CatchmentSIM allows users to apply common spatial distribution rainfall patterns through the 'Spatial Distributions' form. The first of these to be implemented is the Generalised Short Duration Method (GSDM) for estimation of Probable Maximum Precipitation (PMP). A document describing this method can be obtained from the Bureau of Meteorology
Click to Enlarge Image
The first step is to develop the roughness grid (Terrain Category). This grid will label each cell as either 'Smooth' or 'Rough' based on the following criteria:
•Does the elevation of other pixels within a range of X m (default 400) vary by more than X m (default 50), if so then this pixel is rough
•Is there a rough pixels within X km (default 20), if so then this pixel is rough by association
Once the roughness grid has been produced (by clicking 'Add Terrain Category Grid'), you should see it displayed in CatchmentSIM.
At this point, you can already undertake a GSDM analysis using the GSDM_PMP macro script (refer Macro Wizard). However, if you wish to apply the ellipse based spatial distribution, you will need to continue to use this form.
When you first loaded the form, you may have noticed that CatchmentSIM has tried to fit the spatial distribution pattern to the catchment (or subcatchment if selected in the 'Configuration' box). It automatically estimated the correct rotation and centroid coordinates.
However, you may need to adjust the spatial distribution manually to ensure that the catchment is contained within the smallest ellipse possible. To adjust, select 'Reposition over Display' and click and drag the ellipses on the screen, click 'Finished' when complete. Next, click 'Create and Add Vector Layer' and ensure 'Create Grid Layer' is checked. This will add both a vector layer and raster layer containing the ellipses.
At this point, you can undertake some zonal statistics to view the characteristics of the zones within each ellipse and each terrain category (roughness).
For a more detail spatial GSDM analysis, please refer to the 'GSDM_Spatial_PMP.cst' script.
