Use of statistical methods to characterize the influence of groundwater in the thermal regime of rivers in Normandy (France). Comparison between a river influenced by the chalk table in the Paris Basin, the Touques, and another not, the Orne.

The study of the temperature of two rivers in Normandy (France), the Orne and the Touques, over a period from 2013 to 2018, allowed to determine the main control factors regulating their thermal regime. The analysis was conducted by coupling different statistical treatments: linear regression between water and air temperatures, Independent Component Analysis (ICA), Principal Component Analysis (PCA) and multiple linear regression model. The temperature of the two rivers is mainly controlled by climatic factors but secondary regulation factors have been demonstrated to play important roles : runoff for the two rivers, groundwater for the Touques. Hydrogeological differences may explain the different temperature controls. The Touques actually crosses a very aquiferous chalk plateau on the western edge of the Paris Basin. The groundwater contribution in river temperature appears to occur throughout the year according to a seasonal cycle based on the periods of recharge and discharge of the aquifers of the catchment area. The influence is at its maximum in the early spring and increases from upstream to downstream. The Orne monitoring network, insufficiently dense, did not allow ICA and PCA to highlight the influence of groundwater in the thermal regime of the river.
Keywords : stream temperature, groundwater temperature, baseflow index, runoff, hydrology, ICA, PCA,chalk, Armorican Massif, Paris Basin De :Frédéric Gresselin, Bruno Dardaillon, Christine Bordier, Fabrice Parais & François Kauffmann