I agree with RWF7437 - it depends on what type of reservoir. In some parts of my country, a reservoir is an above ground steel tank (sometimes conc or PSC). In other parts, a reservoir is an excavated basin in soil, & lined with a layer of concrete (usually 150 mm thick). Depths from 10 to 15 m.
All of our 'reservoirs' are also roofed - up to 50,000 m2.