Eutrophication is a phenomenon associated with the influx of nutrients in the water. Intensive agricultural activities in catchments result in an increasing influx of nutrients in open water bodies. This study provides a proof of concept for early warning on nutrient runoff in open water bodies using satellite and Earth Observation (EO) data. The early warning system is based on 3 main components. The first involves crop-type mapping to estimate the amount of nutrients per crop type, while the second involves the hydrological model which will transform rainfall into runoff, which carries the nutrients at the basin outlet. The third component is the estimation of the Chl-a concentration by combining in-situ and satellite measurements to validate the proposed method. Satellite and EO data are used to provide the necessary data at all stages of the methodology.