5th Swiss Geoscience Meeting, Geneva 2007
Challenges in the modelling of Bernese Oberland runoff
and Brienz-Thun lake level variations in August 2005
Molnar Peter, Witschi Tobias & Burlando Paolo
Institute of Environmental Engineering, ETH Zürich, CH-8093 Zürich
(molnar@ifu.baug.ethz.ch)
The management of hydrological risk for the future requires not only thorough
analyses of observations of past flood events but also an assessment of the
modelling capabilities to simulate those events, with the purpose of evaluating
possible mitigation strategies. Here we present a post-event simulation of the
August 2005 event in the Bernese Oberland region by a modelling system
which was not explicitly developed for flood prediction and was not calibrated
for this event (Molnar et al. 2007). Nevertheless the simulations are satisfactory
and illustrate some fundamental challenges in modelling such spatially
extensive flood events which may be useful to learn from for the future.
The Bernese Oberland region is a large complex landscape with considerable
heterogeneity in topography and hydrological processes. The headwaters on
the main Alpine ridge are dominated by snow melt and rapid surface runoff, and
at mid-basin altitudes subsurface water in shallow soils and forest cover play an
important role in runoff generation. These complexities are major challenges for
watershed modelling. The study area covers almost 2500 km 2 and consists of
four major drainage basins and two basins contributing directly into the Lakes of
Brienz and Thun (Figure 1). Streamflow is recorded at all of the main basin and
lake outlets and at a number of stations within the basins (12 stations). The
directly contributing areas to the lakes are ungauged. Precipitation stations are
located throughout the basin mostly at lower altitudes (18 stations).
The modelling system consists of the continuous conceptual semi-distributed
watershed model PRMS (Precipitation-Runoff Modeling System) and the lake
water balance model LARESIM (LAke REtention SImulation Model) (Oosenbrug
2003). The watershed is divided into hydrological response units (HRUs) with
similar geophysical properties in terms of runoff production. PRMS simulates all
relevant hydrological processes at the daily scale (interception, potential and
actual evapotranspiration, snow accumulation and melt, soil moisture, surface,
subsurface and groundwater runoff), for each HRU and the basin as a whole.
LARESIM simulates daily lake level variations in the Brienz and Thun Lakes as
a function of inflows, precipitation and evaporation from the lake surfaces, and
lake regulation rules in the form of stage-outflow relationships. PRMS and
LARESIM were combined and calibrated with observations in the period 19762000 to produce the best parameter set and verified on the period 2001-2005,
including the August 2005 event (Molnar et al. 2007).
The performance of the modelling system is very good on a daily and monthly
scale for the entire study area. An example of the daily simulated water levels
for the Brienz Lake is shown in Figure 2 for the August 2005 event. Inflows and
water levels were simulated well in the Brienz Lake. However, inflows and lake
levels in the Thun Lake were overestimated (peak flood stage in the lake was
5th Swiss Geoscience Meeting, Geneva 2007
about 50 cm higher than observed). In this paper we illustrate what we think
were the reasons for the over-estimation: a) unknown contributions to the lake
from direct basin drainage; b) incorrect input of precipitation on the lake surface;
and c) departure of actual lake outflows from the regulation rule. In particular,
Meteoswiss radar data for the event are used to show that an incorrect
assessment of the spatial distribution of rainfall may have led to substantial
errors in runoff and water level predictions at this spatial scale.
Figure 1. Location of the study area, 4 study basins and 2 directly contributing
basins draining into the Brienz and Thun Lakes.
568
0
567
40
60
566
80
100
565
Hsim
Hobs
120
140
564
160
180
563
01.05.05
200
01.06.05
01.07.05
01.08.05
01.09.05
Figure 2. Daily observed and simulated water levels and estimated precipitation
on the lake surface from 1 May - 31 September 2005 for the Lake of Brienz.
REFERENCES
Molnar, P., Witschi, T., Burlando, P., Oosenbrug, E. & Ruf, W. 2007: Evaluating
potential impacts of climate change on floods and water levels in an Alpine lake
system in Switzerland, Hydrology and Earth System Sciences, in preparation.
Oosenbrug, E. 2003: Einzugsgebiets- und Seeretentionsmodellierung im Berner
Oberland: Hochwassersimulation durch Kopplung zweier Modelle, M.S. Thesis,
ETH Zürich, 112pp.
PRECIPITATION (mm)
WATER LEVEL (m)
20