Performance measures

4.2.1 Application uniformity 4.2.2 Application efficiency

4.2.3 Water requirement efficiency 4.2.4 Deep percolation ratio

4.2.5 Tailwater ratio

4.2.6 Integration measures of performance

Among the factors used to judge the performance of an irrigation system or its management, the most common are efficiency and uniformity. These parameters have been subdivided and defined in a multitude of ways as well as named in various manners. There is not a single parameter which is sufficient for defining irrigation performance. Conceptually, the adequacy of an irrigation depends on how much water is stored within the crop root zone, losses

percolating below the root zone, losses occurring as surface runoff or tailwater the uniformity of the applied water, and the remaining deficit or under-irrigation within the soil profile following an irrigation. Ultimately, the measure of performance is whether or not the system promoted production and profitability on the farm. In order to index these factors in the surface irrigated environment the following assumptions can be made, the consequences of which are that performance is based on how the surface flow will be managed:

i. the crop root system extracts moisture from the soil uniformly with respect to depth and location;

ii. the infiltration function for the soil is a unique relationship between infiltrated depth and the time water is in contact with the soil (intake opportunity time); and iii. the objective of irrigating is to refill all of the root zone.

4.2.1 Application uniformity

When a field with a uniform slope, soil and crop density receives steady flow at its upper end, a water front will advance at a monotonically decreasing rate until it reaches the end of the field. If it is not dyked, runoff will occur for a time before recession starts following shutoff of inflow. Figure 40 shows the distribution of applied water along the field length stemming from the assumptions listed above. The differences in intake opportunity time produce applied depths that are non-uniformly distributed with a characteristic shape skewed toward the inlet end of the field.

Application uniformity concerns the distribution of water over the actual field. A number of technical sources suggest the Christiansen coefficient as a measure of uniformity. Others

argue in favour of an index more in line with the skewed distribution shown below. For example, Merriam and Keller (1978) propose that distribution uniformity be defined as the average infiltrated depth in the low quarter of the field, divided by the average infiltrated depth over the whole field. This term can be represented by the symbol, DU. The same authors also suggest an 'absolute distribution uniformity', DU_a which is the minimum depth divided by the average depth. Thus, the evaluator can choose one that fits his or her perceptions but it should be clear as to which one is being used.

Figure 40. Distribution of applied water along a surface irrigated field showing also the depth required to refill the root zone (after Walker and

Skogerboe, 1987)

4.2.2 Application efficiency

The definition of application efficiency, E_a, has been fairly well standardized as: (26)

Losses from the field occur as deep percolation (depths greater than Zreq) and as field tailwater or runoff. To compute E_a it is necessary to identify at least one of these losses as well as the amount of water stored in the root zone. This implies that the difference between the total amount of root zone storage capacity available at the time of irrigation and the actual water stored due to irrigation be separated, i.e. the amount of under-irrigation in the soil profile must be determined as well as the losses.

4.2.3 Water requirement efficiency

The water requirement efficiency, E_r, which is also commonly referred to as the storage efficiency is defined as:

(27)

The requirement efficiency is an indicator of how well the irrigation meets its objective of refilling the root zone. The value of E_r is important when either the irrigations tend to leave major portions of the field under-irrigated or where under-irrigation is purposely practiced to use precipitation as it occurs. This parameter is the most directly related to the crop yield since it will reflect the degree of soil moisture stress. Usually, under-irrigation in high probability rainfall areas is a good practice to conserve water but the degree of under-irrigation is a difficult question to answer at the farm level.

4.2.4 Deep percolation ratio

The loss of water through drainage beyond the root zone is reflected in the deep percolation ratio, DPR, defined as:

(28)

High deep percolation losses aggravate waterlogging and salinity problems, and leach valuable crop nutrients from the root zone. Depending on the chemical nature of the

groundwater basin, deep percolation can cause a major water quality problem of a regional nature. These losses can return to receiving streams heavily laden with salts and other toxic elements and thereby degrade the quality of water to be used by others.

4.2.5 Tailwater ratio

Losses from the irrigation system via runoff from the end of the field are indicated in the tailwater ratio, TWR:

(29)

Runoff losses pose additional threats to irrigation systems and regional water resources. Erosion of the top soil on a field is generally the major problem associated with runoff. The sediments can then obstruct conveyance and control structures downstream, including dams and regulation structures.

4.2.6 Integration measures of performance

With the five measures of performance defined above, a broad range of assessments is possible and specific remedies identified. Application efficiency is the most important in terms of design and management since it reflects the overall beneficial use of irrigation water. In later sections, a design and management strategy will be proposed in which the value of application efficiency is maximized subject to the value of requirement efficiency being maintained at 95-100 percent. This approach thereby eliminates E_r from an active role in surface irrigation design or management and simultaneously maximizes application uniformity. If the analysis tends to maximize E_a, distribution uniformity is not qualitatively important and may be used primarily for illustrative purposes. Of course, some may prefer performance discussed in terms of uniformity or be primarily involved in systems where underirrigation is an objective or a problem. For these cases, uniformity is still available. The assumption of

maximization of application efficiency in effect states that losses due to deep percolation or runoff are equally weighted.