On the slopes of Hawaii Pacific University’s Windward campus, there is a garden.

A rain garden.

Rain gardens are flat-bottomed depressions in the ground that capture excess water and pollutants from rooftops, driveways, sidewalks, parking lots and streets, keeping them from reaching streams and the ocean.

The garden, about 160 square feet, is on a slope next to the nursing annex, overlooking the student residences and dining hall. It’s filled with native plants such as pohinahina, kupukupu ferns, ohai, carex and ahuawa grass.

A pipe delivers rainwater runoff from the nursing annex roof’s gutter spout into the garden.

Regina Ostergaard-Klem, associate professor of environmental sciences at HPU, and students recently volunteered to install the garden, along with Hui o Ko‘olaupoko, a nonprofit group with a mission to protect ocean health by restoring the aina (land) from mauka to makai.

Ostergaard-Klem said it was a great way for her students to get hands-on experience with natural resources management, and they had fun. Students take turns watering the plants and maintaining the garden.

Eventually there will be an informational sign explaining the benefits of the rain garden.

There are several other demonstration rain gardens on Oahu, including at Heeia State Park and Kaiser High School in Hawaii Kai.

Homeowners can build one, too, in their own yards.

Hui o Ko‘olaupoko launched a Rain Garden Co-op and Cost Share Program to help homeowners design and build rain gardens.

The group is looking for private homeowners interested in building rain gardens in their yards. Federal and state funds are available to cover plants and soil, according to Todd Cullison, the group’s executive director, while homeowners, neighbors and volunteers put in the labor. The idea is for homeowners to share their knowledge and help build the next one in the neighborhood.

Besides building rain gardens, the nonprofit works on stream restoration projects — including Heeia and Hakipuu streams on the Windward side — by planting native trees and vegetation to slow down erosion while monitoring water quality.

Another project is the Popoia storm water retrofit, which aims to improve the quality of storm water runoff in a city parking lot next to Kaelepulu Stream in Kailua with interlocking, permeable pavers, native vegetation and small rain gardens. Lanikai Elementary School students designed interpretive signs for the project.

If you’ve visited Kaha Garden at Kawainui Neighborhood Park in Kailua, which was built as an example of how native landscaping can improve water quality, you’ve seen what the nonprofit does.

A homeowner’s manual to building rain gardens is expected to be available in print and online this month. To learn more, visit www.huihawaii.org/raingardens.

Nina Wu writes about environmental issues. Reach her at nwu@staradvertiser.com. Follow her on Twitter @ecotraveler.