Gerbens-Leenes et al. (1) quantify water use of the most relevant global bioenergy crops except palm oil. This is a welcome addition to the existing literature, which often only focuses on “carbon footprints.” Although carbon footprints have been shown to capture a large portion of overall environmental effects for many products and services, the agricultural sector needs additional consideration of other environmental indicators (2). In particular, land use and water consumption can be of substantial importance. Calculating a water footprint is therefore crucial for agriculture-based products.

As the carbon footprint elaboration has shown, a clear concept and definitions of terms are required for proper communication (3). While in carbon footprinting weighting between various greenhouse gases according to global warming potentials is needed, we think that a distinction between the various origins of water use is useful, because the resulting environmental impacts vary largely from region to region. For instance, 1 L of water consumed in the Colorado watershed does not compare to 1 L of water from the Mississippi, because water is much more scarce in the Colorado watershed. Therefore, an extension of the work performed by Gerbens-Leenes and colleagues seems necessary to us, to capture environmental relevance. To clarify our view, a water footprint scheme consistent to that of carbon footprinting in terms of the relevant features is presented in Table 1. It deviates, in part, from the water footprint as presented by Gerbens-Leenes et al. and shows that a further discussion of the concept is necessary.

In the proposed scheme, only consumptive blue water is considered, because it describes the additional loss of available water resource in a watershed. This is in agreement with the concept presented by Gerbens-Leenes et al., because they quantify irrigation water as the consumptive fraction, and not the total use. “Green water” consumption of agricultural systems may be neglected, similar to biogenic CO₂ emissions in carbon footprinting, if it is comparable to the green water demands of the natural vegetation at the specific site. However, we disagree on the water footprint definition, assuming that the water footprint should reflect the environmental impact. We perceive the reported definition of Gerbens-Leenes et al. rather as a water “shoesize” requiring a specific weight for becoming a footprint. Hence, we suggest a spatially varying water stress index (WSI) to weight the water consumption (shoesize), as a function of water scarcity (4). For illustration purposes, we calculated the stress-weighted footprints of biofuels according to ref. 4 based on the data presented by Gerbens-Leenes and colleagues. We normalized them with the WSI of the Colorado watershed resulting “Colorado-m³-equivalents” in analogy to CO₂ equivalents (Table 2). The resulting volumes show some interesting and relevant alterations to the results of Gerbens-Leenes et al. (1). For instance, jatropha has a considerably lower footprint than rapeseed, maize is now favorable over sugar beet, and cassava has clearly the smallest water footprint. Such differences reflect the global distribution of biofuel production and the according water scarcities. They become even more pronounced if biofuels from specific regions are compared.