Runoff Methodology

The process of developing a surface runoff and accumulation model consists of multiple steps. A clear and organized workflow model provides the most efficient means of communicating the process (model 1). Below the model is an explanation of each step. All tools used throughout this project may be found in the spatial analyst toolbox under hydrology and surface.



 

1) The model begins with adding a digital elevation model (DEM) to the project. My elevation model contained microtopographic relief ranging from 401 meters to 406 meters of the Thiaumont Platform, Verdun, France (figure 1). 

Figure 1: Digital Elevation Model of Thiaumont Platform. The DEM provides all of the necessary elevation data for the entire process.

 

2) The flow direction tool is used with the DEM entity to determine the direction the water will travel across the surface (figure 2). This tool is essentially calculating the aspect, the maximum angle of downslope direction (ESRI).


Figure 2: Flow direction shows the downslope angle with the steepest gradient

3a) Once the flow direction has been determined a tool may be used to locate any sinks within the watershed and fill them. Depending on the application of the research, this tool may or may not be used. For example, when observing the watershed of a large area, you may want to exclude depressions that collect surface flow. For my application, each sink (crater) was the target of my study so this step was excluded. 

3b) The flow accumulation tool is used to calculate areas with the highest rate of movement depending upon the flow direction layer (figure 3). This shows where surface runoff is most likely to occur and where it will accumulate.

Figure 3: Flow accumulation uses the flow direction to determine where water is most likely to flow across the surface.

4) After flow accumulation has been determined, the point pour tool is used to delineate the location of drainage basins (ESRI). The locations calculated in step 3b are used to determine where each microwatershed begins and ends (figure 4).


Figure 4: Pour Point uses the flow accumulation previously calculated to show drainage basins. A hillshade was applied to beter visualize the relationship between elevation and water accumulation.