Editing of correction factors related to land use

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

In principle, the CN-values for this model are based on "hydrological soil classes". The land-use and climate zone (which is assumed to have an effect on soil depth) are introduced as correction factors. Thereby, the correction factor is not directly applied to the CN-value but to the S-value, which is a function of the CN-value:

S=(1000/CN-10)*25.4

The S-value (with the dimension "Millimetres" can be understood as the maximum capacity of water a certain soil can store. Correction factors larger than 1 will increase the S-value and thus decrease the CN-value.

The land use classes and their codes (160, 161, 162, 163) were predefined by the Ministry, and their extent has been outlined under ArcView GIS. In addition, urbanisation is considered. It is stored in a separate layer and combined with the vegetation related land use. The calculation is such, that if urbanisation occurs, it will overrule all other types of land use. In order to maintain this functionality, the class limits are protected against accidental modification.

Areas with dense vegetation (mainly forests and brush) have a higher capacity to retain water, while the opposite is true for urban areas. The surprisingly small effect of agricultural areas is a result of the calibration process.

The present version of the model builder is limited to modifying the correction factors for given land use classes. For the purpose of editing these values, the table is copied to a temporary table.

Pressing the "exit key" will exit the form without saving.

Pressing the "save key" will save the values and exit.