How can computer mapping software (GIS) help you draw the lines for a watershed boundary very accurately?

Computer mapping software, specifically Geographic Information Systems (GIS), draws watershed boundaries very accurately by leveraging detailed elevation data to model the path of water flow across the terrain. The primary input for this process is a Digital Elevation Model (DEM), which is a raster data set where each cell, or pixel, represents a specific elevation value for a particular area on the Earth's surface. Higher resolution DEMs, meaning smaller cell sizes, capture finer terrain details and result in more accurate boundary delineation.

The process begins with a step called "filling sinks." Sinks are small depressions or pits in the DEM where the model predicts water would get trapped, preventing continuous flow. The GIS software identifies these sinks and raises their elevation values to that of the lowest surrounding cell, creating a hydrologically conditioned DEM where water can always flow downhill.

Next, the GIS calculates the "flow direction" for each cell in the filled DEM. This determines the direction water will flow from a given cell to one of its eight neighboring cells, typically choosing the path of steepest descent. The output is a raster where each cell contains a code indicating its flow direction.

Following this, the software computes "flow accumulation." For every cell, it sums the total number of upstream cells that drain into it, based on the previously calculated flow directions. Cells with high flow accumulation values represent areas where a lot of water converges, typically indicating stream channels, while cells with low values are generally found on ridges and slopes.

To define a specific watershed, a user must identify a "pour point" (also known as an outlet or watershed outlet). This is the precise location on a stream or river where all upstream water from the area of interest exits. Once the pour point is defined, the GIS software traces upstream from this point using the flow direction data. It identifies all the cells whose water would ultimately flow through the specified pour point.

The collection of all these identified cells constitutes the watershed. The "watershed boundary" is then automatically generated by GIS as the line that encompasses this entire collection of cells. This boundary precisely follows the natural topographic ridges and divides that separate the drainage area flowing to the selected pour point from adjacent drainage basins. The accuracy of the resulting boundary is directly influenced by the resolution and quality of the input DEM and the precise placement of the pour point.