Unsaturated Flow Modeling in the Seasonally Snow-Covered Roselend Granite (French Alps) Using High Resolution and Long Term Meteorological and Water Flux Time Series
Abstract
Quantifying water flow in an unsaturated and fractured medium is a challenging task which is often performed in media where water is scarce, in the context of studies related to waste disposals. The Roselend research program intends to clarify and to develop adapted methodologies to quantify unsaturated flow in fractured media undergoing strong external solicitations. Among them, the influence of drought and snowmelt recharge alternations is investigated. To do so, the 128 meter long Roselend tunnel that has been drilled in the sixties, in the granite (overburden of 55 meters at the closed end of the tunnel) at the vicinity of an artificial lake, is equipped with sensors recording dripwater fluxes at one hour time step, and water chemistry (major and trace elements) with sampling time varying from 36 hours to 4 days. Water level is also monitored by two boreholes located between the tunnel entrance and the lake. A meteorological station installed nearby the tunnel entrance provides hourly records of meteorological parameters. Additionally, an artificial tracer was injected in the ground surface, above the tunnel closed end. The high-range and steep mountainous environment in which the Roselend site is located is characterized by contrasted precipitation regimes with alternating snow, rain, and drought periods. Seasonal flow dynamics arise from dominant rain and melted snow infiltrations from late summer to mid-spring, increasing water content in the medium, and modulating flow rates of percolating waters. Over four years, infiltration in the medium is assessed considering separately the winter (snow on the ground) and summer (no snow on the ground) seasons. For the summer seasons, hourly potential evapotranspiration is calculated using the Penman-Monteith method, and is summed over each day. The real evapotranspiration is systematically calculated using the Thornthwaite-Mather budget, for various available soil water capacities at a daily time step. For the winter seasons, snow melt flux at the snow pack base is calculated by using the CROCUS model, which simulates the snow pack height and density by the mean of energy and mass budgets, based on meteorological information. The model is calibrated by reproducing the daily snow height, record by the meteorological station. Infiltration (and the corresponding available soil water capacity) is determined considering the time series for which the cross correlation with the daily water flux time series recorded in the tunnel renders the best correlation coefficient. Statistical analysis of daily time series water fluxes and water levels in the boreholes indicates that the ground water system below the tunnel is somewhat disconnected from the lake. Flow modeling of the vadose zone in the Roselend site is performed using the METIS code; it accounts for variable infiltration and groundwater level over time in the tunnel and is calibrated by adjusting hydraulic conductivity-capillary pressure-saturation relations in order to reproduce the main patterns of water fluxes recorded in the tunnel. Transport modeling allows for reproducing the artificial tracer breakthrough.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2006
- Bibcode:
- 2006AGUFM.H11F1331P
- Keywords:
-
- 1838 Infiltration;
- 1866 Soil moisture;
- 1875 Vadose zone;
- 1894 Instruments and techniques: modeling;
- 1895 Instruments and techniques: monitoring