1. Reflections on Exercise 4
There is a lot of stuff going on when we execute the Arc Hydro terrain processing and watershed delineation tools. Lets think a little bit about what this means

2. Duality between Terrain and Drainage Network
Flowing water erodes landscape and carries away sediment
Topography defines drainage direction on the landscape
Threshold drainage area to define a stream: 4.5 km2 (20,000 15m DEM cells)

3. Catchments
For every stream segment, there is a corresponding catchment
Catchments are a tessellation of the landscape through a set of physical rules

4. Raster Zones and Vector Polygons
One to one connection
DEM GridCode
Raster Zones 3 4 5
Catchment GridID
Vector Polygons

5. Area goes to Line Connectivity
For each catchment there is a unique drainage line
GridID is same on
both Catchment and
Drainage Line

6. Watershed
A watershed is the area draining to any point on the stream network
A new kind of connectivity: Area flows to a line on a point

7. HydroID – a unique identifier of all Arc Hydro features
HydroIDs of Drainage Points
HydroIDs of Catchments

8. JunctionID – HydroID Relationship
The JunctionID of a drainage area is the HydroID of its outlet point
Can build and store relationships in the geodatabase to preserve these connections