1 INTRODUCTION

Irrigated agriculture plays a major role in the livelihoods of nations all over the world as the agricultural water use sector being the largest of all water use sectors.

With irrigation efficiency, the approach is that irrigation efficiency should be assessed by applying a water balance to a specific situation (Perry, 1). Reinders et al. (2) pointed out that the purpose of an irrigation system is to apply the desired amount of water, at the correct application rate and uniformly to the whole field, at the right time, with the least amount of nonbeneficial water consumption (losses), and as economically as possible. Water should be considered both as a scarce and valuable resource and an agricultural input to be used optimally to produce crops. Not all the water that is abstracted from a source for the purpose of irrigation, reaches the intended destination where the plant can make best use of it - the root zone (Reinders et al., 2). The fraction abstracted from the water source that can be utilized by the plant to produce, can be called the beneficial water use component. Optimized irrigation water supply is therefore aimed at maximizing this component and implies that water must be delivered from the source to the field both efficiently (with the least volume for production along the supply system) and effectively (at the right time, in the right quantity and at the right quality) (Reinders et al., 2). Optimizing water use at farm level requires careful consideration of the implications of decisions made during both development (planning and design), and management (operation and maintenance), taking into account technical, economic, and environmental issues (Reinders et al., 2). The framework can also applied to reassess the system efficiency indicators typically used by irrigation designers when making provision for losses in a system and converting net to gross irrigation requirement.

2 THE WATER BALANCE APPROACH

An article by Perry (1) proposed a developed framework for irrigation efficiency, which was approved by the International Commission on Irrigation and Drainage (ICID) (Reinders et al., 2). In that article, Perry details the history and subsequent doubts about the calculation and interpretation of so-called irrigation or water use “efficiency” indicators. Based on the principle of quality continuity, the proposed terminology and framework is scientifically sound, which facilitates the analysis of irrigation water use situations or scenarios instead of simply calculating input-output ratios as in the past, to reveal underlying issues that can be addressed to improve water management.

The framework is based on the idea that any water withdrawn from a catchment for irrigation contributes to the storage change, the consumed fraction, or the nonconsumed fraction at a point downstream from the extraction point. The water consumed either benefits the intended purpose (beneficial consumption) or not (nonbeneficial consumption [NBC]). Water that is not consumed but remains in the system can be either recovered (for reuse) or not (lost to further use) (Perry, 1).

For higher water availability in the catchment, the authorities should focus on reducing NBC and nonrecoverable fractions (NRF): any activities conducted for this purpose can be described as the best management practices (Perry, 1).

A schematic representation of the ICID water balance framework based on Perry's model is shown schematically in Figure 1.

Typical system components of water infrastructure are defined under different possible circumstances to apply this framework to irrigation areas. Most irrigation areas consist of dams or weirs on rivers from which the water is released to be extracted directly or via a canal. Water users can also take the water directly from a shared source, such as rivers or dams/reservoirs, or from a source at scheme level, such as a groundwater aquifer. The water entering a farm may contribute to storage change (in farm dams), get into the farm's water distribution system, or be directly irrigated to the crop by a specific kind of irrigation system (Reinders et al., 2).

The framework by Reinders et al. (3) contains a water management infrastructure of four levels (as shown in Table 1): water sources, bulk delivery systems, irrigation schemes, and irrigated farms, and the associated water management infrastructure.

Although care has been taken to include all possible system components and water destinations, practitioners are encouraged to customize the framework for their specific circumstances. The abbreviations used to classify the framework components are declared in Figure 1.

In the case of irrigation, losses occur at different levels of water management as shown in Figure 2.

Historically reporting of irrigation efficiencies such as “application efficiency,” “system efficiency,” “distribution efficiency,” “transportation efficiency,” and so forth, have resulted in a simplified way to express efficiencies and the real understanding and investigation of the source or causes of losses has faded. There is a widespread illusion that efficiency is fixed by the type of irrigation infrastructure used rather than to the way a particular system has been designed and managed. In the past, improving performance and efficiency was, incorrectly, only associated with an upgrade in infrastructure (e.g., a change in irrigation system).

Table 2 has been drawn up from an irrigation system perspective and there is not much that the practitioner can do to recover water in some of the infrastructure components.

To improve water use efficiency for irrigation, actions should be taken to reduce the NBC and NRF. Table 3 shows the expected ranges for the NBC and NRF components to help evaluate the results obtained when building a water balance for the first time.

These values should be based on the actual relevant data from a local area. However, considering the varying conditions in different irrigation areas, water managers at all levels should better evaluate the performance of a specific system component against the data from previous years for the same component for continuous improvement, not against other system components (seemingly similar) in different areas.

To quantify the different components, the challenge lies in the lack of data availability. However, by presenting the results for combined water destinations, a water balance may be built with limited data. For example, by building a water balance at the irrigation system level, it is often easier to first measure or calculate the combination of beneficial consumption and the recoverable fractions (transpiration, leaching requirement, drainage water, and so forth), and then to determine or calculate the NBC or NRF. Finally, the legal allocation by water users should be evaluated on the farm side to promote on-farm efficiency. Losses in water transmission, distribution, and surface storage need to be monitored by the Water User Association (WUA) or other responsible organizations at the scheme level, with acceptable ranges set and agreed with WUA where the system provision should be made to cover the losses.

3 APPLICATION OF IRRIGATION SYSTEM EFFICIENCY

The framework can also applied to reassess the system efficiency indicators typically used by irrigation designers when making provision for losses in a system and converting net to gross irrigation requirement. A set of system efficiency (SE) values for design purposes are illustrated in Table 4. These values must not be confused with Table 3's values because Table 3 provide the water balance framework from a holistic point of view and Table 4 provide only the irrigation system efficiency values.

The system efficiency defines the ratio of net irrigation requirement (NIR) to gross irrigation demand (GIR). The NIR is the water amount that the planned irrigation system should deliver to the crop, and the GIR is the water amount supplied to the irrigation system that will be affected by the assumed in-field losses. Nonbeneficial spray evaporation and wind drift, in-field conveyance, filter, and other minor losses have been taken into account, all of which make up the value in the “Total losses” column. The default value of system efficiency in the last column is obtained by subtracting the total losses from 100%. For this purpose, the system must also function optimally and be managed correctly to obtain these desired results. To evaluate an irrigation system, the system efficiency values can be compared with these default values, and possible significant water loss components identified to be improved. Therefore, this approach is more flexible and easier than the original efficiency framework in which the application is limited by definition. It should always be noted that the efficiency of water application to a system will vary between irrigation activities with varying climatic, soil, and other influencing conditions. Therefore, the SE indicator should be cautiously applied as a baseline, as it is not prepared for irrigation management practices. The losses can be determined as the ratio of the amount of water lost to nonbeneficial spray evaporation and wind drift, in-field conveyance, filter, and other minor losses to the amount of water entering the irrigation system in a given period, and can also be expressed as the depth of water per unit area instead of the volume. Thus, improvement can only be achieved through optimized management practices and functions.

4 IMPROVED UNDERSTANDING OF DISTRIBUTION UNIFORMITY

The type of irrigation system used is characterized by irrigation uniformity and the criterion for the design, operation, and maintenance of a particular system, which may also be affected by soil infiltration characteristics and land preparation.

The traditional method of interpreting the lower distribution uniformity (DU_lq) can lead to what is commonly referred to as default irrigation efficiencies, that is, furrow irrigation being assumed to be 65% efficient and center pivot irrigation being assumed to be 85% efficient.

Unfortunately, these assumed efficiencies are seldom considered for their rationale, that is, typical or assumed nonuniformity, and it is often assumed that water will “disappear” with the assumed low efficiencies. However, if the water balance approach is adopted, the water apparently will not “disappear” but contribute to increased deep percolation that may eventually occur further along the drainage system as backflow.

The bottom line is to ensure high irrigation uniformity as the goal for good design and maintenance procedures. Low crop yields caused by uneven irrigation are unlikely to be improved with assumed low irrigation efficiencies and a corresponding increase in water applications.

If poor uniformity leads to low crop yields, the uniformity needs to be corrected to improve system performance. It is unlikely that crop yields will be improved by simply compensating for partially underirrigated fields through increased water application—most fields will now be over-irrigated, and there will be an increased risk of long-term problems due to a rising groundwater table.

In this case, it is preferred to specifically address the problem of poor uniformity. Therefore, for the planning purpose, the GIR at the edge of the field should be calculated as the product of the NIR and the system efficiency.

5 CONCLUSION

It can be concluded that, the “measure, assess, evaluate, improve” approach resulting from water balance has facilitated an investigative water balance approach to improving irrigation efficiency, helping managers and designers to use this developed information and tools integrating detailed investigation with flexible application at any level to improve the performance of irrigation systems.

The framework can be applied to reassess the system efficiency indicators typically used by irrigation designers when making provision for losses in a system and converting net to gross irrigation requirement.

It should always be kept in mind that a system's water application efficiency will vary from irrigation event to irrigation event, as the climatic, soil, and other influencing conditions are never exactly the same. Care should therefore be taken when applying the SE indicator as a benchmark, as it does not make provision for irrigation management practices.

It is recommended that system efficiency be assessed in terms of the losses that occur in the field. This can be determined as the ratio between the volume of water lost to nonbeneficial spray evaporation and wind drift, in-field conveyance, filter, and other minor losses, and the volume of water entering the irrigation system, for a specific period of time. The losses can also be expressed as a depth of water per unit area, rather than a volume.

ETHICS STATEMENT

None declared.