Pluri-annual recharge assessment using vertical soil temperature profiles: Example of the Seine river basin (1984–2001)

Among the various methods used to determine vertical water seepage in unsaturated soils, thermal convection presents significant advantages: temperature measurements are simple to perform and record, and a wide range of time scales can be considered. The authors analysed the data recorded by the meteorological stations of the Seine river basin, at three different depths: 20, 50 and 100 cm. As the measurement sensitivity was limited to 0.1 K, long series of data needed to be stacked in order to obtain sufficient precision to quantify the convective component of heat transfer, in a predominantly conductive context. For the period from 1984 to 2001, it was possible to determine the average recharge at each station, and the recharge variation between groups of three-year periods. By interpolating these data over the whole basin, a global assessment has been made and compared to the exported flow rate at the river mouth: the resulting value of 94 mm yr^?1 lies between the lowest annual rate, 52 mm yr^?1, and the mean total exported value of 252 mm yr^?1.     

Monitoring Soil Water Content by Vertical Temperature Variations

The availability of high sensitivity temperature sensors (0.001 K sensitivity platinum resistors), which can be positioned at intervals of a few centimeters along a vertical profile in the unsaturated zone, allows short‐term in situ determinations—one day or even less—of the thermal diffusivity. The development of high data storage capabilities also makes this possible over long periods and the relative variations in thermal diffusivity allow the monitoring of the variations in water content. The processing of temperature measurements recorded at different depths is achieved by solving the heat equation, using the finite elements method, with both conductive and convective heat transfers. A first set of measurements has allowed this approach to be validated. Water content variations derived from thermal diffusivity values are in excellent agreement with TDR measurements carried out on the experimental site at Boissy‐le‐Châtel (Seine et Marne, France).