Open access journal for water resources
Open Access (OA) is not necessarily free for authors (but free for readers). OA concerns free access to the publication. Many OA journals charge for submitting a manuscript, see for example HESS which could be an OA journal for your paper. It is possible to publish without an affiliation with a university.

Along with OA has appeared a darker side, often referred to as predatory OA journals. So choosing a journal should under any circumstances be made with care. If you are uncertain, I suggest you contact a professional at your (former) university to get some advice.