Over the last two decades, the dramatic increase in the computer power available to hydrologists has led to signiยฎcant developments in the way that hydrological research and operations are conducted. This special issue of Hydrological Processes focuses on one area of such developments, the applications of GIS (geographical information systems) to the solution of hydrological problems. Over this period, GIS applications in environmental modelling have proliferated to take advantage of the spatial data representation capabilities of linking GIS systems to process-based models, and the examples presented in this volume represent many dierent levels of coupling between the GIS and the hydrological models employed to solve particular problems.
Hydrological applications of GIS are extremely varied. Whilst hydrological scientists have progressed in their representations of hydrological processes from lumped through semi-distributed to distributed hydrological models, water resource managers have followed a parallel route in the increasing spatial resolution with which assets, particularly infrastructure, have been represented, interrelated and managed. Common to both the research and management arenas is that the desire for increasing spatial resolution makes it attractive to work within a GIS framework. These developments are considered by Clark, who highlights some of the technical and ethical ramiยฎcations of data quality and, in particular, of increasing spatial resolution. Furthermore, varied hydrological applications can be driven by dierent users accessing the same pool of information. As a result, the structure of the database that supports the GIS, the quality of the data and the way in which the database is managed lie at the heart of the development of many GIS applications. Roberts and Moore elaborate on some of these issues in the context of the development of a multidisciplinary database to support the United Kingdom's Natural Environment Research Council (NERC) Land Ocean Interaction Study (LOIS).
With the increasing availability of high resolution digital elevation models (DEMs), the most widespread application of GIS in hydrology is the identiยฎcation of drainage pathways and runo contributing areas based on topographic form, and their coupling with hydrological models. Such applications frequently face problems concerning the accurate description of terrain. A fundamental issue is how to deal with topographic depressions and ยฏat areas which may or may not be artifacts of the DEM. Papers by Martz and Garbrecht and by Rieger review causes of such problems and propose some solutions. Papers by Watson et al., Storck et al., and de Roo examine the use of distributed hydrological models within GIS applications that address, respectively, the impact of precipitation patterns and the representation of vegetation and topography on estimated water yield; the impact of forest harvest on peak stream ยฏow; and the estimation of catchment-scale soil erosion and sediment yield. The paper by de Roo further develops the theme of coupling GIS to process-based models by illustrating the way in which runo and soil erosion models can be both loosely coupled and embedded within a GIS.
Although catchment-scale hydrological modelling represents an important GIS application within hydrology, GIS has relevance to the solution of many other hydrological problems at local, catchment and regional scales. Other hydrological applications of GIS presented in this volume include the modelling of slope stability and landslide activity at both the site (Miller and Sias) and regional (Montgomery et al.) scales; the regional-scale assessment of non-point source groundwater contamination (Loague and Corwin); and an appraisal of the factors controlling coastal sediment plumes (Mertes et al.).
One of the great attractions of GIS is that it provides a framework for integrating data from disparate sources. The papers in this volume illustrate integration of raster data from remotely sensed sources and ยฎne DEM grids; vector data derived from contemporary and historical map and air photograph sources; and point data from hydrological and ecological monitoring networks. GIS permits the complex representation