Geomorphological Zoning An Improvement to Coupling Alpine Hydrology and Meteorology

CARMEN DE JONG1, PETER ERGENZINGER2, MARTIN BORUFKA3, ARNE KOCHER4 AND MARTIN DRESEN5 1 Geography Department, University of Bonn, Germany, 2 Berlin-Bonn Environmental Research Group, Bornheim-Uedorf, Germany, 3Institute of Geographic Sciences, Free University of Berlin, Germany, 4 Institute of Geographic Sciences, Free University of Berlin, Germany, 5geoSYS,

Berlin, Germany

17.1 INTRODUCTION

To solve complex hydrometeorological problems in mountain catchments under the impact of climate change (Price & Barry 1997), a new generation of models and tools is required. Integrated Watershed Management can benefit from coupled models through improved parameterisation of the earth's properties by geomorphological zoning. In mountain areas, investigation of energy and/or mass exchange does not require only data on incoming and outgoing radiation, precipitation, evapotranspiration (ET) and discharge for every unit ofthe surface but also submodels on sediment dynamics. These interrelations and processes are highly variable since, for example, radiation and water are directly linked to plant life, whereas sediment dynamics are linked to individual geomorphological and geological zones. Crosscutting models describing the dynamics of meteorology, hydrology and geomorphology in alpine valleys therefore require other parameters for describing surface and subsurface properties. It is important that one's focus should be placed on river bed changes in mountain torrents because of varying erosion and accretion. Apart from fluvial sediment transport, hillslope dynamics also needs consideration in hydrological catchment models. This demands a shift in focus on new problems and techniques such as detailed geomorphological zoning of a basin. In contrast, most meteorological models run at small scales. Mountains are described crudely as bulged terrain, an umbrella-like surface characterised by grid nodes with distances of several kilometres. Such approaches may suffice for the description of general air movement, but in situations in which, for example, a stagnating local thunderstorm triggers a large flood, the related model must be adjusted to a different spatial and temporal scale. Clearly, there is considerable contrast in scale between meteorological models on the one hand and hydrological models, including sediment transport, on the other.

Hydrological models should not be restricted to the definition of surface runoff as a function of precipitation alone but should also include regional ET and the

Climate and Hydrology in Mountain Areas. Edited by C. de Jong, D. Collins and R. Ranzi © 2005 John Wiley & Sons, Ltd changing storage conditions of soils, regolith and aquifers (Molnar etal. 1990). The parameters required for the description of regional ET differ from those required for surface runoff, slope water or groundwater. This is relevant for mountain slopes where water is predominantly transported laterally. In this context, geomorphological-geological zoning is particularly important and the description of surface properties should not to be restricted to the vegetation cover, distribution of soil classes and topography alone (Duan et al. 2001). In addition, there is a growing demand to describe the reaction of the basin during extreme events, including the prognosis of river bed stability. This can only be modelled using more sophisticated process conceptualisations with parameters describing sediment sources, grain size, bed structure and the dynamics of river bed material.

Figure 17.1 The Solk (Austria) and Dischma (Switzerland) study areas

Table 17.1 Characteristics of study areas

Catchment

Braeualmbach/Soelktal, Austria

Dischma, Switzerland

Location

Tauern/Styria

Graubiinden, E. Alps

Size (km2)

7.8

43

Length (km)

2.6

14

Altitude (m)

1100-2600

1500-3100

Average grad. (°)

27

30

Glaciers (km2)

None

Scaletta (0.66)

Chualp (0.3)

Geology

Slate, schist, amphibolite, marble

Mainly gneiss, some amphibolite

Soils

Regosols, brown soils, podsols

Regosols, podsols

Vegetation

Forest, dwarf pine, meadows,

Mainly alpine grass, dwarf shrubs,

shrubs and alpine pasture

forest (spruce, larch, and pine)

Annual aver. snow cover

205 daysa

225 days4

Mean rainfall (summer/annual) (mm)

850 (April to August)/1200a

500 (mid-June to mid-Sept)/12004

Mean annual temp. (°C)

1.6 (at 1110 m)a

1.1 (at 2000 m a.s.l)4

Mean discharge (summer/annual) (mm)

610/860a

800/12004

Mean evapotranspiration (mm)

340 (at 1200 m a.s.l)a

300 (at 2000 m a.s.l)4

The goal of this contribution is to recommend the importance of geomorphological zoning for future development of watershed management models dependant on hydrological, meteorological and geomorphological coupling. This approach will be discussed using case studies from the Dischma valley in Grisons south of Davos (Switzerland) and from the Solk valleys in Styria, situated in the Tauern Mountains east of Schladming (Austria) (Figure 17.1). These valleys represent geological and geomorphological aspects typical for the metamorphic central ranges of the Alps with unstable slopes, active sediment sources and significant fluvial sediment transport (Table 17.1).

17.2 SCALES AND DIGITAL ELEVATION MODELS

It is inevitable that scales have to be shifted according to different goals. Timescales of days, weeks and years are commonly used for analysing many different regional hydrological aspects such as stage or discharge in accordance with the interests of hydrological management (Baumgartner etal. 1983). In contrast, forecasts of precipitation and associated floods under mountain conditions often require much higher resolution time steps of tens of minutes or at least half-hourly intervals. Since water levels in torrents and rivers rise rapidly during storm events, time intervals of approximately 10 min are preferable. When aspects such as hazard and risk analysis are mandatory, new model approaches are necessary. For this purpose, precise measurements of regional precipitation and snowmelt are required to simulate discharge and to enable detailed description of their influences on the main water storages in mountain slopes and valleys.

Corresponding spatial descriptions are equally important. For many model approaches, coarse grids (25 by 25 m) are sufficient to describe driving parameters (Grayson & Bloeschl 2001). However, when roughness and geometry of slopes and torrents have to be considered because of, for example, their influence on snow distribution or evaporation, smaller grid sizes or landscape units have to be incorporated. In the Swiss Alps, topographical plans are typically available at a scale of 1:10,000, forming a good basis for digital elevation models (DEM) with grid sizes of 5-10m. At present, this scale is complemented by a new generation of remote sensing images such as ASTER or SPOT. Considering the stability of rivers and torrents, it is not advisable to model with a uniform spatial resolution but to apply a dual-nested approach with a higher spatial resolution for selected areas sensitive to rapid changes in surface discharge. For a daily prognosis of discharge, coarser scale models suffice.

17.3 PARAMETERISING SURFACE PROPERTIES BY GEOMORPHOLOGICAL ZONING

The traditional approach of hydrological models, where surface characteristics are described only in terms of topography, vegetation cover and soils, is not adequate for modelling in mountain terrain (Price & Heywood 1994, Duan etal. 2001). Parameters that are necessary for the description of surface properties, especially geomorphological zones, will be discussed for the following three types of models.

1. Evapotranspiration models

2. Slope models

3. Precipitation-runoff models and sediment transport

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