This note is based on a number of discussions that took place in Bari and Bangalore and as the W4C project carries in its title the term “water use efficiency” and WP3 is dedicated to Water Use Efficiency, it is timely that we have a clear idea about the meaning of this term and how it differs from “water productivity”.
Water use efficiency (WUE) and water productivity (WP) are two different terms. However, they seem to cause some confusion only among agronomists. The fact is, both terms do exist and both have different definitions and applications. Unlike engineers, agronomists are likely to confuse the two and some consider the productivity is efficiency. The simple definition of any efficiency in our daily life is referred to as a ratio or percent. For example when considering energy, work performance and heat storage efficiencies, we say 90% or 70% efficiency which represents the percentage or ratio of output divided by input, both with the same units. In irrigation, for example, if one adds 10 mm of water to the plant and the plant used 8 mm through the root water uptake followed by transpiration and 2 mm is lost by drainage below the root zone or via bare soil evaporation from the surface, then the water use efficiency here is 80%. Productivity is a different term and refers to what we can produce from a unit of input, it is also a ratio of output to input but both do not need to have the same units, e.g. WP is 50kg grains per 1 m3 of water.
These two terms are clear in the mind of the majority of researchers and if asked about the definition of WUE and WP they will answer correctly, giving the two different meanings. However, this is not the case with a minority of researchers who are still following literature perhaps dating back to the 1960’s when some authors confused the meaning of both terms. Those who are still using WUE with the meaning of WP, if asked about WUE, the answer is: “so many kg per cubic meter of water”. This in reality is water productivity (WP), not water use efficiency (WUE). Subsequently, if asked what the meaning and units of water productivity are, they do not have an answer and in some cases an excuse is provided by saying someone published it that way in journal A or B. However, this is not a good enough reason and definitely not a recipe to follow as, what is built on something wrong is also wrong. WUE is the % of water supplied to the plant that is effectively taken up by the plant, i.e., that was not lost to drainage, bare soil evaporation or interception. Read “A note on Water use efficiency and water productivity” full report