A literature review of citizen science for hydrological monitoring - with specific focus on lessons learned in developing countries

Citizen science has emerged as a significant approach to engage the public in scientific research, data collection, and environmental monitoring, around hydrology. This meta-review examines the scientific literature on the involvement of citizens, recruitment and training, spatiotemporal distribution of cases, methodologies or approaches, the precision of citizen science data with controlled measurements, data transfer and accessibility, usage of citizen science data, and who uses the data and equipment used in citizen science projects for hydrological monitoring. The systematic review identified 14 review papers published since 2015, summarising 492 case studies, with a concentration of evidence based on regions in high, upper, and low-middle-income countries. Key findings of this meta-review indicate substantial evidence of citizen participation in hydrological data collection, especially for water quality monitoring, and to a lesser degree, surface hydrology (e.g., streamflow and water levels) and aquatic biota. Measurement of physio-chemical parameters such as pH, temperature, turbidity, and dissolved nutrients, as well as biological indicators like E. coli in surface and groundwater were most frequently described. Some studies also addressed monitoring flooding, rainfall, water flow, and water levels. Successful citizen science programs were characterized by effective recruitmentof participants. Training was identified to be essential for high-quality data collection, enhance participants’ capacity, and build skills while adhering to established protocols. Most studies in the reviews focused on water quality monitoring and employed a range of technologies from basic test kits and manual water level or rain gauges to advanced sensors and social media tools. The type of technology used was found to significantly influence data quality and credibility, with more advanced equipment offering higher precision but requiring greater technical expertise. Ethical considerations were fundamental to the success of citizen science projects, ensuring transparency, consent, and mutual respect between researchers and participants. Addressing participants' concerns and obtaining ethical approval were crucial steps in maintaining ethical standards, but they were not always transparent in the reviewed literature. It was noted in several of the 14 reviews that most citizen science initiatives were researcher-designed, and much fewer examples exist in full co-design mode between scientists, project implementers, and the citizens and communities executing data collection. Notably, this meta-review found that most of the ‘lessons learned’ in scientific review papers focused on data collection. Emerging gaps identified from the 14 reviews include (i) limited quantitative assessments of the precision of citizen science data, (ii) inadequate documentation of ethical considerations and data ownership, and (iii) insufficient focus on data accessibility and post-project utilization of collected data by beneficiaries, iv) weak description on the process to enable co-creation of data collection protocols and methods. This meta-review highlights citizen science's potential to provide cost-effective and highquality hydrological data. However, it emphasizes the need for greater attention to ethical issues, particularly around data ownership and accessibility, and the need for advanced training. Addressing these gaps, especially in underrepresented regions, can significantly enhance global water resource management and support sustainable development goals.