Water use efficiency
The term ‘’water use efficiency’’ is widely used and propagated in irrigation agriculture literature.
Unfortunately, the term ‘water use efficiency’ is used in different meanings in the literature and by stakeholder groups in agriculture, creating confusion in discussions.
Batchelor et al. (2017) in an FAO publication recommends that the term be avoided, and replaced by ”water productivity” and ”irrigation efficiency” for the following reasons:
…a universal definition has yet to be agreed and adopted (Steduto, 1996; Pereira et al., 2002; Hsiao et al., 2007; Perry, 2007, Van Halsema et al., 2012; Pereira et al.; 2012). In the water sector, the term ‘water use efficiency’ is generally understood to be a dimensionless ratio between water use and water withdrawn, while in the agriculture sector it is often used to measure the efficiency of crops (irrigated or rainfed) to produce biomass and/or harvestable yield (Pereira et al., 2012). The net result has been many miscommunications and misunderstandings at the policy level in both the agriculture and water sectors. Therefore, it is recommended that the term ‘water use efficiency’ be avoided, and use should be made only of either ‘irrigation efficiency’ or ‘water productivity’.
Water use fractions
Since the definitions of irrigation efficiency and water productivity refer to water use fractions, we include a figure that explains the different types clearly. See the figure on the right.
Batchelor et al. (2017) notes that
It is now widely accepted that, while irrigation losses appear high, with on average about 40 percent of the water supplied to agriculture reaching plant roots, a large part of these ‘losses’ returns, locally or downstream, in the form of return flow or aquifer recharge. As important, this return flow may be re-used and/or serve important environmental functions (FAO, 2012).
According to Batchelor et al. (2017), irrigation efficiency measures engineering or management efficiency, which distinguishes it from water productivity.
Irrigation efficiency for a specific domain is defined as a ratio:
Irrigation Efficiency = QuantityReq / QuantityDiv
The numerator refers to the volume of water required (QReq) for irrigation (including water needed for crop transpiration, leaching to prevent salinisation, weed control, etc.) and the denominator refers to the volume of water diverted (QDiv) from the source of supply.
They recommend that the term be limited to measure conveyance efficiencies and application efficiencies.
The term ”irrigation efficiency” can also be used to describe the efficiency of irrigation water management at scheme level. In this context, the numerator refers to the volume of water available to irrigators for abstraction in terms of their authorisations and the denominator refers to the volume of water diverted from the source. System losses are classified as non-beneficial water use fractions that may be non-recoverable (for example, evaporation from a canal) or recoverable (for example, seepage from unlined canals) (Batchelor et al., 2017: 85).
Batchelor et al. (2017) defines water productivity as follows:
Water Productivity = Crop yield (Actual) / Volume water consumed
Water consumed can be fractioned into the beneficial fraction (mainly transpiration) and the non-beneficial fraction (mainly evaporation from soil).
Crop yield does not necessarily refer to volumes or tonnes produced; it is also used to refer to value of the crop in monetary terms. For this reason, growers might decide to improve water productivity by changing to a higher value crop.
In this regard, Batchelor et al. (2017) note that the equation is particularly useful for water valuation approaches, because it can be applied in a wider sense by replacing the numerator with economic, social or environmental attributes. For example, Rand/volume water consumed, Jobs/volume water consumed or biodiversity/volume water consumed.
Batchelor et al. (2017) point out the risk of misconceptions if the denominator is replaced by volume water used, because it would then add the non-consumed fraction, which includes beneficial return flows and aquifer replenishment.
This tends to hide rather than explain the potential trade-offs and reallocations of water uses and users in a water scarce basin when increases in agricultural production are propagated. (Batchelor et al., 2017:86)
Batchelor, C., Hoogeveen, J., Faurès, J. & Peiser, L. 2017. Water accounting and auditing: A sourcebook. Rome: FAO.