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 above-right example 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 flow 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 on 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 above-right example 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 river networks are extracted as the cells with flow accumulation above a certain threshold (blue on the below figures). In this case, depressions in the terrain will impede flow leading to a disconnected river 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 river networks. The SCALGO Simplify software package sold as an add-on to SCALGO Hydrology (and also as part of the SCALGO Model software package) can be used to only remove (fill) "insignificant" depressions that are likely to fill easily, leading to more realistic river networks (bottom-right figure).

Watersheds are, for example, used to quickly determine the depression where water on a given cell eventually ends up. Note that watersheds are only defined when each cell has at most one flow direction (since a cell can only belong to one watershed).

Bluespots can be a valuable supplement to the output from the flow accumulation module since it gives a (rough) estimate of areas that could be vulnerable to flooding during extreme rain (where water gathers in depressions).

In many cases only major depressions above a certain size are of interest. Using the SCALGO Simplify software package sold as an add-on to SCALGO Hydrology (and also as part of the SCALGO Model software package) it is possible to remove (fill) small depressions based on their depth, area or volume. The figure to the right illustrates how two insignificant bluespots are removed by removing two small depressions before computing the major depressions.

When modeling how water flows on the surface of a raster terrain model, river networks are often extracted as the cells that transport a lot of water (blue on the below figures). In this case, depressions in the terrain will impede flow leading to a disconnected river network (left figure). Therefore depressions are often filled (the terrain flooded) before modeling flow to simulate a situation where no surface water gathers in depressions (middle figure). However, filling all depressions can lead to loss of a lot of detail (and very large flat areas) and therefore unrealistic river networks. The SCALGO Simplify software package sold as an add-on to SCALGO Hydrology (and also as part of the SCALGO Model software package) can be used to only remove (fill) "insignificant" depressions that are likely to fill easily, leading to more realistic river networks (right figure).

On the above-right example, note that not all parts of the input terrain below a new sea-level are flooded. However, the parts of 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 cells in the input raster terrain model considered to be sea is 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.