Perspectives of GIS Modelling in Hydrology.

Swati Grover
GIS Development
[email protected]

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.

Focus on GIS Modelling In Hydrology
Modeling draws to greater or lesser degree upon the geographic tradition that hydrology, catchment and fluvial system interact closely and casually in time and space. The elements of hydrological modeling predate GIS by more than a century. Geographic Information Systems (GISs) are highly specialised database management systems for spatially distributed data. Chow et al. (1988) offered a taxonomy of hydrological models based on the randomness (deterministic/ stochastic), spatial variation (lumped/ distributed; space independent/space dependent) and time variation (steady flow/ unsteady flow; time independent/time correlated) – thereby drawing the attention to the pivotal position of the spatial dimension explored the several possible applications of linking GIS with the hydrological models:

• Hydrological Assessment to represent hazard or vulnerability (through weighted and summed influences of significant factors rather than through physical laws)
• Hydrological Parameter Determination, whereby the GIS provides inputs to the model in terms of parameters such as surface slope, channel length, land use and soil characteristics.
• Hydrological Modeling within the GIS, provides feasible time snapshots or temporal averages are involved, not time – series.
• Linking the GIS and hydrological modelsto utilise the GIS asan input and display device, including real time process monitoring if the necessary (remotely sensed) observations are available.

Fig. 1: A Conceptual Structure for GIS Applications in Hydrology

However relate these set of assumptions as a spectrum of significant role of GIS, with full linkage to modeling being seen as the ultimate expression of GIS contribution, but according to him these set of assumptions would be misleading the value of GIS not necessarily determined by the extent to which it is in control of the processes concerned, the role of GIS can be detrimental. Further Maffini (1997) argues that the database has been replaced by GIS as the integrating core of corporate information handling, which will further upgrade by network computing capability. GIS integrates different elements like automated mapping (AM), facilities management (FM), remote sensing, land information systems (LIS) and spatial statistics, also at the same time it serve as an input to the management information systems (MIF) in the corporate domain and modeling / control systems in the research domain (see Figure 1). As the corporate sector emphasizes the database as the key to the management information systems , in the same way Maidment (1993) tries to focus on the data model which is the key to the GIS modeling in hydrology concluding

” It is probably true that the factor most limiting hydrologic modeling is not the ability to characterize hydrologic processes mathematically, or to solve the resulting equations, but rather the ability to specify values of the model parameters representing the flow environment accurately. GIS will help overcome that limitation.”

Application Domains of DEM In Hydrology
Digital Elevation Models are used in number of sub-domains in hydrology. Varied hydrological applications can be driven by different users accessing the same pool of information. As a result the structure of the database that supports the GIS, quality of the data and the way in which the database is managed lie at the heart of development of many GIS applications. The DEMs have proved to be very efficient in extracting the hydrological data from the DEM by analysing different topographical attributes (elevation, slope, aspect, relief, curvatures) for modeling purposes. There are number of models developed in past which has been useful individually and combined applications in various combinations. DEMs have potentially proved to be a valuable tool for the topographic parameterization of hydrological models especially for drainage analysis, hillslope hydrology, watersheds, groundwater flow and contaminant transport etc.

Discussion
Most of the hydrological models are mathematical based,where they integrate existing knowledge into a logical framework of rules and relationships. GIS technology has been integrated with these several surface /subsurface hydrological models now for more than a decade understanding the utility and signifiocance of topographic attributes of the terrain for various hydrological applications. The reason of adopting GIS technology is because it allows the spatial information to be displaced in integrative ways that are readily comprehensible and visual. The spatial information collected are further subjected to continous GIS analysis. Further summarising the review of the historical models developed in past years and their significance with computing compatibility and suitability with GIS , discussing the different types of modeling – deterministic and stochastic approach to accomplish a complete linkage between GIS and hydrological models would require GIS to have time dependent data structures so that the evolution through time spatial distribution of hydrologic phenomena could be observed. The GIS techniques have the potential for widespread application to resource evaluation, planning and management. Finally modeling provides the opportunity for realistic representing the three – dimesional nature of natural landscapes in hydrologic modeling under the constraints of maintaining physical rigour , simplifying the governing equations that must be solved and reducing the computational requirements.

Looking into the future of infornation technology is a high risk activity for possible evaluation of priorities of GIS in hydrology. Technologically, the effective acquistion and manipulation of high resolution spatial data will become commonplace to every individual for assessing the development factors in an area. Further increase in efficiency and limitless access to information may lead to real detriment and GIS will be strengthened but progressively subsumed to modeling management and operational control systems.

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