The Water Tank Trail starts at an electrical substation about five miles outside of Lake Havasu City, AZ. From the hike descriptions it wasn't totally clear what this "water tank" was. Sometimes in desert country anything from a mud puddle to a large pond or lake is refered to as a "tank".
The hills on either side of the trail are known as the Aubrey Hills.
It turned out that the "tank" was literally a steel water tank with a system of corrugated metal rainfall collection areas. Rainfall collects in a gutter on the low side of the 60ftx60ft collection area shown in the foreground.
The water in the collection gutter then feeds into the tank through a pipe.
A corrugated metal roof over the tank keeps debris out of the tank and allows water collected on the roof to drain into the tank.
Steel tank and roof.
The water from the tank is used to fill a downstream water trough that animals can drink from.