How to model shallow water-table depth variations: the case of the Kervidy-Naizin catchment -France
The aim of this work is threefold: (1) to identify the main characteristics of water-table variations from observations in the Kervidy-Naizin catchment, a small catchment located in western France; (2) to confront these characteristics with the assumptions of the Topmodel concepts; and (3) to analyse how relaxation of the assumptions could improve the simulation of distributed water-table depth. A network of piezometers was installed in the Kervidy-Naizin catchment and the water-table depth was recorded every 15 min in each piezometer from 1997 to 2000. From these observations, the Kervidy-Naizin groundwater appears to be characteristic of shallow groundwaters of catchments underlain by crystalline bedrock, in view of the strong relation between water distribution and topography in the bottom land of the hillslopes. However, from midslope to summit, the water table can attain a depth of many metres, it does not parallel the topographic surface and it remains very responsive to rainfall. In particular, hydraulic gradients vary with time and are not equivalent to the soil surface slope. These characteristics call into question some assumptions that are used to model shallow lateral subsurface flow in saturated conditions. We investigate the performance of three models (Topmodel, a kinematic model and a diffusive model) in simulating the hourly distributed water-table depths along one of the hillslope transects, as well as the hourly stream discharge. For each model, two sets of parameters are identified following a Monte Carlo procedure applied to a simulation period of 2649 h. The performance of each model with each of the two parameter sets is evaluated over a test period of 2158 h. All three models, and hence their underlying assumptions, appear to reproduce adequately the stream discharge variations and water-table depths in bottom lands at the foot of the hillslope. To simulate the groundwater depth distribution over the whole hillslope, the steady-state assumption (Topmodel) is quite constraining and leads to unacceptable water-table depths in midslope and summit areas. Once this assumption is relaxed (kinematic model), the water-table simulation is improved. A subsequent relaxation of the hydraulic gradient (diffusive model) further improves water-table simulations in the summit area, while still yielding realistic water-table depths in the bottom land.