Breach Depressions When Pre-processing a DEM for Watershed Delineation Using WhiteboxTools

When performing a watershed delineation in GIS, a DEM usually needs to be pre-processed by “filling” any depressions or sinks (an area lower than its surroundings) to create a hydraulically connected DEM. Sometimes the operation of filling depression will significantly change the original DEM by filling too large or too deep an area, which may cause an issue to the following watershed delineation.

Breaching a depression is a low impact alternative to fill depression (Figure 1). A DEM can first be breached to get rid of majority of the depressions and then be filled to eliminate any remaining depressions, which in fact may be a better practice. If you only Fill, you risk artificially raising low-lying valleys (like quarries or wide dams) and flattening the landscape too much. If you only Breach, deep, localized noise or pits might force the algorithm to cut unnatural, deep trenches that skew flow directions.

WhiteboxTools has an very effective depression breaching tool – BreachDepressions (Figure 2).

The breached DEM may be be further “filled” to eliminate any remaining depressions (Figure 3). Since it is believed that the breach tool has gotten rid of the majority of the depressions, the following fill depression operation will have a much less impact on the original DEM than if it is performed without the DEM being breached first. WhiteboxTools has several fill depression tools and the FillDepressionsWangAndLiu is recommended here.

After the above pre-processing steps, a watershed delineation can be conducted using WhiteboxTools as explained in this post.

To further minimize the impact to the original DEM, some discussions on the internet suggest using WhiteboxTools‘ FillSingleCellPits (Figure 4) or BreachSingleCellPits (Figure 5) to remove small depression areas by filling or breaching just one single cell before applying the breaching or filling depressions tools which usually are meant for large depressions features. Note these two tools are optional, and therefore use them at your own discretion.

It always prudent to investigate different DEM pre-processing methods and their resulted DEM – Fill DEM only, Breach DEM only, and Breach/Fill DEM, and use the one that generates the most reasonable watershed delineation results.