Sperbelgraben, Rappengraben, Emmental, Switzerland, 1903-
Location and Scale
Sperbelgraben and Rappengraben are two first-order catchments of 0.54, and 0.60 km2 area, respectively, located in the Emmental region in the Swiss Pre-Alps. They constitute the first and longest continuous runoff measurements in small catchments in Switzerland. Probably, they can also be denoted as the first “paired watershed study” worldwide.
Construction of the runoff stations started in 1900, but it was not until April 1903 that self-recording devices allowed continuous runoff observation in the two catchments. In autumn 1927, new stations, each with a Thompson cross-section (an equal-leg triangle) were installed to replace the formar Bazin stations. On this occasion, the Rappengraben station was moved 300 m upstream onto solid rock. The hydrological and meteorological measurements in the Sperbelgraben and Rappengraben were operated by the Swiss “Zentralanstalt für das forstliche Versuchswesen”, which was the predecessor institute of the Swiss Federal Research Institute WSL, from 1903 to 1957. Since then the runoff measurements have been carried out by the Swiss Federal Office for the Environment FOEN, and the meteorological measurements by the Federal Office of Meteorology and Climatology MeteoSwiss, respectively.
Daily mean air temperature typically varies between -10 oC in winter and +20 oC in summer, with a longterm mean of 6.7 oC. In the 20th century the soil was typically snow covered from December to April. In the past 40 years, snow cover duration has shortened substantially at this elevation. The recorded mean annual precipitation is 1’636 mm in the Sperbelgraben and 1’713 mm in the Rappengraben. According to the Hydrological Atlas of Switzerland HADES, a typical order of magnitude for the annual evapotranspiration in the Emmental amounts to 550-600 mm.
The catchments are situated in the molasse zone and geologically consist of conglomerate layers crossed by marl layers. While the clay content of the soils varies with the fraction of marl in the bedrock, their lime contents are generally low. The Sperbelgraben and Rappengraben are principally characterised by Cambisols, with an intermediate water-storage capacity and moderate permeability. Water-saturated soils, typically Gleysols, are largely restricted to gentle slopes with a high clay content, situated on the terraces typical of both catchments (outcrop of the marl layers).
The Sperbelgraben catchment has an elevation range of 911 to 1’203 m a.s.l., and the Rappengraben catchment of 1’141 to 1’256 m a.s.l. Both catchments have a similar channel density of approximately 5 km km-2 and an identical circumference of 3.0 km. However, the shape of the Rappengraben is quite circular, whereas the Sperbelgraben is rather elongated.
Vegetation / Land Use
The Sperbelgraben catchment is entirely covered with forest. Main tree species include fir (Abies alba), spruce (Picea abies), beech (Fagus sylvatica), and sporadically maples (Acer Pseudoplatanus). In contrast, forest covers only about half of the Rappengraben catchment. The remaining area is used as alpine pasture. The fraction of forested areas, however, has been steadily increasing at the expense of the agricultural areas. In the first half of the 20th century, approximately one third of the catchment was forested (Engler 1919).
No continuous measurements of runoff from small first-order catchments of just a few km2 are available from the 19th century. In 1900, however, the Swiss Federal Institute WSL installed two gauging stations in the Emmental region (central Switzerland) and started a long-term observation resulting in the longest continuous runoff data series of small catchments in Switzerland - and possibly worldwide. The motivation for recording these measurements had to do with the fact that Swiss forests had been in rather bad condition throughout the 19th century. A series of major flood events caused extensive damage in many regions of Switzerland (e.g. 1834, 1839, 1860 and 1868) and led to the hypothesis that over-harvested forests were one of the reasons for the flooding (e.g. Landolt 1869). Several experts claimed that forests play a prominent role in mitigating such rainfall events. It was hoped that studying the runoff behavior in the two catchments would throw light on this question, as one is completely forested (Sperbelgraben) and the other to a large extent covered by alpine grassland (Rappengraben).
Hydrological Knowledge Gained
The first published analyses by Engler (1919) and Burger (e.g. 1934) showed that the forested basin had lower peak flows, but higher baseflows. This confirmed the prevalent belief that forests helped to reduce flood flows and sustain baseflows. According to Engler (1919), the mitigating effect on peak discharge was especially high during short and intense thunderstorms, whereas only slight to no effects were observed during long-duration rainfall, depending on the water content of the soil before the event. This last finding has been referred to much less in subsequent scientific literature. Even though Engler stressed the fact that it was the forest soil rather than the vegetation cover that reduced flood peaks, this did not lead to changes in forest management. The oversimplified result that "forests reduce floods" was what most influenced forest hydrology during the several decades when Engler and Burger were working on the data. Later hydrological studies in these two catchments investigated the impact of wind storm deforestation on the runoff generation at various scales (Badoux et al., 2006a, 2006b), as well as the impact of soil frost on runoff formation in such pre-alpine catchments (Stähli, 2017).
Photos from the archive of the Swiss Federal Research Institute WSL, Birmensdorf
- Badoux, A., Jeisy, M., Kienholz, H., Lüscher, P., Weingartner, R., Witzig, J., and Hegg, C. 2006a. Influence of storm damage on the runoff generation in two sub-catchments of the Sperbelgraben, Swiss Emmental. European Journal of Forest Research, 125 (1): 27-41. doi: 10.1007/s10342-005-0102-6
- Badoux, A., Witzig, J., Germann, P.F., Kienholz, H., Lüscher, P., Weingartner, R., and Hegg, C. 2006b. Investigations on the runoff generation at the profile and plot scales, Swiss Emmental. Hydrological Processes, 20 (2): 377-394. doi: 10.1002/hyp.6056
- Burger, H. (1934). Einfluss des Waldes auf den Stand der Gewässer; 2. Mitteilung; Der Wasserhaushalt im Sperbel- und Rappengraben von 1915/16 bis 1926/27. Mitteilungen der Schweizerischen Anstalt für das forstliche Versuchswesen, 18(2), 311–416.
- Burger, H. (1943). Einfluss des Waldes auf den Stand der Gewässer; 3. Mitteilung; Der Wasserhaushalt im Sperbel- und Rappengraben von 1927/28 bis 1941/42. Mitteilungen der Schweizerischen Anstalt für das forstliche Versuchswesen, 23(1), 167–222.
- Burger, H. (1954). Einfluss des Waldes auf den Stand der Gewässer; 5. Mitteilung; Der Wasserhaushalt im Sperbel- und Rappengraben von 1942/43 bis 1951/52. Mitteilungen der Schweizerischen Anstalt für das forstliche Versuchswesen, 31(1), 9–58.
- Engler, A. 1919. Einfluss des Waldes auf den Stand der Gewässer. Mitteilungen der Schweizerischen Zentralanstalt für das forstliche Versuchswesen, vol. 12, 626 pp.
- Penman, H.L. 1959. Notes on the water balance of the Sperbelgraben and Rappengraben. Mitteilungen der Schweizerischen Anstalt für das forstliche Versuchswesen, 35(1), 99-109.
- Stähli, M., Badoux, A., Ludwig, A., Steiner, K., Zappa, M., and Hegg, C. 2011. One century of hydrological monitoring in two small catchments with different forest coverage. Environmental Monitoring and Assessment, 174: 91-106.
- Stähli, M. 2017. Hydrological significance of soil frost for pre-alpine areas. Journal of Hydrology, 546: 90-102. doi: 10.1016/j.jhydrol.2016.12.032