Water is the backbone of human development. However, a major impediment for sustainable development is the limited amount of data available to support evidence-based decision making on water resources and catchment management. Recently, participatory approaches to environmental monitoring have become more popular, and are being promoted as a potential pathway to address long-standing data gaps. I hypothesised that such a participatory approach to monitoring water resources is an efficient way to reduce data scarcity and to support decision making in the context of catchment management. To test this hypothesis, I studied one of the largest bottom-up initiatives of participatory environmental monitoring in the world: The Regional Initiative for Hydrological Monitoring of Andean Ecosystems (iMHEA). iMHEA is a partnership of academic and non-governmental institutions who instrumented a participatory hydrological monitoring network of headwater catchments in the tropical Andes. After a rigorous quality control of the generated data, I used them to analyse the impacts of land-use changes on the hydrological response of different Andean biomes, to develop a statistical model to predict such impacts in ungauged basins, and to evaluate a particular catchment intervention, i.e. pre-Inca artificial infiltration systems. Lastly, I put my findings in a broader context, testing similar approaches in the Ethiopian Highlands, and developing a simple method for the hydro-economic evaluation of nature-based solutions for water. I find that the data generated using such a participatory approach meets quality standards comparable to those of scientific research and, furthermore, stakeholders are more incentivised to provide open access to their data and to participate in a process of knowledge co-creation and its further assimilation. Strengthening the evidence body over which decisions are based can contribute to the improvement of water resources management as well as to the sustainable development of local communities.