• DEM creation results in artificial pits in the landscape
• A pit is a set of one or more cells which has no downstream cells around it
• Unless these pits are filled they become sinks and isolate portions of the watershed
• Pit filling is first thing done with a DEM
+ =
Take a mapped stream network and a DEM Make a grid of the streams Raise the off-stream DEM cells by an arbitrary elevation increment Produces "burned in" DEM streams = mapped streams
“Burning In” the Streams - Improve accuracy based on photograph
The Flow Direction Grid
32
16
8
64
4
128
1
2
Eight Direction Pour Point Model
67 56 49
53 44 37
58 55 22
1
67 56 49
53 44 37
58 55 22
1
26.162
4467=
−14
1
5367=
−Slope:
Direction of Steepest Descent
2 2 4 4 8
1 2 16
1 2 4 8 4
128 1 2 4 8
2 1 4 4 4
11
Flow Direction Grid
Flow Direction Grid- Red is NE
32
16
8
64
4
128
1
2
Grid Network
The Flow Accumulation Grid
0 0 000
0
0
0
0
0
0
0
0
03 2 2
11 1
1 152 5
24 1
Flow Accumulation Grid
Grid Network Count the # of contributing cells
0 0 00 0
0
0
0
0
0
0
0
0
0
3 2 2
11 1
1
12
15
245
Flow Accumulation Grid
Defining the Stream Grid
0 0 00 0
0
0
0
0
0
0
0
0
0
3 2 2
11 1
1
12
15
245
Flow Accumulation > 5 Cell Threshold
Stream Network for 5 cell Threshold Drainage Area
0 0 000
0
0
0
0
0
0
0
0
03 2 2
111
115
2 5
241
Streams with 200 cell Threshold(>18 hectares or 13.5 acres drainage area)
9895101504 (>1000)
1000 Cell Threshold Exceeded at Stream Junction
Defining Stream Segments
0 0 000
0
0
0
0
0
0
0
0
03 2 2
111
115
2 5
241
Stream Segments
55
11
1
3
22
334 4 4
4 45
5
666
Stream Segments in a Cell Network
Creating Subwatersheds
Watershed Outlet
Watershed Draining to This Outlet
Watershed and Drainage Paths Delineated from 30m DEM
Automated method is more consistent than hand delineation
Same Cell Value
Subwatersheds for Stream Segments
Vectorized Streams Linked Using Grid Code to Cell Equivalents