Der schnelle Anstieg des Grundwassers nach Starkregen

In hydrological practice, as well as in scientific work, groundwater recharge by precipitation is usually seen as a slow process of seepage through soil layers of uniform texture in analogy to the laws of Darcy and Richards. In most catchment areas, however, infiltrating water from heavy rainfalls passes within a few hours via macropores and preferential flow paths to the near-surface, unconfined aquifers. Recharge phases essentially extend over a few days, during which the groundwater level rises, thereby increasing the baseflow in the rivers. In the present study, groundwater recharge and storage are calculated from baseflow, which was separated by the nonlinear reservoir algorithm from time series of daily runoff at gauging stations in Northern Germany. The results are compared with measured groundwater level fluctuations in the catchment areas and daily infiltration rates at two lysimeter stations. The hydrographs of the seepage to the saturated zone with their rapid rising are generally consistent. However, while the event-related recharge determined from baseflow ends when baseflow resumes its typical recession, seepage at a lysimeter can take significantly longer. The volume of lysimeter seepage is generally slightly higher than groundwater recharge in catchments, due to the different vegetation, but especially because the rim around the lysimeter surface prevents surface runoff. In addition, the walls of the devices allow only vertical fluxes. Without correction, seepage rates at lysimeters should therefore not be reported as groundwater recharge at sites with significant lateral interchange processes or of entire catchment areas. It is also to be noted that the forms of the derived response functions of recharge to a precipitation unit are practically time-invariant, but with a strong seasonal variation of the ratio of recharge to precipitation of the events.