The Tcl/Tk interface developed for the hydrologic model is an example of loose integration, 
but does not provide the flow paradigm. The GUI can be exploited to unify and to arrange the 
various operations required to produce a set of inputs for the model. Furthermore, a Tcl/Tk script 
can include any required control or restriction on the input parameters. These restrictions can be 
enforced in an intuitive way and will avoid user errors in these very first steps of the modeling 
process. 
Tcl/Tk itself does not provide any sort of database structure. The most practical way to 
operate within this framework is to maintain the original formats in which the data are stored, 
and to perform all needed conversions within the scripts. This is a very pragmatic approach that is 
suitable for simple applications or in the early stages of development for more complex applications. 
Overall performance is a secondary issue at this stage since the main concern is to simplify the pre- 
and post-processing components of the modeling application. For later development stages, a more 
efficient management of the data will be required. 
5.2 NELSIS CAD FRAMEWORK 
XELSIS is a design flow-based CAD framework developed at the Delft University of Technol 
ogy [ten Bosch 1995]. It is a flexible light-weight framework that can be used to quickly build an 
integrated engineering environment from a set of cooperating tools. Via its high-level integration 
services, one can easily integrate in-house and other vendor's tools to form a consistent design 
system. Although the roots of NELSIS are in the integrated circuit design process, the basic CAD 
framework principles are general enough to be used in other application areas as well, such as 
software testing, high level synthesis, and system simulation. 
The idea behind design flow management is to transform the informal idea of the structure of 
the design process as it resides in the user's mind into a formal description (a design flow) which 
is then used to further assist the user. In our case the design flow describes the basic steps in the 
modeling process, such as preparation and checking of inputs, model execution, and interpretation 
and analysis of outputs. The framework allows complex queries in an intuitive way. It doesn't 
impose any restrictions on data organization, nor on the use of graphical user interface tools. 
The NELSIS CAD framework is being used in our hydrologic modeling application. In the 
example discussed here a design flow for the topographic analysis step of the pre-processing sequence 
has been developed. Although integration of the different tools (mainly FORTRAN routines) 
involved in this step could also be accomplished using Unix scripts, this simple example is useful to 
illustrate some of the features of NELSIS as an integration platform. Topographic analysis utilities 
are standard in many popular geographic information systems, and can also be run as stand-alone 
programs. Starting from a digital elevation model (DEM), one can calculate the flow direction, the 
accumulated drainage surface, the slope, and a topographic index for every pixel, as well as the 
catchment boundaries and the stream network. 
Figure 4 shows the Design System User Interface for this implementation. The aim was to help 
the user run the topographic analysis tools, a set of FORTRAN programs, without the user having 
to write or modify any FORTRAN code. The different tools are "wrapped" into the framework 
by small C programs. Since the FORTRAN routines are compiled on the same machine as the