How soil hydrology reacts during strong precipitation events?

In the context of climate change, strong precipitation events tend to increase in intensity and frequency. Such extreme events are associated with the genesis of fast infiltration into the soil, which can generate a very dynamic contribution of the groundwater, as it was observed during the June 2016 flood event in the Paris basin. An issue is that these events are poorly simulated, limiting the ability to monitor and forecast such event. Even with a physically based model such as the Interaction between Soil Biosphere Atmosphere soil multilayer diffusion scheme (ISBA_DF) the hydrodynamic of the simulated groundwater recharge at the basin scale was poor. Various hypotheses can explain these gaps like 1) a lack of physical processes such as preferential flows or double porosity 2) an inadapted characterization of soil properties and soil hydraulic properties 3) a bad representation of the vegetation properties; 4) or a too coarse spatial and temporal distributions of precipitation.

To answer these questions, four lysimeters of the experimental station of the French Scientific Interest Group for Industrial Wasteland (GISFI, http://gisfi.univ-lorraine.fr/) in Homécourt, France are used. Between 2009 and 2016, the total weight, the drained flow at the base of the lysimeter, and suction/moisture content/temperature were measured of each soil column every hour at 50cm, 100cm and 150cm depth. Ten intense rainfall events were selected by the daily rainfall quantile 99. These lysimeters are cylinders (2m depth*1m diameter) and contain monoliths industrialized soils with a silty-sandy texture. Lysimeters 1 and 2 have no vegetation, contrary to the lysimeter 3 with alfalfa cover, and lysimeter 4 with Noccaea caerulescens cover. In this study, ISBA_DF was used to simulate each lysimeter. The local scale and high frequency evolution of soil moisture, temperature profile, drainage rate and water storage were assessed, based on soil-water retention and conductivity curves from the Brooks and Corey model. For each lysimeter, hydrodynamic parameters were determined based on the observations. The values found for these particular soils are very different from the values expected by the literature.

With the fitted soil parameters, ISBA_DF shows a high performance in reproducing temperature profiles (R² & NSE>0,9) and has good scores for other parameters (R² & NSE>0,6). During intense precipitation, we can reproduce the drained flow at 2m depth; nevertheless, differences on the drainage period and the maximum intensity of drainage were noticed: drainage duration is generally longer for simulations with vegetation and maximum intensity seems too important for the simulations.

Finally, this multi-parameter analysis at a local scale and high frequency demonstrates the good infiltration and vegetation processes of the ISBA_DF model, even during intense rainfall. This study seems to show that there is no need to modify the physics of the model processes, but that efforts should focus on the characterization of soil properties.