Perform hydrological modeling on massive raster terrain models with SCALGO Hydrology

In the SFD model each cell is assigned a flow direction to the steepest-descent neighbor cell. Using the MFD model directions are assigned to all lower height neighbors, and using the aspect decomposition model directions are assigned to (at most) two lower neighbors based on the local aspect of the terrain.

The example to the above right illustrates how the flow directions module assigns directions to a terrain with no flat areas. The example to the right illustrates how the flow directions module assigns directions to a flat area where some cells have at least one downslope neighbor (spill points). On such a plateau both the shortest path and the geodesic model assign directions such that water flows across the plateau to the spill points in a natural way. In the shortest path model directions are assigned such that water follows the shortest path from a cell in the flat area to the closest spill point. In the geodesic model directions are also assigned such that water tends to follow the shortest path to the closest spill point but also converge towards the cells in the middle of the flat area. The example to the right shows where water converges when modeling water flow using shortest path (white) and geodesic (blue) routing. On flat areas without a spill point (a depression) directions are assigned such that water converges to a single cell in the flat area.

If the raster terrain model contains no-data cells these cells are treated as if they have a height lower than all other cells. It is also possible to ignore all no-data cells that are not connected to the boundary of the raster through other no-data cells, that is, regard these cells as having a height higher than all other cells.

In the example above to the right, the flow accumulation value of a cell corresponds to the number of upstream cells. If the initial flow values are set to the area of cells, the accumulation value of a cell will correspond to the area of upstream cells. Note that to model non-uniform rainfall the initial flow values of different cells can be given different non-negative values, and to model the water drainage characteristics of different soil types the initial flow values can be given different negative values. If a cell has more than one flow direction the flow is distributed to the neighbors proportionally to the elevation difference between the cell and the neighbors receiving flow.

Often stream networks are extracted as the cells with flow accumulation above a certain threshold (blue in the figures below). In this case, depressions in the terrain will impede flow leading to a disconnected stream network (bottom-left figure). Therefore all depressions are often filled (e.g. with the Flooding module) before computing flow directions and flow accumulation to simulate a situation where no surface water gathers in depressions (bottom-middle figure). However, filling all depressions can lead to loss of a lot of detail (and very large flat areas) and therefore unrealistic stream networks. The SCALGO Topology software package can be used to only remove (fill) "insignificant" depressions that are likely to fill easily, leading to more realistic stream networks (bottom-right figure).

In the example above to the right, the flow accumulation value of a cell is computed using an initial flow of one on each cell and corresponds to the number of upstream cells. As in the Flow Accumulation module, the initial flow can be user-specified; if the initial flow values are set to the area of the cells, the accumulation value of a cell will correspond to the area of upstream cells.

The output from the Stream Segmentation module is a raster where a cell on a stream segment corresponding to the input raster contains a label (and nodata otherwise). The label of cells on the same segment are the same and labels increase downstream, that is, a segment has a larger label than any upstream segment it receives flow from. The Stream Segmentation module can also be used to output the stream network in vector format (the drainage lines). The drainage lines are a collection of polylines following stream segments between (and including) critical cells. The module can also output the drainage points of the stream network. These are points corresponding to cells that have stream junction cells as their downstream neighbor or cells that are stream end cells. In the figure, circles represent drainage points.

In both the Strahler and Shreve stream orders, stream segments that do not receive flow from any other stream segment have order one (are numbered one). In the Strahler order (left in the figure), the order of a stream segment downstream from a junction is the same as the order as the maximal order of stream segments upstream from the junction, unless two or more of these segments have the same maximal order, in which case the downstream segment order is one larger. In the Shreve order (right in the figure), the order of a segment downstream from a junction is simply the sum of the orders of the segments upstream from the junction.

As in the Flow Accumulation module, the initial flow on each cell is normally one, but it can be fully user-specified. The output from the Stream Ordering module is simply a raster where a cell on a stream segment corresponding to the input raster contains the (Strahler or Shreve) order number of the segment (and nodata otherwise).

Left: Strahler order, right: Shreve order.

Watersheds are, for example, used to quickly determine the depression where water on a given cell eventually ends up.

The Watersheds module can also be used to compute a catchment raster for a given set of labelled drainage points (e.g. as computed from the Stream Segmentation module). Each drainage point is mapped to the cell it is contained in (a drainage cell). The catchment of a drainage cell is the region upstream of (and including) the drainage cell but excluding the regions upstream of any other drainage cells.

Left: Catchments computed based on the flow directions from previous examples and drainage points from the Stream Segmentation module. Right: Catchment polygons produced from the same input.

We refer to the catchment itself as the outlet catchment. In the module it is optional whether to include the outlet catchment in the adjoint catchments or not. Note, that if outlet catchments are not included then some catchments will not have adjoint catchments. The module outputs adjoint catchments as polygons containing the label of their corresponding input catchment.

Left output: Adjoint catchments excluding outlet catchment. Right output: Adjoint catchments including outlet catchments. Example of Adjoint Catchments for a set of input catchment polygons. Note that in this example, there is only one non-empty adjoint catchment if outlet catchments are not included.

Flooding removes all depressions in a terrain. The Depression Mapping, Depression Identification and Depression Filling modules in the SCALGO Topology package allows for a much more detailed analysis of depressions and a very precise removal of only "insignificant" depressions.

In the example above to the right, note that not all parts of the input terrain below a new sea-level are flooded. However, the parts of the orange terrain below the new sea-level correspond to the flooded parts of the input terrain. Thus it is easy to compute the flooded parts of the input terrain for any possible sea-level from the orange terrain. The Masks module in the SCALGO Utility software packages e.g. provides an easy way of computing a set of mask rasters that indicate flooded cells for a set of equidistant sea-levels (for example for every 10cm from 0 to 10 meters).

The cells in the input raster terrain model considered to be sea are usually all no-data cells, but it is possible to distinguish between the set of no-data cells that are connected to the boundary of the raster through other no-data cells and those that are not, and only designate one of these sets as sea. It is also possible to designate all cells of a specific height as sea.