Basic grid data

When a new model is set up (or an existing model fed with improved data sets), basic grid data need to be prepared under GIS.

First of all, the general grid characteristics (grid extents and inner spacing) need to be defined and entered into the configuration file as well as in table App_Params.

Basic grid files need to be produced for the following data levels:

• an empty grid file (all values set to 0) (file flatland.grd)
• hydrological soil type (file hysoil.grd)
• land use (file lusegrd.grd)
• urbanisation (file urbgrd.grd)
• climatic zones (file czgrd.grd)
• groundwater basins (file gwbasins2.grd) for post-processing of groundwater recharge into groundwater safe yield.

The separation of land use and urbanisation into different layers is done, because urbanisation may change faster (and overrules all other / previous land use).

Preparation of grid files is done partly outside of the model. Typically, the following steps are undertaken:

1. Vector layers for the individual thematic layers need to be prepared in the GIS (e.g ArcView). The various features (such as hydrological soil types, land use and climatic zone) need to be distinguished by numeric identifiers. Urbanised areas are set to "1". Care needs to be taken in order to cover the entire country, unclassified areas are set to "0".
2. Raster analysis characteristics need to be set as defined to coincide with the general grid characteristion in the GIS (e.g. under ArcView with Spatial Analyst). Doing this, it is to be considered, that Spatial Analysis rasters cells are differently defined from Surfer grid nodes. Raster cells refer to the center of the raster cell, while grid nodes are corner points of the raster cells.
   It is recommended to initially use 1000 m grids and simplify these later. With this concept, analysis grid extends are 500 m larger (on each side) than modelling grid extends.
   
   
   The above settings would result in a surfer grid of the following dimensions:
   
3. Convert the themes to grids (use the identifier as discussed above as grid value)
4. Convert the ARC/INFO grids to ASCII raster files using the File/Export Data Source... command and store them into the model (system data) directory.
   
5. Import the ESRI raster files by using the ESRI2GRD interface of the rainfall/runoff model.