The model builder is used to modify Curve Numbers (CN-values) assigned to individual Grid cells of the raster based rainfall/runoff model.

• Principle assignment of curve numbers according to the "hydrological soil type"

...is set, the grid file with the CN-values for percolation (GW-recharge) are generated (or recalculated).

This calculation is performed using Surfer commands and it is invoked by pressing the "Surfer calc key" .  The calculation is performed in a few seconds, indicated by the hour-glass cursor.

The original grids as obtained from ArcView are usually in JTM with 1000 m spacing. These grids are located in the "System Data" path as indicated at the bottom of the Core Runoff model form. The following grid file is required:

• hysoil.grd (hydrological soil map).
  The same file is used for surface runoff calculation. However, the individual tables with CN-values, which are different for surface runoff and percolation, will lead to fairly different grids for CN-values.

Note:The creation of this directory and the generation of the original grid from ArcView are presently still managed outside of this model. ArcView is able to generate ascii-grids in ESRI format. The conversion from ESRI to Surfer format is done within this model using the ESRI2GRD utility of the Core Runoff model.

Processing of the grid for percolation is simple.

1. The original grid hysoil.grd is loaded and reclassified according to the table earlier edited. Since Surfer does not have a built-in reclassification command, this is performed by a loop through all the classes resp. table records.
2. Finally, the grid of 1000 m is recalculated to 2000 m to enhance modelling speed. The previous grid size of 2500 m is not used any more - the cubic spline algorithm Surfer applies would increase the extreme values in this case of interpolation.
3. After the grid has been recalculated, the range of CN-values is shown in the form. This may serve as a control.