The estimation of groundwater recharge as subterranean and highly variable runoff component on an adequate time resolution is problematic and therefore often neglected in water balance studies. The aim of this work was to estimate groundwater recharge at the forest meteorological test site Hartheim in southwest Germany (200 m a.s.l) for a three-year period. Tracer techniques are the most precise methods known for estimating groundwater recharge. In this work environmental and artificial tracer methods were applied to the saturated and unsaturated zone. A main focus was the application of deuterium and oxygen-18 isotopes in water as environmental and artificial tracers. The derived results were compared with water balance calculations on daily time resolution. The Bowen Ratio Energy Balance (BREB) method was used for calculation of evapotranspiration.

In comparison to long term observations, the three-year period studied shows relatively wetter conditions. These results reflect high rainfall amounts during the year 1999. Water balance calculations show 5 mm recharge during the hydrological year 1998, 200 mm for 1999 and 100 mm for 2000. All water balance components show a high temporal variability. Isotopic variation in precipitation, soil water and groundwater measured at the test site show a clear seasonal variability. Tracer methods prove that direct recharge occurred but could not be evaluated quantitatively. However as opposed to some earlier hypotheses, it could be concluded that direct groundwater recharge is not negligible at the test site. In fact, in dependence to the variable water balance components precipitation and evapotranspiration it is considerable.

Keywords: tracer methods, oxygen-18, deuterium, deuterium excess, groundwater recharge, water balance

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