Understanding the flow intermittence dynamic of headwater streams using periodic observations of river flow state in South-East France

International audience; Due to their upstream position in river networks, many (HS) experience recurrent flow cessation and/or drying events. They have many ecological values since they are located at the interface between terrestrial and aquatic ecosystems and contribute to good status of rivers (sediment flux, input of organic matter...). However, the understanding of HS remains limited because gauging stations are preferentially located along perennial rivers and, consequently, the proportion of intermittent rivers (IR) is highly underestimated. In France, the observation network ONDE ("Observatoire National Des Etiages", in French) was designed by ONEMA to complement discharge data from the conventional French River Flow Monitoring network (HYDRO) to better understand HS dynamics. ONDE provides visual observations of flow state at 3300 sites along river channels located throughout France since 2012. In this work, we focus our analysis on the 750 sites located in south-east of France. One observation is made every month between April and October and the frequency of observations may increase during drought period to 4 visits / month. One of the following flow states is assigned at each observation: "flowing", "no flow" or "dry". The objective of this work is to combine information from both networks for a better understanding of the spatio-temporal pattern of intermittent rivers in the south-east of France. A first analysis of the ONDE network shows that 50% of sites have shown that at least one zero-flow event between 2012 and 2016 (against only 8% of gauging stations available in the HYDRO database). The proportion of zero-flow events observed at 57 ONDE sites was higher than 50% and at least one zero flow is observed every year at 85 sites mostly located in the south of the region near the Mediterranean Sea. Dry events mainly occur during August and September and the most impacted years were 2012 and 2016 where more than 600 zero-flows were observed. The second step consists in converting the sample of discontinuous observations into continuous time series of river flow states in order to study how flow intermittence develops in both time and space. For this purpose, 30 hydro-ecoregions (HER) homogeneous in terms of climate, topography and geological features were selected. The proportion of no-flow within each region with respect to hydrological regime was studied. Linear regressions have been fitted to link the observed discharges, the groundwater levels and the proportion of no-flow. A strong relation is noticed for 70% of HER with a determination coefficient higher than 60%. Regions with high probabilities of intermittent river segments are close to the Mediterranean Sea where the proportion of no-flow modelled may exceed 50% between July and October. Finally, geostatistical methods have been developed to assess the spatial distribution of IRs and to explore how flow intermittence patterns are related to environmental drivers.