The model builder is used to modify Curve Numbers (CN-values) assigned to individual Grid cells of the raster based rainfall/runoff model.For surface runoff, the following parameters are used to determine the CN-values:
The forms for editing these values are opened pressing the "table edit key"
After the CN-values and their parameters have been set, the grid is calculated (or recalculated). This is done by surfer commands. The original grid as obtained from ArcView is usually in JTM with a 1000 m spacing, for modelling a grid size of 2000 m is recommended. The previous grid size of 2500 m is not used any more, Surfer has difficulties to interpolate this spacing out of a 1000 m grid.
The Surfer calculation is invoked by pressing the "Surfer calc key" The calculation lasts about one minute and is indicated by the hour-glass cursor.
After the grid has been recalculated, the range of CN-values is shown in the form. This may serve as a control.
Move with the cursor over the image. When the cursor changes from arrow to hand, you can click to go to the detailed description.