Seasonal and event-based changes in the flowing stream network of temporary rivers in the Reppisch catchment: Comparison of manual mapping and data from sensors

Abstract

Temporary streams are streams and rivers that lack surface flow during certain periods of the year. They are present all over the world and their occurrence is predicted to increase in the future. However, their importance for hydrological and ecosystem functions has only recently been investigated. As temporary streams start to flow, the flowing stream network expands and as they dry up the stream network contracts. This thesis aims to understand flowing stream network expansion and contraction for two neighbouring catchments, the Diebis and Bleiki in the Reppischtal in the Canton of Zurich, Switzerland. The study used two methods: field mapping in both catchments, during which the sections of the stream network were classified according to flow type, and automatic data collection with a monitoring system in the streams of the Diebis. These methods were applied for two timescales, seasonally, with regular mapping and measurements over an 11-week period, and after single-events to determine the suitability of these methods for the study of temporary streams. The behaviour of the flowing stream density and the patterns of expansion of the two catchments differed on a seasonal scale. This difference can be explained by the topographic characteristics and by the difference in the levels of human disturbance of the two catchments. The analyses for single-events could not be performed as planned because of problems due to the malfunctioning of the flow sensors and the reduced functioning of the electrical resistance sensors from the monitoring system. However, the study highlights the large differences in stream network expansion and contraction for neighbouring catchments. Thus, it is necessary to conduct studies in multiple catchments to come to a robust understanding of the hydrologic behaviour of temporary streams in a region. The methods used are complementary and suitable if they work as planned. The field mapping has a good spatial resolution which helps in particular for the seasonal analysis but is not suitable for the analysis of single-events. For this, the high temporal resolution of the monitoring system is very useful. However, the uncertainties of these two methods have to be considered as well. In particular, the subjectivity of the determination of the flow type and the variability of the conditions during the field mapping sessions have to be considered. Regarding the sensors, the largest uncertainties are due to the need for spatial interpolation because the data are recorded only at specific locations and due to external disturbances of the registration of data because of sediments or other changes in the stream conditions