2.1.1 Critical Soil Water Content
2.1.2 Field Capacity
2.1.3 Infiltrability and Infiltration Rate
2.1.4 Average Application Depth
2.1.5 Uniformity of Application Depth
2.2.1 Farm Distribution System Efficiency
2.2.2 Application Efficiency
2.3 Measures of the Adequacy of Irrigation
2.4 The Effectiveness of Irrigation
2 Irrigation System Performance Measures
Irrigation performance measures are outputs, the results of design and operating decisions. To be useful they need to predictably quantify the effects of design and operating decisions on the extent to which the purpose of irrigation is achieved, and how efficiently the purpose was achieved. They must do this in a way that assists designers and operators to make appropriate decisions.
Unfortunately it is not uncommon for irrigation design courses to focus on one performance measure – irrigation efficiency – and to use it as an input to the design process, rather than using it as one measure of system performance that is to be optimised, along with others, during the design process. It is not surprising, therefore, that performance in the field rarely exceeds, and typically does not match, the assumed performance.
Performance measures, if they are to serve their purpose, must take full account of the following important physical factors. These are:
- Optimum crop growth and yield depends on the maintenance of a specific level of water pressure within the plant. This implies maintaining soil water pressure above a certain threshold.
- At the time of irrigation, the soil’s capacity to store additional water is limited.
- There is a limit to the rate at which water can infiltrate soil, and this limit varies over time.
- Irrigation almost always results in the infiltrated depth of water varying throughout the field. Irrigation is not uniform.
The purpose of this section is to describe the key concepts and terms that are the basis of taking proper account of the above physical factors, and then to describe selected performance measures that are relevant to the design and operation of irrigation application systems. The performance measures will be discussed in three categories. The first deals with the effectiveness of irrigation. The second concerns the adequacy of irrigation, or the extent to which the purpose of irrigation is achieved. The third deals with how efficiently the irrigation system did its job.
2.1 KEY CONCEPTS & TERMS
2.1.1 Critical Soil Water Content
The assumption is made that the goal of irrigation is to avoid loss of income due to crop stress induced by low soil water content. To achieve this goal it is necessary to irrigate, at the latest, when the soil water content has dropped to the point where the crop comes under sufficient stress to cause loss of income. We will refer to this as the critical soil water content. In general, it varies with crop type and soil type.
To determine the critical soil water content it is necessary to know the leaf water pressure (or potential) at which leaf stomata begin to close. If leaf water pressure falls below this threshold the stomata begin to close to reduce evaporative water loss and hence maintain plant tugor. Closure of the stomata also means a reduction in the assimilative capacity of the plant and hence its productivity. It is assumed, for the purpose of irrigation design and operation, that the soil water pressure at which the stomata close is the same as the critical leaf water pressure.
To determine the critical soil water content it is also necessary to know the soil moisture characteristic for the soil to be irrigated because the minimum allowable soil water pressure occurs at different soil water contents for different soil types. The soil moisture characteristic is the relationship between soil water pressure and soil water content.
In summary, the critical soil water content depends on the critical leaf water pressure of the crop to be grown, and the soil moisture characteristic of the soil in which the crop is to be grown. Different combinations of crops and soil(s) will lead to a range of critical soil water contents on each farm.
2.1.2 Field Capacity
Field capacity is an estimate of the maximum volume of water that may be temporarily stored in the soil profile for plant use (Skaggs et al., 1980).
Field capacity is defined to be the water content in a field soil after the rate of drainage beyond a specified soil depth has become small. The soil profile should initially be at saturation, and there should be no evaporative losses during the drainage period. The time for drainage to decrease to a small amount varies from a few hours, for coarse-textured soils, to several days for fine-textured soils. The soil water content at field capacity depends on the soil moisture characteristic and the unsaturated hydraulic conductivity for each of the layers that make up the soil profile of interest, the depth to the water table, gravity, and the definition of what is a small drainage rate.
Field capacity has value, as a concept, because it simplifies estimation of the depth of water that might usefully be stored in the soil profile, and the depth of water that may drain from the profile after application of a given depth of water.
2.1.3 Infiltrability and Infiltration Rate
Infiltrability (or infiltration capacity) is the rate that water will infiltrate soil when the rate is limited by soil factors only. Infiltrability decreases as the pressure difference, or hydraulic gradient, across the infiltration surface reduces.
The infiltration rate is the smaller of the rate at which water is applied and the infiltrability. As long as the infiltrability is greater than the application rate the water will infiltrate as quickly as it is supplied. If the infiltrability decreases with time, which it often does, to be less than the application rate then the soil surface will become ponded. The infiltration rate will then be controlled by the soil profile.
2.1.4 Average Application Depth
Application depth is generally calculated from measurements of flow-rate, irrigation time, and the area irrigated (intended or actual). Because irrigation is non-uniform, this depth is strictly the average application depth. Average application depth can be determined for sprinkler irrigation systems by using a grid of catch-cans to measure the application depth at specific points throughout the wetted area and calculating the areally weighted-average of the measured application depths.
2.1.5 Uniformity of Application Depth
The critical water content and water retention characteristics of the soil, along with crop water use, vary spatially within a field. Ideally, irrigation would apply water in a manner that accounts for this spatial variability. However, this level of precision is currently uneconomic and the assumption is made that the soil water deficit at the time of irrigation is the same over the whole field. This leads to the requirement that irrigation systems apply water in a manner that results in an infiltrated depth that is uniform throughout the field. This is rarely achieved in practice and the non-uniformity of application depth markedly affects design and management decisions.
In practice, some areas may receive more water than is necessary to return soil water content to field capacity. The excess water drains beyond the effective root zone and becomes of no value to the crop. Other areas are under-irrigated in the sense that the soil water content is not returned to the target soil water content. Consequently these areas will reach the critical soil water content much sooner than areas that were returned to the target level (i.e. were fully irrigated). If irrigation proceeds on the basis that all areas are fully irrigated, crop yield or quality reduction will occur on those areas that were under-irrigated.
The degree of variability of application depth about the average application depth is the most commonly used measure of application uniformity. The uniformity coefficient calculated by the following formula is referred to, in the irrigation industry, as Christiansen’s uniformity coefficient. It is the most widely used measure of application depth uniformity.
where xi = application depth for subarea i
X = mean application depth over all subareas i.
2.2 IRRIGATION EFFICIENCY
True measures of irrigation efficiency take account of the spatial uniformity of application depth, the average application depth, and the soil’s capacity to store more water at the time of irrigation. Irrigation efficiency varies with each water application throughout the season, and with site, soil type, and application system.
There are many definitions of irrigation efficiency. The irrigation efficiency principles put forward by Painter and Carran (1978) are conceptually very sound, but are too detailed for general use. For practical purposes it is useful to simplify matters by combining two of their efficiency measures – application efficiency and distribution pattern efficiency. Combining these provides a measure of how much of the water that is applied is actually retained within the effective plant root zone in an irrigation event. By "applied" we mean water leaving the nozzle of a pressurised system, or passing over the sill for border-strip systems. We have retained the term application efficiency to describe
The overall on-farm irrigation efficiency is determined by combining the effect of the application efficiency and the on-farm distribution system efficiency.
2.2.1 Farm Distribution System Efficiency
Farm distribution system efficiency is a measure of how much of the water supplied to a farm is actually applied. It is a function of losses incurred in conveying water from its point of entry onto the farm, to the application device. Quantifying this efficiency requires measuring flow rates through the turnout or pump and over the sill or out of sprinkler nozzles. This efficiency is typically high.
While there are likely to be differences in the distribution system efficiency between piped and open channel systems, the differences are not expected to be as great as for application efficiency. The distribution system efficiency of a piped distribution system, or an open-channel distribution system on a NZ border-strip irrigated farm, is not likely to be significantly affected by application system design decisions, other than the decision to use a sprinkler or surface application method.
2.2.2 Application Efficiency
Application efficiency is a measure of how much of the water that is applied is actually retained in the root zone, in the target area, after an irrigation event. It is principally a function of the soil water status before irrigation, the depth of water that infiltrates the soil, and the soil’s water retention characteristic. All of these factors have a considerable degree of spatial variability and this significantly affects the efficiency. It is also a function of evaporative losses, spray drift off the target area, and run-off.
Application efficiency is defined as follows:
This definition takes into account losses due to spray drift, evaporation, run-off and drainage beyond the effective root zone.
Sometimes a so-called efficiency is calculated using the average application depth to represent the volume of water applied, and the soil water deficit at the time of irrigation to represent the change in the volume of stored water. As will be seen below, this approach over-estimates the real application efficiency because it does not take account of the non-uniformity of application depth.
Measurement of the spatial variation in application depth resulting from sprinkler irrigation, under a wide range of conditions, has shown the distribution of application depth to be normally distributed. Therefore the change in the volume of water stored in a given depth of soil can be calculated from the mean application depth, Christiansen’s uniformity coefficient, and the soil water content at the time irrigation commences (Bright, 1986). The volume of water applied is calculated from the average application depth and the area irrigated.
The relationship between application efficiency and mean application depth is shown in Figure 2-1 for a range of uniformity coefficients. For the purpose of this figure it has been assumed that evaporative, drift, and run-off losses are negligible. The soil water deficit at the time of irrigation is 50 mm.
Figure 2-1: The Effect of Application Depth and Uniformity on Application Efficiency
For irrigation systems that apply water perfectly uniformly (Uc = 100 percent), and for the assumptions stated, it is clear that the application efficiency is 100 percent until the application depth exceeds the soil water deficit – the depth needed to bring the soil to field capacity. Increasing the mean application depth beyond that point results in a linear decrease in application efficiency.
For irrigation systems that apply water with a uniformity coefficient of 70 percent, for example, the decrease in application efficiency begins to occur at mean application depths significantly less than the soil water deficit. When the mean application depth equals the deficit the application efficiency is about 85 percent. Work by John et al. (1985) suggests that sprinkler irrigation systems in NZ are likely to be achieving application uniformity coefficients of around 70 percent. Therefore assessments of the application efficiency of spray irrigation that ignore spatial variability are likely to over-estimate efficiency by about 15 percent.
There are currently no field measurements of the depth distribution for NZ border-strip systems that would enable field-based measurement of application efficiency.
The potential uniformity of irrigation applications is largely determined by design decisions. Operating conditions such as wind (sprinkler) and surface roughness (border-strip) subsequently modify it. Application system design decisions significantly affect the application efficiency.
2.3 MEASURES OF THE ADEQUACY OF IRRIGATION
In theory, application efficiency of greater than 95 percent could be achieved with application systems that apply water reasonably uniformly. However, to do so in practice requires small average application depths and significant soil water deficits after irrigation. Irrigation under these circumstances might not be very effective as a tool for improving crop yield. Losses due to evaporation and spray drift also become a much higher proportion of the volume of water applied.
Application efficiency is therefore not a sufficient measure of irrigation performance. Some measure of the adequacy of irrigation is required. By this is meant some measure of the extent to which soil water content is restored to some management defined level. Two definitions of adequacy are described below, representative of the two main options available.
2.3.1 Adequately Watered Area
The concept here is to determine the proportion of the field for which the soil water content is restored to a level that equals or exceeds a set level, or target soil water content. An acceptance level of 80 percent of the field being irrigated to at least this level (adequately watered) may be appropriate for pastoral agriculture. A higher level may be required for high value process crops. The acceptance level is a management decision. The appropriateness, in terms of financial performance, of specific levels of adequately watered area has not been investigated, to our knowledge.
The area adequately watered by an irrigation event is a function of the average application depth, the uniformity of irrigation, the soil water content prior to irrigation, and the target soil water content. The nature of the relationship between application depth and uniformity is illustrated in the Figure 2-2, again assuming a 50 mm soil water deficit at the time of irrigation.
Figure 2-2: The effect of Application Depth and Uniformity on the Adequately Watered Area
It is clear that as the average application depth increases, so too does the adequately watered area. However, referring to Figure 2-1, application efficiency decreases. It should be noted that as the uniformity of irrigation decreases, the average application depth required to achieve a specified level of adequately watered area increases. To achieve full irrigation over 90 percent of the area requires a mean application depth of about 95 mm if the uniformity coefficient is 70 percent. The resulting application efficiency is about 51 percent.
2.3.2 Application Adequacy
The adequacy of irrigation is not strictly a simple function of the area that achieves or exceeds a specified soil water content. More correctly, it is a measure of the degree to which the soil water deficit over the whole field is met – a volumetric measure analogous to application efficiency. By deficit it is meant the difference between the current soil water content and the target soil water content. The application adequacy is defined to be (Bright, 1986):
Compared to the adequately watered area concept, this takes account of the water added to the effective root zone, but which does not raise soil water content to, or above, the target level. The effect on the application adequacy of the application depth and uniformity is illustrated in Figure 2-3.
Figure 2-3: The effective of Uniformity on Irrigation Adequacy
Achievement of an application adequacy of 90 percent requires a mean application depth of about 58 mm if the uniformity coefficient is 70 percent. The resulting application efficiency is about 77 percent.
2.4 THE EFFECTIVENESS OF IRRIGATION
The effectiveness of irrigation is a measure of the extent to which economic loss, due to insufficient soil water, was avoided. A simple measure is the ratio of actual income to potential income. The practical difficulty with this, and alternative measures of effectiveness, is that not all components of this performance measure can be measured, notably the potential income.
On the basis of the discussion in Section 2.1.1, the key to achieving highly effective irrigation is timing. The latest time an irrigation event can occur without incurring economic loss is the time at which the soil water content drops to the critical soil water content. Furthermore, the timing must be right for each irrigation event, all season. The effectiveness of irrigation is therefore a measure of irrigation performance that integrates the effects of a number of irrigation events.
A simple measure of the effectiveness of irrigation, for design purposes, could be the proportion of the days per season that the soil water content was above the critical soil water content. This measure would not take account of the non-uniformity of irrigation. To do so requires a measure such as the proportion of the days per season when the soil water content over 80 percent of the irrigated area exceeds the critical soil water content. In practice, it is the performance over the whole of the irrigated area of a farm that is relevant. Therefore the measure of effectiveness must integrate over all irrigation events on the farm each season.
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