When delineating a watershed using a Digital Elevation Model (DEM), what specific characteristic of the terrain defines the boundary lines, and why is ignoring subtle changes in this characteristic problematic?

A watershed, also known as a catchment area or drainage basin, is a geographical area where all precipitation and surface water runoff ultimately drains to a common outlet point, such as a river, lake, or ocean. When delineating the boundaries of such an area using a Digital Elevation Model (DEM), which is a raster-based representation of terrain elevation where each cell contains an elevation value, the specific characteristic of the terrain that defines these boundary lines is the drainage divide. These are also commonly referred to as ridge lines or local topographic high points. A drainage divide is the line of highest elevation that separates two adjacent watersheds, effectively dictating the direction of surface water flow. Water falling on one side of this divide will flow into one watershed, while water falling on the other side will flow into an adjacent watershed. The DEM uses its elevation data to determine the path of steepest descent for water, and these ridge lines represent the paths from which water diverges in different directions, thereby forming the natural boundaries of the drainage basin. Ignoring subtle changes in this characteristic, specifically minor variations in elevation along these drainage divides, is problematic because it directly leads to inaccurate determination of flow direction and consequently, an incorrect catchment area. If a subtle dip, saddle point, or small undulation along a ridge line is not accurately captured by the DEM, perhaps due to insufficient resolution or smoothing during processing, the computed flow path of water can incorrectly cross what should be a watershed boundary. This error means that land that naturally drains into one watershed might be mistakenly assigned to another, or vice versa. For example, a small, uncaptured depression in a ridge could cause the model to route water that would otherwise flow into watershed A to instead flow into watershed B. This misallocation of upstream area directly compromises the accuracy of subsequent hydrologic calculations, such as estimating runoff volumes, peak flow rates, and pollutant loads within the watershed. Inaccurate watershed delineation can lead to significant errors in applications like flood prediction, the design of water management infrastructure such as culverts and dams, and the identification of pollution sources, as the true hydrological connectivity and contributing areas would be misrepresented.