An energy technology's water use, sometimes referred to as its water footprint, is the amount of water that is used in order to produce a unit of energy. The water used as fuel for generating hydroelectricity is returned to the water course and remains available for other purposes.
The issue of consumptive use of water, on the other hand, examines how much water is consumed and is, therefore, no longer available. For hydropower, evaporative losses are increasingly considered to be consumptive losses.
However, most watercourses already lost water through evaporation prior to the introduction of a reservoir. Therefore, a net approach is the only way to accurately assess the evaporative losses of a reservoir.
A net evaporation evaluation takes into account the pre-existing characteristics of the site by factoring out the natural evapotranspiration of plants in the flooded area and the evaporation of pre-existing water bodies, both of which occur naturally in any landscape.
Furthermore, reservoirs typically make water available to the system that was previously not available when needed. Wet and dry seasons lead to an uneven hydrograph, with too much water for part of the year and not enough in other parts of the year.
Through the strategic use of a reservoir, hydropower facilities are able to smooth the annual variations in runoff, conserving excess water in the reservoir during high-flow periods for use furing times when natural inflows are insufficient.
This implies a credit may be needed in any true assessment of hydropower's water footprint, recognising the value of making water available when it is most needed.
Our work on water footprint
A common and accepted framework for reporting on hydropower’s net water footprint has not yet been established due to inconsistencies in the currently available methodologies for calculating net consumptive water losses, and a lack of clear guidelines on the allocation of these losses in the case of multipurpose reservoirs.
We are working to examine the issue of quantifying net consumptive water evaporative losses associated with hydropower projects, considering the conditions prior to and after project implementation.
We are also working to establish guidelines on how to apportion these water losses among the multiple services provided by hydropower development, taking into account the seasonality of water value.
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