Effects of Initialization on Response of a Fully-Distributed Hydrologic Model

Knowledge of initial conditions is very important to correctly model the basin response at the storm event scale. Of particular interest is the influence of topography and soil type on the principal hydrologic variables and runoff generation mechanisms as a function of antecedent wetness conditions. This study addresses the influence of initial states on the short-term hydrologic response and characterizes the effects of topography and soils on the dissipation of the influence of the initialization conditions. Two case studies are considered: a synthetic two-dimensional planar hillslope with various assumed slope magnitudes and soil types; and a real basin (∼800 km2) with actual land-surface characteristics. These case studies have been chosen in order to investigate the hydrological behavior at two different scales. Various precipitation rates and durations are used to force both domains, each initialized assuming different positions of spatially variable water table. The results illustrate the existence of complex mechanisms that modulate the watershed response depending on the catchment initial state and rainfall forcing. Factors contributing to the unsaturated zone soil moisture movement are discussed in relation to different soils and topographic properties. The role of rainfall intensity in determining the degree of control exerted by initial conditions on the total runoff as well as its partition into various mechanisms is examined for both case studies. Differences in the sensitivity of certain terrain locations to the initialization and the rainfall rates are revealed. In particular, the real case study points out that the effects of initial conditions strongly depend on rainfall intensity and that the spatial hydrologic variations is influenced by the initial groundwater position.