The application of stable Oxygen-18 (δ¹⁸O) and Deuterium δ²H isotopes as a tracer for uxes between dierent compartments of the water cycle is subject of the present work. Within the framework of the study these tracer applications are limited to a maritime temperate climate zone. During a three year period, temporal data from a wide range of water cycle constituents was collected from the Skjern River catchment. The presented applications focus on the isotopic
input signal to the hydrosphere in the form of precipitation, the isotopic output signal and related dynamic processes at a coastal saltwater-freshwater interface and the temporal development within a given lowland headwater catchment.
In order to utilize ¹⁸O/²H as a hydrological tracer for specic regions, its input composition needs to be known. A two year dataset of monthly accumulated precipitation was collected, to study major isotopic changes occurring on a temporal and spatial scale. From that, an overall local meteoric water line (LMWL) summarizing the input function for the Skjern river catchment was developed and is expressed as: δ²H=7.4 δ¹⁸O + 5.36 0/00. Even on a small scale and at terrain
the isotopic composition is shown to vary signicantly. As one of the rst, this study can show that under maritime temperate climate conditions, post condensation processes will substantially aect the isotopic compensation of precipitation. Humidity is correlated to be the major driving factor for the observed spatial and temporal changes in δ¹⁸O/δ²H, while temperature in
uences the δ¹⁸O/δ¹²2H in precipitation to lesser extent.
Combining δ¹⁸O/δ²H with salinity and other hydrogeological and geophysical methods the temporal saltwater- freshwater dynamics of a coastal aquifer adjacent to a lagoon system were mapped and quantied. This study underlines the usefulness of δ¹⁸O and salinity as combined tracer in order to obtain stable end-member compositions and minimize mixing errors related to varying end-member concentrations. From the dual-tracer approach the saltwater-freshwater interface is suggested to vary seasonally in response to recharge dynamics. In periods of higher recharge, the tracer composition shows a refreshing of the near coastal aquifer. Low recharge periods are accompanied with saltwater intrusion processes, especially seen below the lagoon bed. Those results,
together with additional measurements of lagoon stage and salinity, were incorporated in a numerical framework in order to test and extrapolate previous ndings and study the temporal behavior and contributing hydrogeological processes to this density-dynamic zone. Numerical results could not reproduce the eld observations. The recharge ux is shown to only modify the very shallow parts of the aquifer, while the saltwater wedge is simulated to be maintained at any time of the year.
Relating the δ¹⁸O input isotopic concentrations to output δ¹⁸O concentrations obtained from a one year dataset of stream discharge in a headwater catchment of Skjern River, we were able to obtain estimations on mean transit times (MTT) for dierent sub-catchments. However, the dierent approaches applied, gave diverse results. Commonly used advanced methods based on convolution
integrals, did not converged with satisfying performance criteria. Relating all MTT estimates to catchment descriptors, did not show signicant relationships. The MTT results stand in contrast to earlier residence time estimates obtained from groundwater samples beneath near stream areas. Therefore from δ¹⁸O/δ²H compositions the conceptualization of hydrological conditions in the Holtum stream is suggested to be revised.