Digital Elevation
Models have been proved to be a valuable tool for the topographic
parameterisation of hydrological models
Geographical distributions on the earth surface has been inherently complex, revealing more information at higher spatial resolution apparently without limit. Modeling these distributions into geographical reality is a process of discretization that converts a finite number of database records or objects. The environmental processes in the real world are computer based, mathematical models that simulate spatially distributed and time dependent, which are increasingly recognized as fundamental requirements on the reliable, quantitative assessment of complex environmental issues of local regional and global concern. Spatial representation is critical to environmental problem solving, but GIS currently excels the predictive and related analytical capabilities necessary to examine complex problems at least to some extent. The environment system which is emphasized here is to understand the water system of the earth.
GIS provides representations of
these spatial features of the Earth, while hydrologic modeling is concerned with
the flow of water and its constituents over the land surface and in the
subsurface environment. Groundwater vulnerability to contamination from the land
surface has become an important area of hydrogeological research. To date most
of this research has focused on small sites of special interest using intensive
sampling techniques and complex modeling procedures . There is also a need
however for more extensive evaluations of groundwater vulnerability that might
help guide land planning and managemment at regional scales. Hydrologic models
are mathematical representation of the flow characteristics. These models are
successful in dealing with time variation assuming uniform spatial properties.
There are three main basic issues in Hydrology which are necessary to model to
understand the problems faced by the environmental system. These issues are :
- Pollution Control and mitigation for both
groundwater and surface water.
- Water Utilization for water supply for
municipalities, agriculture and industry and the competing demands for
instream water use and wildlife habitat.
- Flood Control and mitigation.
GIS with its upcoming advanced technology has been a great asset to Hydrologic Modeling. The goal of this essay is to outline the rational basis for the linkage between GIS and hydrologic modeling, to indicate the type of model that could be incorporated within GIS and which are best left as independent analytical tools linked to GIS for data input and display of results; to examine the object oriented data model as an intermediate link between the spatial relational model inherent in GIS and the data models used in hydrology; and to look at some future directions of hydrologic models that have not been possible before but that might now be feasible with the advent of GIS. Digital Elevation Models or DEMs are increasingly becoming the focus of attention within the larger realm of digital topographic data. The quality and caliber if DEMs has been extremely valuable in the hydrological applications. DEM provides a digital representation of a portion of the earth’s terrain over a two dimensional surface.
Breif Histrorical Review of Hydrological Modelling
During the last decade there has been tremendous development in Hydrologic Modeling using GIS. The use of digital terrain models have showed there potential to a number of analysis in hydrology. As said earlier that hydrologic models have been mathematical for more than a century now, since the Darcy’s Law (the fundamental equation governing groundwater flow) was discovered in 1856, the St. Venant equations describing unsteady open channel flow were developed in 1871, and a steady stream of analytical advances in description of the flow of water has occurred in the succeeding decades. Transport of constituents in natural waters was sparsely treated before about 1950; during this time of 1950s Digital Terrain Models were used for various geoscience applications. Computer models began to appear by the mid of 1960s, first for surface water flow and sediment transport, then in the 1970s for surface water quality and groundwater flow, then in 1980s for groundwater transport). Then the decade of 1990s when people realised the utility of incorporating GIS with hydrologic modeling. Petts (1995) attempted to identify the essence of the ‘modern’ geographical approach to hydrology by proposing the basis for the catchment and fluvial systems to interact in such a way that could be addressed through GIS. Brown (1995) focusing on GIS in Hydrology quoted defining assertion that hydrological GIS represents a modeling opportunity, which is unambiguously he views as the hydrological focus lying within the modeling function, with sampling, measurement, scale and accuracy representing key issues for evaluation of status of GIS in hydrology.