Key words: Digital elevation model, open sources GIS, Arc Info, Terraflow, and 
Hydrological model

Nowadays most of the distributed hydrological models available are using GIS 
functionality for handling with pre- and post-processing procedures by using 
importing and exporting functions. A digital elevation model (DEM) or sometimes 
referred to as a digital terrain model (DTM) is a quantitative representation of the 
topography of the Earth (or sometimes other surfaces) in a digital format. The 
resolution of the DEM, or the distance between adjacent grid points (often the size 
of the cell or pixel), is a critical parameter in determining the amount of detail 
that a user should expect to represent in the DEM. The smaller the resolution, the 
more details or features that will be present. For assessing the more accurate 
results in the hydrological modeling the high resolution digital elevation models 
are used. At the same time the size of the data is very high. In order to achieve 
this, data sets of high resolution to be handled by the open source GIS software.
The 25m resolution DEM is used for hydrological function computation.  This paper 
compares the time required for computing hydrological function like flow direction , 
sink filling and flow accumulation for different size of data sets using Terraflow 
module in GRASS open sources GIS and Arc Info hydrological extensions. The two GIS 
software out puts are compared with respect to hydrological model aspects. Also it 
will explain the Massive data import and export in open source GIS through 
Geospatial Data Abstraction Library (GDAL).