The Origin of Streams : Stream cartography in Swiss pre alpine headwater

University essay from Uppsala universitet/Institutionen för geovetenskaper

Abstract: Temporary streams have received undeservedly little scientific attention and as a result their role in hydrological, biogeochemical and ecological processes is not yet fully understood. The ultimate goal of the research was to gain a better understanding of the temporary stream network and the processes that control it and determine how the active and connected stream length change with catchment wetness conditions to find simple methods to map seasonal and event-based changes in temporary flowing stream networks. Streams, springs and wetlands of four relatively small headwater catchments (11.7 – 25.3 km2) and one wetland in the steep and remote Zwäckentobel catchment in Alptal, canton Schwyz (Switzerland), were mapped and stream segments were classified by flow type during different weather conditions using direct observations. The mapping was performed by an elite orienteer with mapping experience. The variation in streamflow was analysed and related to the catchment wetness and topography using the TWI-values and the upslope accumulated area of the stream segments. As the catchments wetted up in response to fall rainfall events after a dry summer the flowing stream density increased up to five times and the connected stream density increased up to six times with a 150-fold increase in discharge. Also the number of flowing stream heads increased up to ten times. The best description of the pattern of stream expansion is a combination of the variable source area and the element threshold concepts, where surface topography, particularly TWI (Topographic Wetness Index) and upslope accumulated area (A), and local storage areas controls where streamflow is initiated and how flow in different stream segments connects. Streams in the Alptal show a seasonally bottom up or disjointed connection pattern. Mapping the temporary streams in steep and remote watersheds as a function of hydrological conditions is not an easy task. It is however necessary in order to fully understand where water is flowing or not. A combination of field observations with monitoring equipment can facilitate this extensive work by providing a more detailed temporal resolution.

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