ABSTRACT: Linking GCMs and surface hydrology: the challenge of spatial scale.

CGRG Bibliography of Canadian Geomorphology
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Abstract

Despite substantial advances in our understanding of the problems of scale and scaling in hydrology and hydrological modelling, for many questions we only have incomplete answers. One of the key questions is: What is the scale range over which hydrological models can be extrapolated, and what factors determine this scale range? A partial answer to this question is that sometimes the scale range is controlled by the geomorphological features of the basin. Pilgrim (1983) discusses an example where alluvium in the gently sloping downstream reaches of the channel network affects baseflow characteristics and prevents extrapolating from small to large drainage basins. De Boer (1992b) termed such an impediment on scaling a 'morphological constraint' and contrasted this kind of constraint, resulting from the presence of morphological elements (e.g., a floodplain and alluvium) in the large scale basins and their absence in the smaller scale ones, with 'functional constraints' which arise from the water and energy flows in the system. An example of a functional constraint is provided by channel routing, which has little significance when modelling daily flows in small basins, but gains in importance as basin scale increases. A second key question is: How are small scale and large-scale hydrological processes linked together? Hydrological processes occur over a wide range of scales (Figure 2), and every hydrological process is part of a nested hierarchy that can be investigated in two fundamentally different ways. The reductionist approach is based on the premise that a system at one level of the hierarchy can be understood by studying the subsystems at lower levels, i.e., the 'whole' is investigated by examining the parts, and the combined knowledge of the individual parts provides insight into the whole. Reductionism has been extremely successful in a broad range of sciences. At the same time, however, it has become clear that reductionism has its limitations and that some systems cannot be understood as the simple sum of their parts. In recent years, this view has been strengthened by the study of non-linear dynamical systems. Such systems may display complexity, which describes system behaviour rather than system structure. This arises from the interaction of the system's components, but cannot be predicted from the rules describing the components and their interaction. As a result, such a system possesses emergent properties, which are apparent only when the system is viewed as a whole. While complexity and the related concepts of emergence and self-organization have been applied in ecology (Pahl-Wostl 1995) and geomorphology (Phillips 1999b, De Boer in press), there have been relatively few applications in hydrology (e.g., Rodríguez-Iturbe et al.1998; D'Odorico and Rodríguez-Iturbe 2000). One of the ways of studying the interaction between different scales is to use regional climate models nested within a GCM. Goyette et al. (2000) investigated the impact of the Great Lakes on the regional climate by linking the Canadian Regional Climate Model (CRCM) to the second generation Canadian Global Climate Model (GCMII). The models were linked by using GCMII output data as the boundary conditions for the CRCM runs. Laprise et al. (1998) employed a similar approach to investigate regional scale climate and climate change in western Canada. Advances in the development of a theoretical framework and computational techniques need to go hand in hand with the collection of data that can be used for testing theories and models. The application of remote sensing in hydrology has led to the availability of data at much larger spatial scales than previously possible. Maps of snow cover are now readily available for many parts of the world and provide a valuable data source for snowmelt runoff studies. Many hydrological parameters and variables, however, cannot be measured using remote sensing and are not available for large areas. Advances in data collection must accompany advances in theory and modelling to ensure the continued development of hydrology as a science and to face the environmental challenges of the 21st Century.

Bibliography of Canadian Geomorphology