The contribution of Distributed Temperature Sensing (DTS) in streams to assess spatial runoff processes in a moraine dominated agricultural catchment

Evaluating impacts of site-specific changes in land use and land cover on catchment processes is significantly complicated by spatial heterogeneity and the long and variable time lags between precipitation and the responses of streams and groundwater. In this study, a 1-D soil-plant-atmosphere model (Daisy) was used to calculate the water balance of a moraine dominated agricultural catchment (42 km2) in Denmark, and a Distributed Temperature Sensing (DTS) system was installed in the stream Elverdam to assess the spatial variations in lateral inflows to the stream. The Daisy model was set up using a high spatial resolution (10-30 m) land use map which includes agricultural crops, forest, wetlands and inhabited areas, and spatial variations in soil types, geology and tile drainage were represented. The DTS system measured diurnal variations in water temperature each meter along a stream reach of 1800 m over 812 hours ( 34 days) in all four seasons of a full year. Data from two water level loggers quantified flow contributions over the study reach, and a climate mast provided local meteorological data. Multiple discrete locations with stream-groundwater interactions were identified, as indicated by low temperature amplitudes, but only some discrete groundwater inflow sources were large enough to allow quantification based on heat and mass flow conservation equations. The trend in downstream water temperatures at night was useful to quantify total groundwater inputs. The routing and flow accumulation of water from individual grids within the catchment to the stream was calculated using a Lidar based (1.6 m resolution) digital elevation model. Many locations with observed (DTS-based) discrete lateral inflows were in good agreement with stream locations receiving extra large inflows from the upland contributing land areas, but riparian land use, tile drains, geology and stream slope were also important determinants of reach-scale lateral inflow