Geographical information systems provide appropriate methods for efficient storage, retrieval, manipulation, analysis, and display of large volumes of spatially referenced data. Accordingly, GIS consist of four basic components: data input and editing, storage of geographic databases, data analysis and spatial modeling, and data visualization and presentation (Fig. 2). The data may be collected from fieldwork, extraction of map data, air photo interpretation, and interpretation and classification of remotely sensed images. Data input may be carried out by manual digitization or computer-assisted semiautomatic methods. The data are organized into a series of spatially co-registered layers, with each layer relating to a particular theme or a set of layers relating to temporal variation of a theme.
Data input and structuring is one of the most time-consuming and expensive tasks in the creation of a GIS. Remotely sensed data can be put to the best use if they are incorporated in GIS. A GIS, therefore, when combined with up-to-date data from a remote-sensing system, can assist in the automation of several operations (e.g., interpretation, change detection, map revisions). The hydrological system is a dynamic entity; the information stored in a GIS is only a static representation of the real world and therefore has to be updated for the temporal coverage (i.e., a third dimension) on a regular basis. Remotely sensed satellite data offer an excellent input in this context to provide repetitive, synoptic, and accurate information of the changes of a watershed.
Integration of GIS with hydrological models is necessary to better explain the complexity of hydrological processes arising from spatial heterogeneity of inPut
Retrieval & Analysis
Entity Transformation & Formatting
Figure 2 Submodules of a GIS for input, storage, retrieval, analysis, modeling, and presentation of spatial and nonspatial data.
parameters such as topography, soil types and characteristics, vegetation, and antecedent soil conditions. A GIS can be employed to supply physical and hydrological parameters to a hydrological model. Model simulation results can be analyzed using a GIS. This requires a continuous flow of information to and from both the GIS and hydrological model. One of the useful possibilities of linking GIS with hydrological modeling is the capability of dynamic spatial visualization of the model simulation results, including user interaction. Real-time or near-real-time visualization of simulated hydrological processes could greatly improve existing analysis of simulation results.
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