Good question, it doesn't really need to be, those numbers are partly just to visualize it. But we do have some reasons to keep it with more surface area:
- less digging
- less reinforcing needed - it's more stable
- In some cases we're interested in floating solar on top of the reservoir which wouldn't work well if the reservoir was too deep.
But it's certainly not out of the question to go deeper instead.
But it's certainly not out of the question to go deeper instead.