ewce09.azraftaohid.me/L4T1/EWCE 464/TUTORIAL 03_WATERSHED DELINEATION/C01 - Class note.md

---

Created: 2026/06/30 08:16

Introduction

Watershed: DEM: Fill: To pass water, it much reach upto the elevation of next obstacle; fill does that

Flow direction:

• Eight direct pour point -> among the neighboring 8 cells (3x3 grid), directs to the cell with largest slope; calculates slope with farthest points.
• Coding -> 2 indicates bottom-right, 128 indicates top-right; for a cell, the system calculates largest slope that assigns the constant values to indicate direction

Flow accumulation:

• For a particular cell, from how many cells (cumulatively) water is coming from

Stream link: Assigns unique number to each link. Stream order:

Procedure

• Ensure checkbox under "Geoprocessing" > "Geoprocessing Options" > "Background Processing" is enabled

1. Fill

• Toolbox > Spatial Analyst > Hydrology > Fill
• Z limit: to keep certain structures below given elevation. E.g., to keep a lake (10m) from filling, enter 6m
• Continue with default values

2. Flow direction

• Toolbox > Spatial Analyst > Hydrology > Flow Direction
• Use filled raster as input
• Use D8 method

3. Flow accumulation

• Toolbox > Spatial Analyst > Hydrology > Flow Accumulation
• Use output data type integer

4. Build raster attribute table

• Toolbox > Data Management > Raster > Raster Properties > Build Raster Attribute Table
• Use the flow accumulation raster to get its integer values

5. Stream creation

• Check cell size: cedar_dem > Properties > Source > Cell size (30 x 30)
• Threshold area: 25 km^2 (area discharging)
• Cell number: $\frac{25\times {10}^6}{30\times 30}=27,778$

5.1 Set null

• Toolbox > Spatial Analyst > Conditional > Set Null
• Use Flow accumulation raster as input
• Use expression Value <27778
• Use false raster to 1
• Specify more sensible name to output raster (e.g., Stream)

6. Stream link

• Toolbox > Spatial Analyst > Hydrology > Stream Link
• Use Stream as input
• FlowDir_Fill1 as direction raster
• More sensible name StreamLink

7 Stream basin

• Toolbox > Spatial Analyst > Hydrology > Basin
• FlowDir_Fill1 as direction raster

8. Raster to polygon

• Toolbox > Conversion Tools > From Raster > Raster to Polygon
• Use basin as input

9. Stream order

• Toolbox > Spatial Analyst > Hydrology > Stream Order
• Use Stream as input
• FlowDir_Fill1 as direction raster

10. To feature file

• Toolbox > Spatial Analyst > Hydrology > Stream to Feature
• Use Stream as input
• Use FlowDir_Fill1 as direction raster
• Use more sensible name: StreamPolygon

11. Flow length

• Toolbox > Spatial Analyst > Hydrology > Flow Length
• Use FlowDir_Fill1 as direction raster

12. Create watershed

• Import the provided "Points" shapefile
• Use the points attribute table has unique IDs (no duplicate)

12.1 Ensure points are onto stream line

• Toolbox > Spatial Analyst > Hydrology > Snap Pour Point
• Use Points as pour point data
• Use Id as pour field
• Use FlowAcc_Flow1 as accumulation raster
• Use given snap distance (e.g., 200, meaning search within 200m)

12.2 Watershed

• Toolbox > Spatial Analyst > Hydrology > Watershed
• Use input params as directed

13 Subwatershed

• Add more points to Points
• Ensure unique IDs are assigned
• Use snap pour if required
• Create watershed