A reservoir is a body of water that is stored for some purpose, such as drinking water supply, irrigation supply, flood control or hydropower.

A run-of-river system has a small head pond, but no storage reservoir. When the feeding catchments are frozen in winter, and in the long dry months of summer, flow in a natural river may be very low and inadequate for hydropower. In the spring freshet, the melting snow swells the rivers and the flood flows are normally much larger than can be used for power generation. The benefit of a reservoir is that the high inflows are stored to provide continuity of flow over the drier months of the year.

When the water level in a reservoir rises, the surface area and volume of stored water both increase. The area-elevation curve is a chart that shows the variation of surface area with elevation. These days, reservoir areas are extracted from the topographic data for each metre rise of elevation using GIS. This data is placed in a table and plotted as the area-elevation chart.

The reservoir volume is also computed at each metre of elevation (using the areas and height between them) and integrated to create a storage-elevation table. This is plotted as the storage-elevation curve for the reservoir. 

While these curves are useful for owners and operators, reservoir management systems use the underlying data table instead.

A reservoir has “dead storage” (below the lowest operating level); “live storage” (from this level up to maximum operating level) and flood storage (from this level up to the maximum flood level).

Published by hydrogray

Hydropower specialist

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