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.