Which type of storage facility is used primarily for maintaining constant water pressure?

Elevated tanks are specifically designed to maintain a consistent water pressure throughout the distribution system. By being positioned high above ground level, these tanks utilize gravitational force to generate pressure that pushes water down into the piping system. This elevation enables them to serve not only as storage facilities but also as a means to ensure that there is always adequate pressure available for consumers, even during peak demand times.

In contrast, ground level storage tanks, while useful for storing large volumes of water, do not provide any pressure increase; they rely on pumps to move water into the distribution system which may lead to fluctuating pressures. Surge tanks, on the other hand, are primarily used to absorb pressure surges and mitigate water hammer in pipelines rather than to maintain consistent water pressure. Clear wells are designed for sedimentation and storage of treated water before it enters the distribution system, focusing more on water quality than pressure maintenance.

Thus, elevated tanks are essential for achieving stable pressure levels in municipal water systems, making them the correct answer for this question.