One of the classic ways to increase circulation to a magazine that has a specific, do-good mission is to make the publication the centerpiece of the greater cause and promote subscriptions to your magazine as the first tangible benefit of support. Think Audubon magazine and the wildlife movement. Or Smithsonian magazine and American history.
General interest in the issues of water and wastewater may allow you to position and market your magazine as a center for public conservation and personal stewardship of the environment. In that way, you may attract a larger audience that responds to your magazine as a primary driver of useful information and urgent action in support of the cause.
To explore such an approach, do an A/B test. Create one promotion that offers your magazines alone as a traditional low cost / high value subscription. And create another that offers your magazine as part of membership in a group or organization concerned with issues of water and wastewater — as part of a larger mission and movement that involves your entire cause.
If Test B is successful, you will not only increase subscription sales, but open up other related marketing opportunities. You may also find larger audiences and new revenue for magazine-sponsored events, travel, educational programs, corporate partnerships, books, tools, gear, and more.
Be bold and creative in your testing!
And all the best—Richard