Exercise 4a: Surface flow and neighborhood operations
Due date:
Lectures: http://courses.washington.edu/geog460/lec_schedule.htm
Lab Syllabus: http://courses.washington.edu/geog460/Lab/Geog460 lab_syllabus.doc
Lab Schedule: http://courses.washington.edu/geog460/Lab/Lab%20assignment%20schedule.doc
Deliverable: Answers to all questions in All Parts. Answer sheet must be typed and all
answers must be complete. Make sure to include your name and section.
Objectives

Encounter and understand surface properties through the preprocessing sequence
for hydrologic analyses.

Adjust a DEM for entry into the preprocessing sequence.

Build a hydrologic preprocessing model.
Part 1: Introducing hydrology surface flow operations
This section is a guide to the hydrologic surface flow commands used in the rest of this
exercise. Index terms for Arc Desktop Help are listed followed by the menu/dialog
sequence from the ArcMAP interface.
This provides an overview of the hydrology functions:
Arc Desktop Help Index: Hydrology toolset >described
Arc Desktop Help Index: Fill tool/command (Spatial Analyst Toolbox)
(This refers to ArcToolbox > Spatial Analyst Tools > Hydrology > Fill)
[Question 1]: What is a sink and what importance does this have for surface functions
like hydrology?
Arc Desktop Help Index: Flow Direction tool/command (Spatial Analyst Toolbox)
(This refers to ArcToolbox > Spatial Analyst Tools > Hydrology > Flow Direction)
Question 2]: What are the possible cell values and what do they mean?
Arc Desktop Help Index: Flow accumulation > calculating
(This refers to ArcToolbox > Spatial Analyst Tools > Hydrology > Flow Accumulation)
Question 3]: The minimum cell output value from the Flow Accumulation tool is zero
and the maximum output cell value is near infinite. What do these extreme values mean?
What does an infinite value tell you about the surface on which you ran the flow
accumulation? How might the resolution of the input grid affect the cell values in the
output grid?
Arc Desktop Help Index: Watershed >delineating
Arc Desktop Help Index: Watershed tool/command (Spatial Analyst Toolbox)
Arc Desktop Help Index: Basin tool/command (Spatial Analyst Toolbox)
(This refers to ArcToolbox > Spatial Analyst Tools > Hydrology > Basin and to
ArcToolbox > Spatial Analyst Tools > Hydrology > Watershed)
Question 4]: What are “pour points”? How does the Basin tool use them differently than
the Watershed tool?
Part 2: Hydrologic preprocessing
Review the introduction to ModelBuilder from Exercise 2
Load datasets:
R:\GEOG460\data\PugetSound\USGS\WA30meterDEM\wwash30n27.bil
R:\GEOG460\data\PugetSound\WADOE\wria\wria_polygon.shp
Select the Lower Skagit-Samish wria and zoom in.
Suggested reading: http://www.ecy.wa.gov/apps/watersheds/wriapages/03.html
Clip a raster with rectangle coordinates.
Clip the 30 meter grid to a rectangle that encompasses the Lower Skagit / Samish WRIA.
ArcToolbox > Data Management Tools > Raster > Clip
The coordinates to do this are:
Top / maxY: 5395909.37
Left / minX: 509493.323
Right / maxX: 591289.424
Bottom / minY: 5343640.82
Direct output to: p:\students\YourFolder\Skj30dem
Build a hydrologic preprocessing model
Using the following diagram as a guide, build a hydrological preprocessing model using
Model Builder. The operations required (fill, flow direction and flow accumulation) are
in ArcToolbox > Spatial Analyst Tools > Hydrology. The “drop” gets created
automatically when you add flow direction. Direct the output files as named in the
diagram to p:\students\YourFolder by double clicking on the output raster layers and
renaming them.
When the model is complete name it hydroprepro and then Model > run.
Direct output to these locations:
P:\students\YourFolder\Skj30fill
P:\students\YourFolder\Skj30fdir
P:\students\YourFolder\Skj30facc
Add the output layers into your Table of Contents in ArcMAP.
Question 5]: Is flow direction an overlay, neighborhood operation, or both? Explain
your answer.
Zoom to layer for skj30fdir.
Make basins
ArcToolbox > Spatial Analyst Tools > Hydrology > Basin
Enter the Flow Direction grid and select p:\students\YourFolder\skjBasin
[Question 6]: In the skjf30dir file, what do the values of “4” and “16” represent? Does
“16” represent four times as much as “4”? What is the level of measurement for this
layer?
[Question 7]: Open the attribute table for skjBasin and look at the ‘Count’ field. What
do the values in ‘Count’ represent and what do those numbers tell you about the area of
the various basins?
[Question 8]: How do the results from the Basin command compare with the WRIA
polygons? What input grid and hydrology function (i.e. toolbox command) could
delineate a watershed identical to the Lower Skagit-Samish? What is it about this
hydrology function that enables it to accomplish this task? (Hint: look at question 4)