RESOURCE NOTES

NO. 9 DATE 10/19/99

Automated

Drainage Network

and Watershed

Delineation from

Digital Elevation

Data

by Jacek Blaszczynski,

Physical Scientist, BLM

NARSC RS-120

Geographic Information Systems

(GIS) consist of sets of computerized map analysis methods for

doing what we have always done

with maps: collect data to organize

geographic information about various aspects of our environment

that we want to know about.

Then, similarly to the map analysis

and map overlay methods that

have been developed using transparent film map overlays, we relate

the geographically distributed data

via conceptual models. These models are in turn based on the accumulated knowledge arising from

observation of naturally occurring

environmental processes.

Geographic Information Systems

perform various types of routine

map analyses more efficiently than

human beings, however ultimate

judgement as to the accuracy of

model results always remains with

the scientist. In that way GIS permits us to quickly perform various

routine mapping tasks so that we

can spend more time on interpretation of results rather than on repetitious data preparation.

One powerful way to utilize GIS as

an environmental analytic tool is

for drainage pattern and watershed

boundary delineation from Digital

Elevation Models (DEMs). The

7.5 minute quadrangle DEMs are

roughly as accurate as the United

States Geological Survey (USGS)

7.5 minute quadrangle topographic sheets. Quadrangle DEMs for

most areas in the United States can

be downloaded at no cost by simply connecting to the USGS DEM

site and using FTP (file transfer

protocol) to download the data

(WEB address:

http://edcwww.cr.usgs.gov/doc/edc

home/ndchb.html). Since resource

analysis work always contains a

component related to streams and

watersheds, the standard GIS capabilities to delineate streams and

watersheds from Digital Elevation

Models (DEMs) provide us with

quick automated ways to model

surface hydrology of the of the

landscape from shape of the terrain.

The methodology was originally

developed by S. Jenson and J.

Domingue in 1987, and when

ESRI, Inc. developed the raster

portion of their ARC/INFO GIS

the methods were incorporated

into the GRID module for analyzing maps in cell data format.

DEMs are essentially lattices of

points that represent centers of

cell. For the USGS 7.5 minute

DEMs the standard are grid cells

the size of 30 m by 30 m on the

surface of the source map that is

being used to create them, usually

the traditional USGS 7.5 minute

quadrangle topo sheets. So each

cell, or each point of the lattice,

represents an elevation value calculated from topographic contours as

representing the center of the cell

or that area on the ground. Thirty

meter DEMs turn out to be quite

accurate for most areas when the

results of terrain analysis to derive

stream networks are compared to

the actual surveyed stream networks available in the Digital Line

Graph (DLG) or Spatial Data

Transfer Standard (SDTS) format,

the latter often being accurate to 2

m on the surface.

Stream network and watershed

boundary modeling requires similar steps that results in maps which

can be utilized over and over again.

These maps are created in the data

preprocessing stage of hydrologic

terrain modeling. As the first step

the users need to calculate flow

direction using the GRID

FLOWDIRECTION function,

but they have an option of applying the function on the original

DEM, which contains internal

drainages (surface depressions), or

on a filled DEM, where all the surface depressions are filled using the

GRID FILL function. To most

easily trace stream and watershed

boundary data from digital terrain

data we should fill the DEM.

Then by comparing the filled

DEM and the original DEM we

can discover the location of all the

surface depressions. We can calculate flow direction from either the

filled terrain map or the original

data.

Once this is accomplished we can

evaluate, based on flow direction,

which cells of the geographic grid

flow from or into which other

cells. In that way we can build up

a flow accumulation count map in

which the value of each cell represents the number of other cells

flowing into it. Naturally the greatest amount of flow concentrates in

the drainages. Therefore by selecting ranges of flow accumulation

values it is possible to model

drainage patterns. The lower the

initial flow accumulation value

used to model a drainage network,

the further upland can we extend

the network. It is a tradition when

calculating watershed morphometric parameters that require

drainage network information (e.g.

drainage density) to delineate the

Range

GIS and GPS
streams and channels all the way to

the watershed boundary. This

approach includes delineation of

the hollows eroded by intermittent

streams, or the generally moist

concave areas which break the continuity of the hillslope. The filled

DEM produces streams that continue throughout the landscape

without draining into a depression

giving us a better network, while

using the original DEMs will result

in flow directions pointing toward

internal drainages.

By knowing flow direction we can

also delineate watersheds, or, more

precisely, areas from any point on

the map (and, approximately, the

ground) up to the nearest divide.

For example, if our watershed outlet is placed near the outlet of a

hollow, the modeled watershed will

include the trough of the hollow

and its side slopes. Same is the case

with valleys of first order streams,

second order streams, and so forth

down the network. To obtain best

results we need to place the outlet

points (or other outlet locations

such as areas, e.g. a lake, or linear

features such as dams) so that they

are registered properly to the flow

accumulation data obtained from

the elevation grid. Point outlets or

watershed pour points need to be

located right on the drainage patterns modeled from terrain, otherwise the watershed delineated will

outline only a limited area flowing

into a point on an adjacent hillslope. The GRID module provides

capabilities for moving or snapping

selected points to the nearest high

flow accumulation value within a

user specified distance (GRID

function SNAPPOUR).

At the NARSC we have developed

several Arc Macro Language

(AMLs) programs that can make

stream network and a watershed

delineation a lot easier. If there is

enough interest in making these

AMLs into an ARCVIEW extension this is also possible. The completed programs include the following:

1. STREAMNETWORK.AML

Automatically delineates streams

and channels from terrain data.

2. REACHSHEDMODEL.AML

Automatically delineates watersheds for each reach (length of

stream between two junctions with

other streams) of a stream network.

3. INTERACTSHEDS.AML

Interacts with the user to delineate

watersheds from point or polygonal outlet locations specified

graphically by the user.

4. POLYGONSHEDS.AML

Converts a grid of modeled watersheds into a polygonal watershed

coverage eliminating diagonally

connected watershed cells since

they create additional polygons.

Other AMLs under development

include:

1. SEEDSHEDMODEL.AML

Delineates watersheds for each

reach of a stream network from

input line stream data such as the

USGS DLGs.

2. MAKEDAMS.AML

Delineates and analyzes reservoirs

for dams or other user specified obstructions delineated in the terrain.

For additional information contact: Jacek Blaszczynski, Physical

Scientist, BLM National Applied

Resource Sciences Center,

Geographic Sciences RS-120,

Denver Federal Center Bldg. 50,

P.O. Box 25045, Lakewood,

Colorado, 303-236-5236.

Range

Range

Range

Resource Notes are intended to be early

announcements of technical and informational topics for

Bureau of Land Management personnel and some of their customers.

Information in this Resource Note is based on the opinion and

experience of the author and has not been peer-reviewed. Conclusions

and opinions expressed herein do not

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