Putting water in its place: a perspective on GIS in hydrology and water managment

Clark, M. J. (1998), Putting water in its place: a perspective on GIS in hydrology and water management, Hydrological Processes, 12(6), 823–834, doi:10.1002/(SICI)1099-1085(199805)12:6<823::AID-HYP656>3.0.CO;2-Z.

The opening article of Hydrological Processes 1998 special edition issue on Geographic Information System opened with an article my Michael J. Clark: Putting water in its place: a perspective on GIS in hydrology and water management. Clark introduces the idea of hydrological GIS and examines the technical and ethical ramifications of data quality, in particular how the increasing of spatial resolution impact hydrological GIS.

Roots of Hydrological GIS

What is interesting about his origins of hydrological GIS, is that he looks back at the geomorphic prinicples that underline modern GIS theory.  Since he does this his roots of hydrological GIS goes back to roots of theory on fluvial systems.  He quotes Charles Lyell's, A Manual Of Elemenatry Geololgy (1852) and T.H. Huxley's Physiography (1880).

The two quotes have contrasted views on the nature of fluvial systems, and shows how the perspective on the role of the rive changed.  Lyell's quote emphasizes the idea that river occupies a part of a valley they flow, in other words the river is defined by the topography and make up of the land it flows in, dictating where it flows.  Huxley's main idea is that river's create the landscape, thus play a role in determining it's own course.  This meant that river's have a place in the valley of there own making.  This idea that the river defines it own place serves as the foundation of GIS in hydrology.

This idea makes sense when examining the toolkit for hydrology present in the Spatial Analyst Extension of ESRI ArcMap. These tools work off of elevation modeling, were topography is taken and used to derive all the hydrological features.  The methods are all derived from modeling water flow on the surface and having the hydrological feature manifest themselves.  This means that the underlying idea is that all aspects of hydrology follow a set of rules that can be simulated in computing environments to model there creation.

For example if you take the tool to define a watershed.  It is based on two components, the pour point and the flow direction raster.  The flow direction raster is a data-layer that gives the direction of the steepest downslope for any cell. This is all base on the idea that water flows downhill.  The datalayer that is used to create the flow direction is a Digital Elevation Model (DEM), which is the topography represented as elevation for each individual cell. The pour point is the outlet of the watershed.  Basically the tool takes the definition of a watershed and use these two datasets to derive it.

A watershed is the upslope area that contributes flow to a common outlet.  So the watershed is defined by taking the outlet and determining everything uphill from it by tracing out the network created by the flow direction raster. Overall hydrological GIS approach is that the terrain can be used to determine hydrological features, weather you see it as the terrain defining the river or the river morphing the terrain.