Molden and Sakthivadivel, 1999; and Molden and Oweis, 2003). These definitions were subsequently refined and applied in numerous articles in the irrigation literature.<sup>14</sup> Water productivity in agriculture was understood as agricultural output per unit volume of water. The numerator of the ratio could be in physical terms, such as kilograms of agricultural production or marketable crop yield; or, in so-called “economic” terms, a dollar value, such as gross or net value of product. The denominator could be expressed as water supplied or depleted (such as consumed by evapotranspiration, and/or lost in a sink where it cannot be readily reused). Thus, when choosing a particular numerator and denominator, a decision had to be made about ‘which crop’ and ‘which drop’ to include, taking into account factors such as the relevant scale (i.e. field, farm, irrigation system, or basin), the stakeholders, and data availability. For example, it was proposed that at the farm level the ratio of kilograms of yield relative to evapotranspiration would be suitable, whereas at the basin level the ratio of a dollar value relative to the available water would be appropriate (Molden _et al_ ., 2003).

A review of the single-factor studies shows that various methods have been used to measure agricultural water productivity across crops, basins, countries, regions and time periods, with the aim of identifying critical factors to articulate recommendations for policy reform and interventions to “close the gap” in the water productivity findings and, thus, help alleviate rising water scarcity. Substantial differences in spatial and temporal water productivities have been documented—depending on the particular definition of “water productivity” chosen, but also with the use of the same definition. For example, water productivity for wheat in terms of kilograms of yield relative to cubic meters of evapotranspiration among farms in an irrigation district in Iran varied from 0.76 to 0.98, while in terms of kilograms of yield relative to cubic meters of transpiration it ranged from 1.11 to 1.26 (Vazifedoust _et al_ ., 2008).<sup>15</sup> Most studies use such findings to argue that a large scope exists to increase yields and/or save water and, thus, to improve water productivity. Many explanations for the differences are presented, usually without further analysis with regard to the significance and/or magnitude of the different factors that may cause the variations. The identified policy implications tend to be wide-reaching, but often not supported with robust evidence from the analyses carried out.

> 14 A search in the databases Water Resources Abstracts, Agricola, and EconLit was carried out using the terms “water productivity,” “crop per drop,” and “water use and productivity” in the title of studies.  This resulted in 257 articles from peerreviewed journals; seven studies were identified in EconLit.

> 15 Some studies thus recommended that further analyses should be carried out to increase the sample size (for example, regarding the number of irrigation schemes, as in Sakthivadivel _et al_ ., 1999).  Also, for better comparisons of estimates, it was recommended that water productivity indicators should be used in a more standardized way (for example, with regard to fresh matter or dry matter for grain yield in the numerator; or the period taken into account in the denominator, such as the entire growing season or only the time from sowing to harvest), and/or that information on these aspects should at least be included in the studies (Bessembinder _et al_ ., 2005).

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