The behavior of water is closely tied with the characteristics of the terrain's surface—particularly the values connected to elevation. In this section, we will use a basic hydrological model to analyze the location and direction of the hydrological network—streams, creeks, and rivers. To do this, we will use a digital elevation model and a raster grid, in which the value of each cell is equal to the elevation at that location. A more complex model would employ additional physical parameters (e.g., infrastructure, vegetation, etc.). These modeling steps will lay the necessary foundation for our web application, which will display the upstream toxic sites (brownfields), both active and historical, for a given location.

There are a number of different plugins and Processing Framework algorithms (operations) that enable hydrological modeling. For this exercise, we will use SAGA algorithms, of which many are available, with some help from GDAL for the raster preparation...