Dynamic streams in a pre-alpine catchment

Abstract

This thesis aims to investigate the factors controlling the dynamics of temporary streams, i.e., streams that do not flow throughout the year. Temporary stream hydrology is a relative unexplored scientific field. The field work for this thesis was conducted in a pre-alpine headwater catchment, the Lümpenenbach, in canton Schwyz, Switzerland. The streams were first mapped and then divided into segments. During ten surveys, each segment was assigned one of six flow types (dry, wet, weakly trickling, trickling, weakly flowing and flowing). By comparing the results from the mapping with topographic indices, in particular the Topographic Wetness Index (TWI) and upslope accumulated area, the effect of wetness and topography on the presence of water in temporary streams could be found. The study was conducted during a limited timeframe and therefore only event-based changes, not seasonal changes, could be studied. The conditions during the field campaigns had a relative small effect on the flowing stream length because of the short timeframe of the study and the relative stable weather conditions and low temperature. The discharge changed 2.8 times between the driest and wettest field campaign. Based on field observations, the stream network reacts quickly to rain fall events, both with a quick response, and a fast drainage and recession. However, there are some smaller areas where the stream initiation mainly is driven by springs, resulting in a slower recession. The Lümpenenbach main stream is a perennial stream, and were always flowing. The flowing stream length varied 1.4 times during the surveys. There were a higher spatial and temporal variation in the stream state for the small tributaries. There is a clear relation between the topographic indices and the likelihood of a stream segment flowing: a higher topographic index value increases the chance for a stream to flow.