Sustainable Engineering

Considering the fate of evaporated water across basin boundaries – Implications for water footprinting

Water consumption along value chains of goods and services has increased globally and led to increased attention on water footprinting. Most global water consumption is accounted for by evaporation (E), which is connected via bridges of atmospheric moisture transport to other regions on Earth. However, the resultant source-receptor relationships between different drainage basins have not yet been considered in water footprinting. Based on a previously developed dataset on the fate of land evaporation, we aim to close this gap by using comprehensive information on evaporation recycling in water footprinting for the first time. By considering both basin internal evaporation recycling (BIER; > 5% in 2% of the world’s basins) and basin external evaporation recycling (BEER; > 50% in 37% of the world’s basins), we were able to use three types of water inventories (basin internal, basin external and transboundary inventories), which imply different evaluation perspectives in water footprinting. Drawing on recently developed impact assessment methods, we produced characterization models for assessing the impacts of blue and green water evaporation on blue water availability for all evaluation perspectives. The results show that the negative effects of evaporation in the originating basins are counteracted (and partly overcompensated) by the positive effects of reprecipitation in receiving basins. By aggregating them, combined net impacts can be determined. While we argue that these offset results should not be used as a standalone evaluation, the water footprint community should consider atmospheric moisture recycling in future standards and guidelines.

An overview on all derived factors (evaporation recycling ratios and characterization factors) is presented below:

Evaporation recycling ratios

BIER
Basin internal evaporation recycling [0,1]:
Average fraction of evaporation re-precipitating in the originating basin

BIERgreen
Fraction of basin internal re-precipitation refilling local green water reserves

BIERrunoff
Runoff-relevant basin internal evaporation recycling ratio [0,1]:
Considers that only a part of the basin internal re-precipitation contributes to the building of new local blue water runoff

BEER
Basin external evaporation recycling [0,1]:
Average fractions of evaporation re-precipitating in the sum of all outlying basins

BEERgreen
Fraction of basin external re-precipitation refilling local green water reserves

BEERrunoff
Runoff-relevant basin external evaporation recycling ratio [0,1]:
Considers that only a part of the basin external re-precipitation contributes to the building of new local blue water runoff

TER
Terrestrial evaporation recycling ratio [0,1]:
Average fractions of evaporation re-precipitating over land (= in the sum of all drainage basins)

TERgreen
Fraction of terrestrial re-precipitation refilling local green water reserves

TERrunoff
Runoff-relevant basin external evaporation recycling ratio [0,1]:
Considers that only a part of the terrestrial re-precipitation contributes to the building of new local blue water runoff

Characterization factors

WDI

Water deprivation index [0.001,1]

Expresses local freshwater scarcity while describing the potential to deprive other users when consuming water in this basin and month

Considers both relative blue water scarcity based on a consumption-to-availability ratio (extended by indexes considering ground and surface water stocks) as well as absolute blue water shortages derived from the ratio of local potential evapotranspiration to precipitation

A value of 1 denotes in this context the highest water deprivation potential, whereas a value of 0.001 refers to the lowest one

Combined characterization factors

WAVE+

Basin internal characterization factor combining the WDI with BIERrunoff

WAVE+ = (1 - BIERrunoff)*WDI

Multiplied with the basin and month specific evaporative water consumptions, WAVE+ factors can be used to determine the basin internal risk of freshwater deprivation

WAVEext

Basin external characterization factor combining the WDI with BEERrunoff over all receptor basins (m)

WAVEext = ∑(BEERrunoff,m * WDIm)

Multiplied with the basin and month specific evaporative water consumptions, WAVEext factors can be used to determine the external freshwater replenishment potential (represent the beneficial supply effects to external receptor basins)

WAVEt_b

Basin trans-boundary characterization factor balancing basin internal risks (WAVE+) with the beneficial effects in outlying basins (WAVEext)

WAVEt_b = WAVE+ - WAVEext

Multiplied with the basin and month specific evaporative water consumptions, WAVEext factors can be used to determine the trans-boundary risk of freshwater deprivation. This describes the potential net impact across all basin boundaries. It could potentially turn negative implying that the positive supply impacts towards external basins outweigh the negative impacts on the source basin itself

The subjacent link leads to the results, which is divided into two results sections.

Dropbox

The section “Factors for blue water consumption” provides the monthly factors for all spatial units and the associated annual averages for basins, countries, world regions and the world as such. The annual averages represent in this context average values aggregated based on total (yr_tot), agricultural (yr_agri) and non-agricultural (yr_non_agri) blue water consumption. With regard to basins, additional kmz-files are available for download, which can be load into a google earth layer. The kmz-files plot average annual values aggregated based on total blue water consumption. The section “Factors for green water consumption”, on the other hand, provides in addition to the monthly factors for basins the annual averages for all spatial units aggregated based on total evaporation (used as a proxy for green water consumption). Depending on the type of study (e.g. blue water footprint for unspecified, agricultural or industrial systems or green water footprint), a practitioner could select the most appropriate aggregation method.

With regard to more detailed information we refer to the following publication:

Andreas Link, Markus Berger, Ruud van der Ent, Stephanie Eisner, and Matthias Finkbeiner
Considering the fate of evaporated water across basin boundaries – Implications for water footprinting
in preparation