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Runoff generation is a result of the interplay of a range of processes, the relative magnitudes of which vary, among other things, with climate, catchment properties, and catchment scale. The variability of runoff generation processes within a mountain catchment and the variability from event to event is one particularly intriguing aspect. A better understanding of these spatio-temporal patterns of runoff generation is critical for obtaining realistic model simulations of events, such as extreme floods, and of run-off behaviour associated with changes in environmental and land use conditions. Estimating runoff generation is very difficult as it involves a high degree of extrapolation. Difficulties in accurately assessing runoff in mountains have been highlighted by local-scale field experiments (e.g. Scherrer 1997), observations in experimental basins (e.g. Anderson et al. 1997; Kirnbauer and Haas 1998; Torres et al. 1998; Müller and Peschke 2000; Uchida et al. 2001), and modelling studies (e.g. Moore and Grayson 1991) that emphasize the spatially highly heterogeneous nature of runoff. Also, different runoff processes may dominate at different spatial scales (see e.g. Blöschl 1996; Uhlenbrook and Leibundgut 1997). Although it is possible to estimate runoff for yet unobserved situations with hydrological simulation models, the reliability of such estimates is notoriously poor, particularly when moving from the plot scale or small catchment scale to medium sized catchments (DFG 1995). There is still a gap between the understanding of runoff generation processes at the plot scale and process-based hydrological modelling at the catchment scale.

Many scientific papers and books demonstrate the direct and indirect effects of human activities on the intensification of soil erosion processes and changes in both sediment and runoff sources (Ives and Messerli 1989). It is well known that deforestation and hillslope farming cause distinct changes in soil properties and infiltration rates, which ultimately affect soil erosion processes and the hydrological cycle at a basin and hillslope scale (Goudie 1986).

The riparian zone encompasses the strip of land between the stream channel and the hillslope and is sometimes referred to as the valley floor, near-stream zone (Cirmo and McDonnell 1997), floodplain (Bates et al. 2000), or buffer zone (Lowrance et al. 1985). Riparian zones have been differentiated from upslope zones by unique hydrology, topography, vegetation, and soils (Hill 1996). Characteristics such as anoxic zones, gleyed soils, distinct soil color, high organic content, breaks in slope, and near-surface water tables often distinguish riparian zones from adjacent hillslopes. Because of their location, riparian zones have significant potential to regulate the movement of water and elements in surface and subsurface runoff that flows from upslope areas to the stream (Hill 1996).

According to the Second and Third Assessment Reports of the Intergovernmental Panel on Climate Change (IPCC 1996; 2001) the increase in mean surface air temperature of the northern hemisphere was larger in the 20 century than in any other period of the last 1000 years. The decade 1990–1999 was the warmest of this time period. It is also believed that this increase in air temperature will be accompanied by intensification of the global hydrological cycle and, in the same chain of cause and effect, by enhanced evaporation and precipitation (Schär and Frei, this volume). However, the scientific community needs to gain a better understanding of the biosphere-atmosphere system before being confident on the predictions of hvdrolodcal processes in a future climate (Frei et al. 2000: Ohmura and Wild 2002).

The western U.S. derives its water resources predominantly from cold season precipitation and storage in snowpack along the narrow Cascades and Sierra ranges, and the Rocky Mountains. Hydroclimate is modulated by the diverse orographic features across the region. Precipitation and runoff generally peak during winter and spring respectively, whereas water demand is highest during the summer. Such phase differences between water supply and demand create a necessity for water management, which is reflected by major developments of reservoirs and dams that regulate irrigation, hydropower production, and flood control during the past 50 years. Because water resources have been essential to the economic development and environmental well being of the western states, it raises concerns when recent studies suggest that global warming may exert significant impacts on snowpack and streamflow, which may seriously affect water resources in the western U.S. in the 21 century (e.g. Leung and Ghan 1999; Leung and Wigmosta 1999; Mile et al. 2000; Leung et al. 2003a; Miller and Kim 2000).

Slopes induce biological diversity, and nowhere else is diversity so important as on slopes. Why the first? Why the second? The inclination of a piece of land causes gravitational forces that structure the surface and climatic vectors that differentiate life conditions across those structures. The resultant multitude of microhabitats leads to a multitude of inhabitants. A major function of those inhabitants is to secure substrate against further action of gravity. Sloping terrain is only as stable as the workforce keeping it in place. It is this endless battle between the force of gravity and biological safeguards against its consequences, which governs mountain biota. If the substrate is gone, so too are most of the plants and animals.

Alpine ecosystems occur on all continents, and potentially serve as sensitive indicators of biotic response to environmental change. Because environmental change associated with resource extraction and development is minimal in most alpine areas, biotic changes in the alpine are reflective of “indirect” anthropogenic environmental effects, including changes in climate, atmospheric chemistry, and transmission of ultraviolet radiation. Plant species respond differentially to these environmental changes, related in part to their ability to alter growth rates as resource supply changes and to changes in biotic interactions with neighbors (Theodose and Bowman 1995; Callaway et al. 2002). Thus, changes in plant species composition are likely to herald environmental change in the alpine. Floristic changes have been noted in some alpine areas, potentially associated with climate change (Grabherr et al. 1994), atmospheric pollution (Rusek 1992), and increased N deposition (Korb and Ranker 2001; see Baron et al., this volume for aquatic biotic responses to N deposition).

High mountain ecosystems are sensitive to climate change (Box 1). Historical records of the flora on high summits in the Alps provide an important baseline against which climate-induced effects on high mountain ecosystems can be assessed. Reinvestigations of these old “monitoring summits” have shown that mountain plants have migrated upwards during the 20 century. An increase of atmospheric temperatures since the late 19 century is the most likely cause of this upward shift (Gottfried et al. 1994; Grabherr et al. 1994; 1995; 2001a; Pauli et al. 1996; 2001a). This “summit study” underlined the importance of long-term monitoring for assessing climate change effects on mountain ecosystems and initiated the establishment of extensive monitoring networks in mountain environments.

The montane and alpine regions of the world cover about 10% of the terrestrial area, a life zone ca. 1000 m above and below the climatic treelines in temperate and tropical latitudes, including some of the biologically richest ecosystems. The alpine life zone above the climatic treeline hosts a vast biological richness, exceeding that of many low elevation biota and covers 3% of the global terrestrial land area (Körner 1995). The overall global vascular plant species richness of the alpine life zone alone was estimated to be around 10,000 species, 4% of the global number of higher plant species. No such estimates exist for animals but based on flowering plants, high elevation biota are, as a general rule, richer in species than might be expected from the land area they cover.

The alpine and subalpine regions of south-eastern mainland Australia are small and restricted, covering an area of only approximately 11,000 km in a continent of 7.7 million km (Williams and Costin 1994; Costin et al. 1999; Williams et al. 2003). The most extensive are the Kosciuszko plateau in New South Wales (NSW), the Bogong High Plains in Victoria, the Central Plateau in Tasmania and the mountains of south-west Tasmania (Kirkpatrick 1994; 1997; Williams and Costin 1994; Costin et al. 1999). These areas are of prime importance as catchments for the supply of high quality water to adjacent lowlands; for hydroelectricity generation; for recreation in both summer (e.g. walking, horse riding) and winter (mainly skiing); and for nature conservation. In Victoria and Tasmania, the high country is also used for the summer grazing of domestic cattle. Because of their unique combination of geomorphic, biotic and land-use characteristics, and despite their limited distribution, Australia’s high mountain regions are of national and international significance (Kirkpatrick 1994). In recognition of this, most of the Australian Alps are designated National Park.