The aim of the project is to be able, ultimately, to model the behavior of the Basse de Courbeligne headstream in response to climate change and anthropogenic pressure (reduction of air pollution, forest management). The Basse de Courbeligne is a headstream flowing on sandstone (mainly Buntsandstein) in the Vosges Mountains (North-East France). Since 2002, the water quality (pH, conductivity, major anions and cations) in its upper section has been monitored monthly within the framework of the Vosges Acidification Observatory (https://deims.org/22915474-7c50-47c1-8239-6c59fa924a1b). Its pH is acidic (5.5, standard deviation (S.D.) = 0.7), its conductivity is low (16.4 µS/cm, S.D. = 5.7µS/cm) and its average concentration in dissolved aluminum is around 240 µg/L (S.D. 125 µg/L) (period 2012-2021). At the headstream source, the aluminum concentration can reach 540 µg/L. In 2016 three plots have been set up on the right steep bank and instrumented to monitor the surface runoff water quality. The pH of each plot runoff is slightly higher than the headstream pH, the conductivity being similar. The main difference with the headstream lies in the dissolved aluminum concentration, which is much lower: 60 to 80 µg/L (S.D. = 50 µg/L) between 2017 and now. In the future, these plots will be submitted to different forest management protocols. Recently the downstream section of the Basse de Courbeligne has been investigated, as an expansion of the monitoring of the headstream to its full watershed is desirable: it has been noticed that the water pH at the watershed outlet (i.e. confluence with the Gentil Sapin stream) might be much more neutral (around 7). A detailed survey of the stream has been done concerning its water quality as well as the watershed geology and soil characteristics. The combined effect of a fault and of lateral springs are playing key roles in the stream flow and its water quality. After summarizing the long-term water quality data collected on the watershed, a first attempt to model its behavior using HYPE will be proposed.