Effect of catchment spatial characteristics on groundwater inflows influencing the thermal regimes of a Danish lowland stream using high resolution temperature measurements

Temperature is a vital stream physical property recognized for its influence on other physical, chemical and biological processes of riverine ecosystems. Discrete groundwater discharge zones in a stream system are known to moderate or alter the thermal regimes particularly during the periods of large diurnal fluctuations. This study examines the effect of catchment spatial characteristics on groundwater discharge zones influencing the thermal regimes of a Danish lowland stream using high resolution temperature measurements and a stream temperature model. An Agilent Distributed Temperature Sensing (DTS) unit (N4386A) with 1.8 km long fiber optic cable was used to collect temperature from Stream Elverdamsåen at 1 m spatial and 3 min temporal resolution. The DTS data collection period was intermittently spread over a year from August 2010 to August 2011 capturing both the spatial and seasonal dynamics of the groundwater discharge zones. In total, 16 discrete groundwater discharge zones were identified from the DTS datasets representing summer, 16 in winter and 19 in spring, albeit with only four interactions contributing in all three seasons. Local heterogeneity of the unconfined fractured clayey till and antecedent precipitation was found to influence the distribution of groundwater water discharge zones thereby directly influencing the stream thermal regimes. A physically based one dimensional stream temperature model was setup for the study reach to fit the observed high resolution DTS data by calibrating the groundwater inflow estimates. With 19 influx locations contributing a total of 13.5 l/s, each inflow source has to be calibrated for an accuracy of less than 1 l/s. To overcome the resultant uncertainty, the inflow locations were grouped into four stream sections exhibiting different thermal characteristics and the model was calibrated for sectional inflow estimates to mimic the respective thermal regimes. The resultant sectional groundwater inflow estimates were 4 l/s for 0-750 m, 3 l/s for 750-1200 m, 1 l/s for 1200-1350 m and 5l/s for 1350-1750 m. The sections are compared with catchment characteristics (geology, topography, land use) and used to set up a spatially distributed hydrological model. This study illustrates the combined use of DTS and stream temperature model to understand the effect of non- uniform groundwater inflows on stream thermal regimes.