Background

This was an academic lab for a course I attended titled Spatial Modeling in Raster.  The purpose of this lab was to work with Hydrological Modeling in ArcGIS using functions from Spatial Analyst.  I was provided with a raster digital elevation model of Rio Fajardo area in Puerto Rico and points representing water stations.

Product

Link to Interactive 3D Model

(Note:  This may take a moment to load. Works best in Google Chrome or Internet Explorer.  Click ‘Help’ to learn the controls to navigate the model)

Link to 2D Map

Process

The first step in Hydrological Modeling is to fill sinks.  Sinks can be inaccuracies created by the resolution of the data or rounding of elevations to the nearest integer value.  This creates places on the raster elevation grid that would falsely seem like water would flow into.  Sinks were filled by running the DEM as an input to the Spatial Analyst Fill tool.  Next I computed flow direction.  This is another Spatial Analyst tool that takes a DEM as the input and outputs a raster giving the direction of flow out of each cell. The Flow Accumulation tool was the next tool used in this process.  This tool calculates the accumulated flow as the weight of all cells flowing into each down slope cell and provides an output raster.  Next, to delineate watershed I used the Watershed tool.  The contributing area is determined with the flow direction tool and locations for catchment areas.  In this case we were provided a point file for water station stream gauges.  Next, I determined streams and their orders using the stream order tool.

After completing this lab, I decided that with a 2D map it was hard to interpret a Hydrological Model.  Therefore, I decided to create a 3D interactive map.  I used QGIS and a plugin called Q2Three.js to create an interactive 3D model of the Hydrological model.  Creating the 3D map helped me visualize the terrain and get a better idea of the flow direction and accumulation.

This was a good exercise in learning that an understanding of the output of a process that one asks a GIS to compute is very important.  ArcGIS has tools that compute many powerful processes, but just pressing buttons without the understanding of the output can lead to problematic or erroneous results.