Estimation of flood discharges are required for design of hydraulic structures, floodplain zonation and
watershed management works. It is required to extend the regional flood models based on station data to
ungaged locations within the same region. The spatial variation of drainage area as the main variable in
regional models may be determined by GIS. Therefore, having the regional model and the corresponding
maps of independent variables, one may extend the model onto the rivers. Specific attention is given to the
role of GIS for model’s spatial extension, derivation of flood maps and extraction of flood profiles in rivers.
Another word, the methodology of spatial extension of regional flood analysis based on data of hydrometric
stations is the main problem in this paper.
The discharge data of 22 watersheds in Gilan province were regionally analyzed. Independent variables
such as area and slope were extracted based on the digital elevation model (DEM). Map of average annual
rainfall was also prepared for the region. A program for spatial mapping was written in Arc View GIS.
This program uses D8 flow tracing algorithm to identify the upslope cells of a given target cells. Average
values of independent variables such as slope and rainfall is determined by having the upslope cell values.
Thus, it is possible to compute flood discharge at the ungaged target cell using the regional model equation
which involves the average values of independent cells. As such, the flood map in river cells is extracted.
One can use these maps to study the variation of flood discharge along the river length. Finally, the
share of each independent variable in the regional flood model was investigated. The methodology to prepare
the base flood maps are the most important outcome of this paper which may be used in design of
hydraulic structures as well as flood control measures.