Water footprint

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.