Watershed Delineation & Mapping
Using ArcGIS Pro and high-resolution DEMs to delineate watershed boundaries, map flow paths, and calculate drainage area characteristics — the foundational GIS work behind any serious stormwater or agricultural water management project.
Background
Before you can design a stormwater system, manage agricultural runoff, or size a pond, you need to know exactly how much land is draining to a given point — and how. That's what a watershed delineation tells you. It's the starting line for almost every water resource project.
This project involved delineating one or more subwatersheds in central NC using LiDAR-derived elevation data from the USGS 3D Elevation Program (3DEP), then characterizing them by land use, soil type, slope, and cover to support downstream hydrology calculations.
Approach
The workflow starts in ArcGIS Pro using the Spatial Analyst hydrology toolset. First, the raw DEM is filled to remove sinks, then flow direction and flow accumulation rasters are generated. A pour point is snapped to the highest flow accumulation cell near the outlet of interest, and the Watershed tool delineates the contributing area upstream of that point.
Once the boundary is established, land cover data from the NLCD, soil data from SSURGO, and slope data from the DEM are clipped to the watershed for characterization. This feeds directly into curve number calculations and the hydrology model.
"The quality of your watershed delineation sets the ceiling on everything downstream — garbage in, garbage out on every flow calculation that follows."
Key Details
- DEM source: USGS 3DEP 1-meter LiDAR, downloaded via The National Map.
- Total delineated watershed area: approximately 340 acres across 3 subwatersheds.
- Dominant land cover: row crop agriculture (~55%), forested riparian buffer (~25%), developed low-intensity (~20%).
- Weighted CN calculated at 74 (AMC II conditions) using NLCD + SSURGO overlay.
- Flow path length and slope extracted using Hydrology toolset for time of concentration calculations.
- All outputs exported as shapefiles and final map layouts produced at 1:4800 scale.
Map Outputs
Outcomes & Status
Watershed boundaries and flow paths are delineated. Land cover and soil characterization are complete. Curve number calculations are finalized. The maps and data are currently being used to support BMP sizing in a companion stormwater design study.
Final deliverable will be a map package and summary table of watershed characteristics formatted for use in a stormwater permit application.