Measurements of tritium and deuterium concentrations in precipitation and runoff were in addition to conventional investigation methods used to provide some further insight into the storage properties of Lainbach valley, a catchment area of 18.7 km2 and 670 to 1801 m above sea level in the Bavarian Alps. The basin was modeled using different simple hydrological models. Information about mean transit times of subsurface water and storage volumes of basin reservoirs is derived from the application of specific flow models to the isotopic output data. Based on fitted mean transit times of 0.8 yr. for the upper subsurface reservoir and 7.5 yr. for the lower subsurface reservoir, storage volume equivalents were obtained of 0.6 and 1.9 m of water depth, respectively.