The Irvine Ranch Water District will start installing water pipes in September for a 2.5-million-gallon reservoir in Whiting Ranch Wilderness Park to provide water to Portola Hills area.
The $3-million construction site is just northeast of the park's Four Corners area.
It which marks the head of several popular trails that lead into the wilderness.
The park will stay open during the, with one trail closed.
The pipeline connecting an existing reservoir, built a decade ago, to the new reservoir will pass under the Whiting Road Trail.
The trail will be closed to the public through December.
The closure will affect mostly mountain bikers, who account for about 80% of park users.
Mountain bikers typically use the trail to exit the park after entering through Lower Borrego Canyon Trail, said Ants Uiga, project manager.