution uniformity and excessive deep percolation
 
In Figure 2,the farmer has acted to prevent excessive deep percolation, without any other changes.  Now part of the field remains under-irrigated.   Under-irrigation usually results in a high irrigation efficiency as most water applied is stored in the root zone, available for plant use.  But it may not be an effective way of growing as the resulting water stress on the crop in some parts of the field will usually decrease yields.  Also, there is the need for some deep percolation for leaching to maintain a salt balance.  Note that the leaching must be uniform over a number of years to prevent areas of excessive salt accumulation.
FIGURE 2 -Depiction of irrigation resulting in poor distribution uniformity and insufficient irrigation in parts of the field
Second relationship between DU and IE:
The second relationship between DU and IE is that good distribution uniformity is no guarantee of good irrigation efficiency.

The second relationship between DU and IE is that good distribution uniformity is no guarantee of good irrigation efficiency .  Figures 3 and 4 show that a good distribution uniformity allows a good irrigation efficiency, but the total amount of water applied must still be controlled.  Figure 3 depicts a good irrigation. There was a high distribution uniformity as indicated by the flatter infiltrated depth line (the dotted-dashed line). About the right amount of water was applied. There is little deep percolation (enough for salt control) and the entire field is wetted sufficiently. It is assumed that surface runoff was minimal or collected for reuse.
FIGURE 3 -Depiction of an irrigation sufficiently watering the entire field with good distribution uniformity and irrigation efficiency

Figure 4 depicts an irrigation with the same high DU (same flat infiltrated water line).  However, twice as much water as needed was applied, resulting in low irrigation efficiency.  Another practical example of this situation is the farmer who is using a well-designed and maintained micro-irrigation system.  The hardware provides the good distribution uniformity and the potential for high irrigation efficiency.  However, if the farmer runs the system twice as long as necessary, the potential is not realized.


FIGURE 4 -Depiction of irrigation resulting in good distribution uniformity but poor irrigation efficiency

In summary, Figures 1 through 4 demonstrate that:
  1. Improved irrigation system hardware may result in higher distribution uniformity and also make it easier to achieve higher irrigation efficiency.
  2. But achieving high irrigation efficiency ultimately depends on the management of the system.
Corollary to relationships between DU and IE: If the whole field is to be sufficiently watered, then the distribution uniformity becomes the theoretical upper limit to irrigation efficiency.
That is why DU is the first aspect examined when trying to improve irrigation performance.

An important corollary to the above relationships is that if the entire field is assumed to be sufficiently wetted during an irrigation, including that water required for leaching, then the distribution uniformity is the upper limit of irrigation efficiency.   And it then follows that the first concern when improving irrigation system performance is the distribution uniformity.

  EVALUATING IRRIGATION SYSTEMS FOR DISTRIBUTION UNIFORMITY
There are rational processes that have been developed for evaluating distribution uniformity of most major types of irrigation systems.  One of the most well-known of these was developed by the Irrigation Training and Research Center at California Polytechnic State University (ITRC).  However, there have been many different evaluation programs developed all over the world.  In addition, although the ITRC programs were the basis for the Mobile Irrigation Laboratories in California, many of the labs have developed their own software and techniques to match conditions in their area.

Any evaluation process is a statistical process.  That is, a sample of the field is taken and the DU for the entire field is inferred from the sample.   Practical limitations regarding costs mean that the estimated number may be plus or minus 10-20% from the actual DU. 

However, the evaluation will:
  1. Point out obvious problems with the system hardware or management.
  2. Should provide further education to the grower on the correct way to operate the system.
  3. Is a rational, repeatable method. That is, even though the calculated DU may not be entirely accurate, relative increases in DU as a result of changes should show up with repeated evaluations using the same techniques.
Note also that there are many different ways that a system can be evaluated.   An irrigation specialist may look at a farm from a holistic point of view and make observations and suggestions that speak to improving obvious deficiencies first.   Then as the big changes are made, formal, numeric evaluations for DU (with their additional cost) would be made.

Any competent irrigation specialist can perform a DU evaluation.  It doesn't have to be a formal Mobile Irrigation Laboratory group.  The software and explanations of the techniques (including on-site short courses) developed by ITRC are available from them for a nominal cost.  For example, the West Stanislaus Resource Conservation District contracts with a private consultant on an as-needed basis for system evaluations in their area.