Delineation of Watersheds

There are two ways to delineate a watershed:

1. Use an existing dataset for delineated basins (Hydro1k)

-use only for large watersheds: this dataset includes the drainage delineations for very small basins, but it is most accurate for the large-scale basins.

2. Use the ArcInfo functions to delineate a basin from a high-resolution DEM

-functions include: flowdirection, sink, snappour, watershed

-use this method for small watersheds

-high-resolution DEMs can be obtained from gtopo30 or hydro1k

-for more information, go to ArcInfo Help

-type help on command line

-Contents>Spatial modeling>Cell-based Modeling with GRID>Surface hydrologic analysis

In this example, the first method will be explained in detail. This example introduces several ArcInfo concepts. These are listed at the end of the tutorial.

To delineate a basin using Hydro1k (e.g. the Columbia river basin):

Obtain the Hydro1k dataset:

click North America

click basin boundaries

download basins dataset

Prepare Dataset for Importing into ArcInfo:

untar:

>tar -xvf na_bas.tar

unzip:

>gzip -d na_bas.e00.gz

Start ArcInfo:

log into plane2

>ssh plane2

setup:

>/usr/local/bin/setup owclient arcinfo81

run arcinfo:

>arc

Import the image into arcinfo:

Arc:import auto na_bas.e00 na_bas

You now have a coverage called na_bas that polygons for multi-leveled drainage basins.

Project the Coverage:

Describe this coverage:

Arc: describe na_bas

You can see that the projection of this basin is Lambert-Azimuthal. Eventually you will want to change the projection of this basin (to geographic in this example). This can be done at this time or after you have picked out your basin. It is easier to determine which basin you are interested in if you change the projection now, but it takes more time to do it this way.

To change the projection, it is easiest to use "arctools":

Arc: arctools

select "Command Tools"

Edit>Coordinates>Project Coverage

Input Cover: na_bas (or right click in space and select)

Output Cover: na_bas_geo (name it whatever you want)

PRJ source: click Define..., Hemisphere, Geographic, enter OK

Close out all the windows

View this coverage:

You can view the coverage either in the module "ArcEdit" (type ae) or using arctools:

Arc: arctools

Select "Edit Tools"

File>Coverage,Open

in right column, select na_bas_geo

in bottom left column, select ARC, hit OK

Clean the coverage:

Everytime you perform an operation on a coverage, it needs to be "cleaned" in order to re-establish the polygons.

in Edit Tools Menu>Arctools>Commands...

ARCEDIT:clean

ARCEDIT:save

quit out of arctools and open again, this time plotting the polygons:

File>Coverage,Open

in right column, select na_bas_geo

in bottom left column, select POLYGON, hit OK

Highlight Basin of Interest:

(NOTE: The basins in hydro1k are described with up to six digits (or levels) called the Pfafstetter system:

see http://edcdaac.usgs.gov/gtopo30/hydro/P311.html

All basins ending in the numbers 2,4,6,and 8 are the four largest tributaries within a basin at that level.

All basins ending in the numbers 1,3,5,7, and 9 are the interbasins between the tributaries.

All basins ending in 0 are closed basins (no outflow).

LEVEL 1 is the the largest scale, and LEVEL 6 is the finest.)

In the Edit Tools menu, go to Edit>Attribute Selection...

A window title Logical Expresssion pops up:

Our objective is to highlight the Columbia River Basin in the Current expression line, enter the following and then hit Apply expression:

:LEVEL1 = 2 (This highlights the Mackenzie River Basin)

:LEVEL1 = 4 (This highlights the Nelson River Basin)

:LEVEL1 = 6 (This highlights the St. Lawrence River Basin)

:LEVEL1 = 8 (This highlights the Mississippi River Basin)

So, the columbia river is not one of the four largest basins in this dataset.

Now, let's try the "interbasins":

:LEVEL1 = 1 (This highlights a large area of Northwestern North America - including the Columbia)

:LEVEL2 = 12 (This highlights the Columbia River Basin)

:LEVEL2 = 14 (This highlights the Fraser River Basin)

:LEVEL2 = 16 (This highlights the Kuskokwim River Basin)

:LEVEL2 = 18 (This highlights the Yukon River Basin)

Continuing in this manner, smaller basins can be highlighted.

Highlight the Columbia basin again:

:LEVEL2 = 12

Select Basin of Interest:

Once you have the basin highlighted, merge all polygons in basin:

on Edit Polygon menu: MERGE

On Feature Slection menu: hit the Switch Selected Set button (two arrows)

on Edit Polygon menu: DEL

now clean the basin as before and save as something different (e.g. colum_dem)

quit out of arctools. You now have a delineation for your basin.

Use this delineation to clip out a DEM for your basin:

for example, if you have a DEM for North America (from gtopo30 or hydro1k) called na_dem, then use the following commands:

enter the grid module: grid

clip the dem using the basin delineation: gridclip na_dem colum_dem cover colum_del

view the clipped dem:

bring up the graphics window: display 9999 or &stat 9999

set the map extent: mape colum_dem

set the color scheme: shadeset rainbow

plot the dem: grids colum_dem # linear

you can also plot the coverage: arcs colum_del

quit grid: quit

export your basin dem to an ascii file: gridascii colum_dem colum_dem.asc

You should now be familiar with:

-how to start ArcInfo

-how to start "help" in ArcInfo

-importing files into ArcInfo

-using arctools to change the projection of a coverage (works the same for a grid)

-using the ArcEdit module through arctools to

-view a coverage

-clean a coverage

-using the attribute selection menu

-using the edit polygon menu: merge, delete

-using the feature selection menu: reverse selection

-how to enter the grid module

-plotting grids and coverages in grid

-using the gridclip command to clip a grid using a coverage

-using the gridascii command (note that asciigrid works in the reverse direction)

-the Pfafstetter code system of Hydro1k