Water Tanks

something to do with gravity and getting the water to your tap under pressure.

Water tanks and water towers don't generally act as storage devices, per se. They are pressure accumulators. Big, industrial pumps are either on or off. The elevation of the tower determines the pressure, the pump does not. The level of water drops to a preset level and a switch, either float operated or pressure sensitive turns the pump on. When the tank is nearly full, the pump is turned off. The water pressure at the faucet remains essentially constant.

and each ten meters, the pressure highens by 1 bar

the manhattan skyline is littered with these roof-mounted units

Thanks, I knew there would be a slew of information here. I always thought these were more storage devices. You learn something new every day.

No snickering kids, but the definition of the energy/pressure imparted to water connected to a source such as a tank is 'head'. Next time you hear a farmer going on about how the head is good at his place, don't get jealous... or curious.