Alptal, Central Switzerland, 1968 -
Location and Scale
Alptal is a pre-alpine catchment in central Switzerland located south of lake Zurich ((47°02’20”N and 8°43’45”E), with a total area of 47 km2. The Swiss Federal Research Institute WSL has been conducting a comprehensive hydrological monitoring of three torrential first-order catchments since 1968: Vogelbach (with an area of 1.58 km2), Erlenbach (0.73 km2) and Lümpenenbach (0.88 km2) – all of them located in the upper part of the valley. The central village is named “Alpthal”, and the principle town at the outlet of the valley is Einsiedeln.
The first examination of the Alptal torrents started in 1963 by manual sampling. Between 1968 and 1971 small (relatively low-resistant) runoff stations were set up at the outlet of eleven first-order catchments. The first continuous runoff measurements started in August 1968 in the Vogelbach. After the destruction of all stations in one severe flood event (June 23, 1974), only the gauging stations of Vogelbach, Erlenbach and Lümpenenbach were rebuilt; this time in a much more robust way. Since then, the three runoff stations have been operated continuously. In addition to the runoff measurements, meteorological variables have been recorded since 1968 on a daily to 10-min resolution, weekly to monthly snow measurements have been conducted along 15 transects since 1969 and sediment transport has been recorded since 1986. Spatial variability in groundwater and channel flow, isotope concentration of rain, snow and stream water, as well as specific torrent processes have been measured since 2008.
The Alptal has a humid temperate climate and is characterized by frequent rainfall (Van Meerveld et al., 2018). The mean annual temperature is 6°C; average monthly temperatures vary from -1°C in January to 14°C in July. The precipitation has a distinct north-south gradient: the mean annual precipitation in the Erlenbach is 2300 mm y-1. Summer months are wetter than autumn and winter months. Roughly one fourth to one third of the annual precipitation falls as snow (Stähli and Gustafsson, 2006).
The study catchments are underlain by Tertiary Flysch, a sequence of sedimentary rocks, particularly argillite and bentonite schists, calcareous schists, marl and sandstone. Most sub-catchments comprise of the Schlieren, Wild- and Wägitaler-Flysch. The bedrock is considered to be (relatively) impermeable. The soils in the flatter areas are mollic Gleysols with typically a permanently reduced Bg horizon. At the steeper ridge locations, the soils are classified as umbric Gleysols with an oxidized Bw horizon. The humus layer consists of mor humus on dry ridges and muck humus in the wetter and flatter areas. Soil depth varies from 0.5-1 m at the ridge sites to 2.5 m in depressions and is weakly related to slope.
The headwater catchments in the Alptal are steep, with an average slope of ~35%. The topography is influenced by soil creep and landslides, resulting in a complex terrain with alternating steeper and gentler slopes. There is also distinct surface micro-topography, e.g. hummocks and hollows in the flatter wetland areas. The drainage density is high. The channels have a step-pool morphology (Molnar et al., 2010) and cut into the alluvium of weathered bedrock (Keller, 1970). However, most of the streams are relatively shallow and only the larger streams are incised by more than 0.5 m. The headwater streams have almost no riparian zone.
Vegetation / Land Use
The study area consists of forests, meadows and wetlands. The forested sites are generally located on the steeper slopes and are dominated by spruce trees (Picea abies L.) and Silver fir (Abies alba) with an understory of blueberries (Vaccinium sp.) on the drier sites and ferns and Equisetum on the wetter sites. In the lower elevations, the forest also contains maple and beech trees. The forest coverage is different in the three investigated first-order catchments: Vogelbach 63%, Erlenbach 39% and Lümpenenbach 19%. Wetlands have formed in the flatter areas and contain a thick organic soil horizon (up to 1 m thickness). Meadows in the upper parts of the catchments are used as summer pastures. The meadows contain Poa trivialis and Carex ferruginea; the wetland sites contain Caltha palustris, Petasites albus, Poa trivialis and Carex ferruginea.
In the beginning of the 1960-ties, the former WSL director Prof. Kurth decided to relaunch research on the forest impact on flood formation in Switzerland. He issued the young forest engineer Hans M. Keller (1936-1993) to establish a new hydrological observatory. Hans had just returned from a study visit at the Colorado State University where he had accomplished a Master of Science in Watershed Management.
Soon he selected the Alptal valley for his endeavor because of the particular geology, the representativeness for large parts of the Swiss pre-alps and – last but not least – the short distance to Zürich. After a few years of exploration with manual sampling and measurements, he and his team installed runoff stations, snow courses and rain gauges in up to 10 first-order catchments.
From the beginning, Hans M. Keller not only limited his investigations to the physical characterization of the water flow, but also explored the water quality of the torrents. Until then, systematic water quality measurements from small, natural catchments were rare or even missing in Switzerland. Later (in 2003) the three Alptal torrents became part of the Swiss National River Monitoring and Survey Programme (NADUF). In the 1980s, with the first EU-projects and Swiss National Research programs, the team of researchers in the Alptal became larger and more diverse. Investigations of sediment transport, nutrient dynamics and runoff formation complemented the original topic of forest hydrology. Over the years, more and more research groups joined and made use of the existing infrastructure and long-term data. They added their own ideas and equipment resulting in a modern environmental field lab. Today, the Swiss Federal Research Institute WSL still takes care of the basic operation and data management of the long-term measurements, but the Alptal research has definitely turned into an international research site supported by ETH Zürich, University Zürich, GFZ Potsdam and many others.
Hydrological Knowledge Gained
The many different observations in the Alptal at scales from plots of few m2 to catchments of 1 km2 have led to a comprehensive understanding of runoff formation in dynamic watersheds with wet soils and steep topography. In particular the role of the topography and the forest cover, as well as the importance of the snow cover for catchment runoff has been clarified.
The research in the Alptal has also brought important insights into the dynamics of sediment transport and concurrent change in the morphology of the torrents. In particular the role of single large floods for sediment supply and redistribution along a steep torrent has been demonstrated.
Comprehensive measurements of deadwood of various size and dissolved organic matter have shown that carbon discharge by a mountain stream is dominated by coarse organic matter (Turowski et al., 2016) .
- Keller, H.M. 1970. Factors affecting water quality of small mountain catchments. Journal of Hydrology (New Zealand) 9: 133-141.
- Molnar, P., Densmore, A.L., McArdell, B.W., Turowski, J.M., and Burlando, P. 2010. Analysis of changes in the step-pool morphology and channel profile of a steep mountain stream following a large flood. Geomorphology 124: 85-94. http://doi.org/10.1016/j.geomorph.2010.08.014.
- Rinderer, M., van Meerveld, I., Stähli, M., and Seibert, J. 2016. Is groundwater response timing in a pre-alpine catchment controlled more by topography or by rainfall? Hydrological Processes 30: 1036-1051. http://doig.org/10.1002/hyp.10634.
- Schleppi, P., Muller, N., Feyen, H., Papritz, A., Bucher, J.B., and Flühler, H. 1998. Nitrogen budgets of two small experimental forested catchments at Alptal, Switzerland. Forest Ecology and Management 101: 177-185. https://doi.org/10.1016/S0378-1127(97)00134-5.
- Stähli, M., Gustafsson, D. 2006. Long-term investigations of the snow cover in a subalpine semiforested catchment. Hydrological Processes 20, 411-428. http://doi.org/10.1002/hyp.6058.
- Turowski, J.M., Hilton, R.G., and Sparkes, R. 2016. Decadal carbon discharge by a mountain stream is dominated by coarse organic matter, Geology, 44(1): 27-30, doi: 10.1130/G37192.1.
- Van Meerveld, H.J, Fischer, B.M.C. , Rinderer, M., Stähli, M., and Seibert, J. 2018. Runoff generation in a pre-alpine catchment: A discussion between a tracer and a shallow groundwater hydrologist. Cuadernos de Investigación Geográfica, 44 (2): 429-452.