Modeling Watersheds

page 1

Tutorial

ModelingWatershed

Geomorphology

with

TNTmips®

WATERSHEDS

Modeling Watersheds

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Before Getting Started

You can print or read this booklet in color from MicroImages’ Web site. TheWeb site is also your source for the newest tutorial booklets on other topics.You can download an installation guide, sample data, and the latest versionof TNTmips.

http://www.microimages.com

The movement of water over land surfaces is an important environmental factorthat is governed primarily by terrain shape. The Watershed process in TNTmips®

analyzes an elevation model and defines drainage networks and the boundariesbetween different drainage areas (watersheds) along with related attributes. Theexercises in this booklet lead you through the main procedures involved in water-shed modeling with the Watershed process.

Prerequisite Skills This booklet assumes that you have completed the exercisesin the tutorial booklets Displaying Geospatial Data and TNT Product Concepts.Those exercises introduce essential skills and basic techniques that are not cov-ered again here. Please consult those booklets for any review you need.

Sample Data The exercises presented in this booklet use sample data that isdistributed with the TNT products. If you do not have access to a TNT productsDVD, you can download the data from MicroImages’ Web site. In particular, thisbooklet uses the sample file WATRSHED in the TERRAIN data collection.

More Documentation This booklet is intended only as an introduction to water-shed modeling. Details of the processes described here can be found in a varietyof Technical Guides and QuickGuides, which are all available from MicroImage’sweb site.

TNTmips® Pro and TNTmips Free TNTmips (the Map and Image ProcessingSystem) comes in three versions: the professional version of TNTmips (TNTmipsPro), the low-cost TNTmips Basic version, and the TNTmips Free version. Allversions run exactly the same code from the TNT products DVD and have nearlythe same features. If you did not purchase the professional version (which re-quires a software license key) or TNTmips Basic, then TNTmips operates inTNTmips Free mode.

The Watershed process is not available in TNTview or TNTatlas. All the exer-cises can be completed in TNTmips Free using the sample geodata provided.

Randall B. Smith, Ph.D., 21 August 2013©MicroImages, Inc., 2000-2013

Modeling Watersheds

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choose Terrain /Watershed from theTNTmips menu

Welcome to Modeling Watersheds

A 3D perspective view of arelief-shaded DEM withwatershed boundaries(orange) and flow lines(blue) produced by theWatershed process.

The Watershed process addresses the influence ofterrain on surface water hydrology by modeling themovement of water over the land surface. The inputdata for the process is a DEM (Digital ElevationModel), a regular grid of elevation values stored as araster object.

The Watershed process computes the local directionsof flow and the gradual accumulation of water mov-ing downslope across the landscape. From theseintermediate results the process then computes thestream network and the boundaries between water-sheds, the areas drained by particular stream systems.Watersheds can be further subdivided into basins as-sociated with particular branches of the streamnetwork. The flow path network, watershed bound-aries, and basins are created as separate temporaryvector objects. You can adjust several processingparameters to vary the level of detail in these objectsbefore saving the final results. Varied attribute in-formation is also created and saved with the flowpaths and watersheds. The information created bythese area-wide computations can be used as inputfor further analysis of water resource issues, floodand erosion hazard, and movement of pollutants.

An interactive tool is also provided to generate flowpaths and basins for particular point locations in theelevation model. This tool is useful for analyzingthe impacts of point-source pollutants within a wa-tershed.

A sequential processing option is available for fill-ing depressions in the elevation model prior todetermining the flow routing. This procedure maybe useful in assessing the influence of any naturaldepressions in the landscape.

Additional terrain analysis processes in TNTmips areintroduced in a companion tutorial booklet Analyz-ing Terrain and Surfaces.

An introduction tocomputing flow paths andwatersheds is provided onpages 4-9, followed by adiscussion of the databaseattributes computed forthese features on pages 10-13. Other objects computedby the watershed processare described on pages 14-16. Pages 17-18 discussmethods for filling anddraining depressions. Theuse of null cells and masksto control processing iscovered on pages 19-20.Pages 21-22 describemethods for segmentingflow path lines. Sequentialprocessing is covered onpages 23-27.

Modeling Watersheds

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STEPSpress the Openicon button in theWatershed Analysiswindowuse the Select Objectdialog to select DEM_W1from the WATERSHD

Project File in the TERRAIN

data collectioncheck that the FillDepressions togglebutton on the Generalpanel is turned onpress the Run iconbutton

Begin Watershed AnalysisStarting the Watershed process opens the Water-shed Analysis and Watershed Analysis Viewwindows. The View window automatically displaysthe DEM you select for analysis and, after process-ing, selected results of the process.

The quickest way to fully delineate watersheds andflow paths in a DEM is to use the Fill Depressionsoption, which is turned on by default. (We will ex-plore the impact of this choice on processing in laterexercises.) The Watershed process creates a seriesof temporary vector and raster objects that presentdifferent aspects of the results. To save any or all ofthese objects, press the Save Results icon buttonon the Watershed Analysis window.Press the Save Results

icon button to choosewhich of the temporarywatershed layers youwant to save to a ProjectFile.

press the LayerManager iconbutton on the Viewwindow to open theLayer Manager

Modeling Watersheds

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The type of layer names shown for the watershedobjects in the Layer Manager depend on your De-fault Layer Name setting in the Display process.The illustrations here show object descriptionsrather than object names as the layer names. Eachtemporary vector object created by the Watershedprocess is displayed with a pre-assigned color. Youcan change the color or line width by clicking theVector icon button for a layer to open its VectorLayer Controls window.

Flow Paths and WatershedsSTEPS

in the Layer Managerwindow, click on the

Show / Hide

checkbox for theSTANDARD BASINS vectorobject to uncheck it andhide this layerturn on the checkbox forthe WATERSHED POLYGONS

vector layer

After you run the Watershed process using the FillDepressions option, two vector objects depictingstandard basins (named STDBASIN) and standard flowpaths (STDFLOWPATH) are displayed over the inputDEM in the View window. Other output vector andraster objects are listed in the Layer Manager win-dow but are initially hidden. We will return to thestandard basins object later, but for the moment wewill hide it and instead show the vector object repre-senting watershed polygons (WATERSHED).

The lines in the flow paths vec-tor (shown in cyan color)represent the computed net-work of actual and potentialstream channels that drain eachwatershed. The boundaries ofthe watersheds are shown asblue polygons in the watershedvector object. The watershedboundaries follow topographicdivides between different drain-age systems.

Use the Show / Hide checkboxesto control which of the availablelayers are currently displayed.

Modeling Watersheds

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STEPSpress theWatershedAttributes icon button onthe toolbar on theWatershed Analysiswindowpress the SelectWatershed iconbutton at the top of theView windowleft-click with the mouseinside the large centralwatershed polygon

Watershed PropertiesThe Watershed Attributes window presents a list ofthe basic properties of the watersheds that were foundby the process. When all depressions are filled, eachwatershed drains to the edge of the raster object (orto the boundary between valid and null raster values,which can be used to represent a coastline). Eachwatershed in this case has neither inflow from anupstream watershed nor outflow to a downstreamwatershed, as indicated by the lack of check marksin the Outflow and Inflow columns in the window.Watersheds that drain beyond the edge of the rasterin the same direction may in fact join downstreamand form parts of larger regional watersheds.

The watershed attributes include elevation values(minimum and maximum elevations within the wa-

tershed, plus minimumboundary elevation), and thearea (in square meters) and pe-rimeter (in meters).

The area and perimeter ofeach watershed are computedusing the georeferenceinformation associated with theDEM or from the raster cellsize if the DEM is notgeoreferenced. If no cell sizehas been specified, the lineand column cell size areassumed to be 1 meter.

The selected polygon ishighlighted in color in theView and in the WatershedAttributes window.

Modeling Watersheds

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Flow Path ParametersSTEPS

close the WatershedAttributes windowclick on the Flowpathand Basin to expose itspanel

If you closely examine the watershed and flow pathvector objects, you will see that flow paths do notextend all the way upstream to the watershed bound-aries. Also, no flow paths are shown for some smallerwatersheds around the edges of the DEM. The pa-rameter values shown on the Flowpath and Basinpanel are default thresholds that were used by theprocess to compute the current flow path and basinvectors. You can modify these parameter values andrecompute new flow path and basin vector objectsthat show greater or lesser detail.

In order to generate flow paths, theWatershed process computes foreach DEM cell the number of up-stream cells that contribute flow tothat cell. These flow accumulationvalues are used in part to trace flowpaths upstream, beginning with thehighest accumulation values wherestreams reach the boundaries of thearea. The Outlet parameter sets theflow accumulation threshold for initiating a flow pathat the edge of the raster. Only boundary cells withflow accumulation values greater than the Outletthreshold are used to initiate a flow path.

The Inlet parameter determines how far upstreameach flow path is traced toward its headwaters. Aflow path terminates when the flow accumulationvalue for the next upstream cell falls below the Inletparameter value.

The Branch parameter controls the upstream split-ting of flow paths at potential junctions betweentributaries (branches). A branch flow path is createdwhen the flow accumulation value at the mouth ofthe tributary exceeds the Branch parameter value.

Increase the value of theOutlet parameter if youdon’t want to create flowpaths for smallerwatersheds around theperiphery of the DEM thatcurrently show them.

Decrease the values of theInlet and Branchparameters if you want toextend flow paths closer tothe upper watershedboundaries and increasethe number of tributary flowpaths shown.

To restore the defaultvalues for the Flow Pathand Basin parameters, clickthe Set to Defaultsicon button.

The Flowpath and Basin threshold values are computed internally with raster cellsas the area units, but you can use the Units menu to choose from a standard set ofarea units (including square kilometers, hectares, and others) to use for the valuesof these parameters

Modeling Watersheds

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The polygons in the standard basins vector object(shown in yellow in the illustration on page 4) showsubdivisions within larger watersheds. Each basin(subwatershed) is the area drained by a network ofbranches of the main stream. The lowest elevationin each basin is the junction between its stream net-work and a larger stream.

The Basin parameter sets an area threshold for gen-erating basin polygons within each watershed. Abasin polygon is created for each branch system thatdrains an area greater than or equal to the value setfor the Basin parameter. Because flow accumulationis expressed as the number of cells contributing flow,the area of a basin is equivalent to the flow accumu-lation value of the cell at the mouth of its tributarysystem.

The changes you make to the flow path and basinparameters in this exercise create a less dense, less

detailed flow path network withless numerous and shorterbranches and less numerous andlarger basins.

Recompute Flow Paths and BasinsSTEPS

use the Show / Hidecheckboxes in the LayerManager window to hidethe WATERSHED vectorlayer and show theBASINS layeron the Flowpath andBasin panel, change thevalue of the Inletparameter to 32 cellschange the value of theBranch parameter to 128change the value of theBasin parameter to 256press the Run iconbuttonif you don’t want tosave the previousprocess results, click[No] in the Questionwindow, otherwiseclick [Yes] and name theoutput objects

If you turn on the SeparateValley Polygons toggle beforerecomputing flow paths andbasins, large elongate basinsassociated with master streamsare partitioned into separatepolygons at branch intersections.The minimum size of thesepolygons is also set by the Basinparameter value.

Modeling Watersheds

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Basins and Flow Paths from Seed PointsIn some instances you may want to know the extentof the watershed upstream from a designated point.For example, the point might represent a streamsample locality where a chemical anomaly was de-tected and the upstream watershed represents theregion containing the potential source. On the otherhand, if a point source of pollution has been identi-fied, you would want to predict the downstreamflowpath along which the pollutant would be dis-persed.

The Seed Points tool allows you to place seed pointsthat can be used to compute a downstream flow path,upstream basin, or both for each point. These op-tions are controlled by the corresponding togglebuttons on the General panel in the Watershed Analy-sis window.

STEPSchoose Manual from theSeed Points menu at thebottom of the Flowpathand Basin tabbed panelpress the iconbutton for theSeed Points tool atthe top of the Viewwindowleft-click in the bottom ofthe major valley near thecenter of the area; across-in-circle cursorappearsreposition the cursor ifnecessary to place it onthe existing flow line,then right-click to addthe point and computethe flowpath and basin(answer [No] in theQuestion window)left-click on a flow pathin one of the smallervalleysright-click to add thepoint and compute anadditional flowpath andbasin for the new point

Cursor markingprospective seedpoint location

Marked seed point

Computed basinsand flow paths fortwo seed points.

when you are ready to return tocomputing flow paths and basinsfor the entire DEM, simplychoose Automatic from the SeedPoints Mode menu

Seed points arelisted on the Manualpanel of the GraphicTool Controls. Youcan delete a poorly-located point by

pressingits Deleteiconbutton.

Modeling Watersheds

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Flow Path AttributesSTEPS

turn on all of the togglebuttons in the Computepane on the Generalpanelpress the Run iconbutton, then [No] inthe Question windowin the Layer Managerwindow, press theExpand / Collapse iconbutton for theSTANDARD FLOW PATHS layerrepeat for the lineelement entry for thislayer to show its tablesturn on the Show Tablecheckbox for theSTREAM_PROPERTIES

tablein the STDFLOWPATH

table window, turn onthe View AllRecordsbuttonrepeat for theSTREAM_DISTANCES

table

The Watershed process can compute many hydro-logic and geomorphic attributes for the flow path linesand the watershed and basin polygons. ASTREAM_PROPERTIES table is produced automaticallywhenever flowpath lines are computed. This tablepresents stream line attributes related to topographyand hydrology: maximum and minimum elevations(elevations at upstream and downstream ends, respec-tively), elevation drop, average slope, line length asprojected to the XY mapping plane, and the sinuos-ity ratio (line length divided by the straight-linedistance between endpoints). All of these values arecomputed fields (as indicated by the blue field names)that read or compute their values from field valuesstored in various other tables.

A STREAM_DISTANCES table is computed whenever youhave turned on either of the Flow Distance optionson the General panel. Since each stream line has afinite length, separate distance values are computedfor each end of a line (MinimumUp and MaximumDown for the upstream end, and MaximumUp andMinimumDown for the downstream end), along withaverage values for a line’s midpoint.

All of the spatial layerscreated and displayed by theWatershed process haveDataTips enabled by default.When you pause the cursorover the View, you get aDataTip listing a cell value orsalient attribute value fromeach layer.

Modeling Watersheds

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Stream OrderSTEPS

turn off the Show Tablecheckboxes for theSTREAM_PROPERTIES andSTREAM_DISTANCES tablesturn on the Show Tablecheckbox for the tableSTREAM_ORDER

in the STREAM_ORDER tablewindow, select Switch toSingle Record view fromthe Table menupress the Selecticon button in theView window andselect a segment of thestream that drainstoward the lower rightcorner of the DEM asshown in the illustration

Scheidegger: This system defines for each segment an “associated integer” (shownin the STREAM_ORDER table) that is twice the Shreve magnitude. The Scheidegger streamorder is the logarithm to the base 2 of the associated integer.

Since stream segments in each watershed join down-stream to form larger streams, the relative importanceof each segment can be expressed as a numerical rankor order within the stream network. The Watershedprocess computes stream order for each line elementin the standard flow path vector object using fourdifferent ordering systems that are described and il-lustrated below. The resulting values are stored inthe STREAM_ORDER table.

Strahler: The smallest headwater segments are assignedorder 1. Order increases downstream by 1 whenever twostreams of equal order join. For example, two streams oforder 2 join to form a third-order stream. But the ordernumber does not increase when a higher-order stream isjoined by a lower-order stream.

Horton: This system begins with the same orderingscheme as the Strahler system, but the main streammaintains the same order number all theway upstream to a single headwatersource. The order of major tributaries istreated in the same way. At each junctionwhere two segments of equal Strahler ordermeet, the longest or most direct upstreamsegment is renumbered to the higher orderof the main stream or branch.

Shreve: The order or “magnitude” of astream segment formed at a junction is thesum of the magnitudes of the two tributaries. For example,the confluence of a magnitude 1 and magnitude 3 streamforms a magnitude 4 stream. The magnitude of any streamsegment equals the number of its magnitude 1 sources,which means that the Shreve magnitude is more simplyrelated to predicted flood flow than other ordering systems.

close the STREAM_ORDER

table

Strahler Shreve ScheideggerHorton

1

11

11

1

1

11

1

32 1

12

1

1

1

1

11 1

11 1

1

1

22

2

2

23

3

3 3

33

32

2

2

2 22

2

3

44

8

9

2

2 2 22

22

2

24

4

4

6

88

16

18

Modeling Watersheds

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Watershed and Basin AttributesSTEPS

press the Expand /Collapse icon button for the STANDARD BASINS

layerrepeat for the polygonelement entry for thislayer to show its tablesturn on the Show Tablecheckbox for theATTRIBUTES tablein the ATTRIBUTES tablewindow, select Switch toSingle Record view fromthe Table menurepeat for the tableATTRIBUTES BY ORDERS

use the select tool toselect one of the basinpolygons in the View

You can have the Watershed process compute a setof hydrologic attributes for basin and/or watershedpolygons when you turn on the Geomorphic Attributetoggle for the respective polygon type on the Gen-eral tabbed panel. The ATTRIBUTES table for eachpolygon type records attribute values for each entirepolygon, while the ATTRIBUTES BY ORDER table parti-tions the attributes for each polygon by stream order,with one record for each stream order represented ina particular polygon.

These tables record several simple attributes such asthe number of stream lines, total stream length inmeters, and average stream length in meters. Theyalso record a number of more complex attributes:

Basin Length: maximum polygon dimension measured from the stream outlet.

Form Ratio: polygon area divided by the square of the basin length.

Elongation Ratio: ratio of the diameter of a circle with area equal to the basin area to thebasin length.

Ruggedness Number: basin relief times drainage density.

Bifurcation Ratio: the mean (for all stream orders) of the number of streams of a givenorder divided by the the number of streams of the next higher order.

Length Ratio: the mean (for all stream orders) of the average stream length of a givenstream order divided by the average length of the next higher order.

Drainage Density: total stream line length divided by thepolygon area.

Constant of Channel Maintenance:polygon area divided by the total streamlength (reciprocal of drainage density).

Length of Overland Flow: reciprocal of (2* drainage density).

Stream Frequency: total number ofstreams divided by the polygon area.

Modeling Watersheds

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Hydrologic Connectivity TablesSTEPS

close the ATTRIBUTES andATTRIBUTES BY ORDERS

tablesturn on the Show Tablecheckbox for the BASINS

and FLOWPATHS tables inthe STANDARD BASINS layerin the STANDARD FLOW

PATHS layer, turn on theShow Table checkbox forthe DOWNFLOW and BASINS

tables

press the UnmarkAll icon button inthe View and close allof the open tables

Stream lines in the standard flow paths vector form aconnected 3D line network in which tributary streamlines merge in the downstream direction. Thesubcatchments (standard basins) associated withthese stream lines likewise are ordered by the direc-tion of stream flow and the merging of tributaries.

The connectivity of these hydrologic elements iscaptured by the Watershed process in a series oftables that are automatically created for the stan-dard flow paths and standard basins vectors. Eachflow path line can flow into only one downstreamline element; the ID number of that line element isrecorded in the DOWNFLOW table for each stream line.A BASINS table is also created for the standard flowpath vector if you have chosen to create the stan-dard basins vector; this table records the ID numberof the basin polygon in which each stream line lies.

Two hydrologic connectivity tables are created forthe standard basins polygons. Each basin polygoncontributes flow to only one downstream basin; theID number of that downstream polygon is providedin the BASINS table. The flowpaths table has a recordfor each flow path line showing the correspondingbasin ID. Larger basins may contain a number ofstream lines, so multiple records may be attached to

a single basinpolygon.

The hydrologic connectivitytables and their recordattachments to flowpath andbasin elements allow you touse the standard flow pathand standard basins vectorobjects to perform higher-level spatial analysis andmodeling that relate specificflowpath elements tocorresponding areas of thelandscape.

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Other Watershed ProductsSTEPS

use the Layer Managerto hide the STANDARD

BASINS and STANDARD FLOW

PATHS layers and showthe STANDARD RIDGES layerhide the STANDARD RIDGES

and DEM layers, thenshow the other rasterlayers discussed below

The Watershed process also produces one other op-tional vector object and several raster objects. Someof these products show particular aspects of the ter-rain and may be useful for specialized analysis, butothers are primarily intermediate objects used to pro-duce the objects discussed previously. Theillustrations below show the northwest (upper left)quarter of some of these objects.

Brighter tones in the Flowaccumulation raster indicate

higher flow accumulationvalues and trace out the

branching pattern of potentialflow paths. This intermediate

object is used to generate the flow path vectorobject and the watershed and basin boundaries.

Lines in the Standard ridges vector object (shown inmagenta) follow the topographic divides that separatedifferent watersheds and basins. Portions of the basinboundaries that cross uniform slopes or flat areas are notincluded as ridge line segments.

The Flat areas and extrema points raster indicates thelocation and type of locally-significant elevation values inthe original DEM. To assess its signficance, each elevationis compared to that of its 8 nearest neighbors. The majorityof cells are not significant and are shown in gray. Cells thathave isolated (single) local maximum or minimum valuesare shown in bright red and bright blue, respectively. Cellsthat are part of contiguous groups forming local maxima orminima are shown in dark red and dark blue. Flat areas arein yellow.

Two other intermediate raster objects are created in theWatershed process. Values in the Flow directions raster

(shown to the right) encode the local direction of flowrelative to the surrounding 8 cells. The direction value

increases clockwise from top right to top. This raster is usedalong with the Flow accumulation raster to produce the final

flow path and basin vector objects. The Watershed raster(not shown) contains a unique cell value for each watershed.

Modeling Watersheds

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Hydrologic Properties of the TerrainToggle buttons on the Compute pane of the Water-shed Analysis window’s General tabbed panel allowyou to choose to produce a number of additional ras-ter objects that are not required to computewatersheds and flowpaths but nevertheless recordsignificant hydrologic properties of the terrain.These four raster objects are described and illustratedbelow.

STEPSuse the Layer Managerto show the additionalraster layers discussedbelow

Specific Catchment Area depicts for each cell the upslopecontributing area per unit flow width perpendicular to theflow direction. This value is computed from the local flowaccumulation and flow direction values and the celldimensions. This property is used as a parameter inmodeling hillslope runoff and resulting soil erosion.

Compound Topographic Index(CTI), also referred to as awetness index, predicts zones

of increased soil moisture where the landscape areacontributing runoff is large and the local slopes are low,such as at the base of hillsides and in valley bottoms.The index value for each cell is computed as:CTI = natural log (Flow Accumulation / tan (slope) ).This property is used in soil landscape modeling and inanalysis of vegetation patterns.

Maximum Upstream Flow Distance maps the longest flowdistance from any part of the upstream watershed boundaryto the current cell. This property can be used in modelingerosion rates and sediment yield.

Downstream Flow Distance maps the flow distancedownstream from each cell to the outlet of its watershed.This distance has applications in the analysis ofdownstream dispersal of pollutants and sediment.

Modeling Watersheds

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Adjusted ElevationDepressions are areas in a DEM that are completelysurrounded by higher elevation values. Some natu-ral processes can form sizeable natural depressions,and human activities such as mining and quarryingcreate smaller depressions. But natural depressionsare rare in common landscapes that are shaped pri-marily by stream flow, and consequently mostdepressions in a DEM do not represent real land-scape features. They are data errors, or result fromthe averaging involved in assigning elevation valuesto cells of finite area. These spurious depressionsinterfere with the correct routing of flow paths dur-ing the watershed analysis, especially in areas of lowrelief.

The Watershed process solves this problem by firstlocating and “filling” the depressions. It increasesthe values of cells inside each depression to the el-evation of the lowest bounding cell (the pour pointor outlet), simulating the natural filling of depres-sions with water to form ponds and lakes. Whenyou run the Watershed process with the Fill Depres-sions option turned on, a depressionless version ofthe DEM (ELEVATION, with object description “Ad-justed elevation”) is created. This depressionlessDEM is used to compute the flow paths, basins, andwatersheds.

DEM_W1 includes no largenatural depressions like theone illustrated below. Itdoes have many smallspurious depressions, butyou would need to look veryclosely to see any notice-able differences between itand the depressionlessDEM.

Perspective view of a relief-shaded DEM with a largenatural depression partiallyfilled by water to form a pond(uniform gray surface incenter). The water level islower than the lowest naturaloutlet (pour point).

Perspective view of thecorresponding area of thedepressionless DEM. El-evations in the depressionhave been raised to thelevel of the lowest pourpoint, simulating completefilling with water.

Computed flow paths over-laid on the depressionlessDEM. Flow paths are routedacross the flat areas createdby filling depressions, linkinginlet streams and outletstreams.

STEPSuse the Layer Mangercontrols to hide theraster objects examinedon the previous pageand show the ADJUSTED

ELEVATION raster (whichdisplays without reliefshading)

Modeling Watersheds

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STEPSpress the Openicon button, then[No] in the Questionwindowin the Select Objectwindow, choose objectDEM_WFD and press [OK]on the General panel,turn off all of the togglebuttons except FillDepressions andFlowpathson the Depression panelturn off the Put null cellat bottom of unfilleddepressions togglein the Fill Maxima PerDepression control groupset the Area value to5000press the Run iconbuttonnote the watershed withyellow pour point in leftcenter of the areaincrease the Area valueto 10000press the Run iconbutton, then [No] inthe Question window

Filling DepressionsThe filling of depressions is controlled by area anddepth thresholds that you set on the Depressionstabbed panel. The Area and Null Area fields allowyou to set the maximum number of cells and the maxi-mum number of null cells, respectively, for adepression to be filled. The Depth field sets themaximum depth allowed for a depression to be filled.An additional depth parameter sets the threshold de-pression size above which the depth limit is applied;smaller depressions of any depth are filled.

The DEM in this exercise includes a shallow depres-sion in the upland area between the major drainages.This depression is larger than the Area value weintially set (5000 cells) and so is not filled in the firstRun. With the Put null cell at bottom of unfilleddepressions toggle turned off, the result is that noflowpaths are computed and only watershed poly-gons are displayed. The watershed polygon with theyellow point symbol on its northern boundary in-cludes the unfilled depression. You can examine theDepression attributes (see page 25) to investigatedepression size. In this case increasing the Areathreshold in the second run fills the depression andallows flowpaths to be computed.

Routing flow across theflat areas formed bylarge filled depressionscan produce artificiallystraight flow pathsacross those areas.

Watershed withunfilled depresssion

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Draining DepressionsSTEPS

press the Openicon button, then[No] in the Questionwindowin the Select Objectwindow, choose objectDEM_WDD and press [OK]on the Depression panel,turn on the Put null cellat bottom of unfilleddepressions togglein the Fill Maxima PerDepression control groupset the Area value to 500set the cell numberthreshold for the Depthsetting to 100press the Run iconbuttonturn on the Watershedpolygons vector layer

Magnified views of several closeddepressions. Arrows indicate nullcells placed in bottom of each.

The DEM in this exercise includes several naturaldepressions caused by chemical weathering of thelimestone that underlies the area and which is ex-posed at lower elevations. Within these depressions,surface water drains inward and some streams dis-appear into sinkholes that connect to cave systems.

In order to replicate the internal drainage of thesedepressions in the Watershed process, we have ad-justed the depression-filling settings to prevent thesesmall depressions from being automatically filled(reduced the maximum depression area and thethreshold for triggering the depth limit). We’ve alsotoggled on the option to put a null cell at the bottomof unfilled depressions. When flowpaths are found,each of these null cells acts as an outlet or drain tonucleate a drainage network within the depression(see the followingpage).

Modeling Watersheds

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Use Null Cells to Limit ProcessingSTEPS

press the Openicon button, then[No] in the Questionwindowuse the Select Objectdialog to select DEM_W2from the WATERSHD

Project Filepress the Runicon button

oceanareadisplayedtransparent

Standard Flow Paths forDEM_W2

The Watershed process traces watersheds, flow paths,and basins to the edge of the DEM raster or to theboundary between valid elevation values and nullcells. A null cell is a “blank” or “no data” cell in araster object. For example, a DEM that has beenresampled to a map projection may be rotated so thatvalid elevation values do not fill the full rectangularextents of the raster object, leaving “blank” areas atthe edges and corners that are set as null cells. (Nullcells may be designated by a reserved numerical valuein the raster or by a null mask.) These null areas areautomatically excluded from processing in the Wa-tershed and other TNT processes, and they aredisplayed transparently in the View window.

In a DEM of a coastal area the ocean is normallyrepresented by cells with a value of 0, which of courseis the elevation value that represents mean sea level.But since real drainage systemsterminate at the coastline, thereis no reason to include the oceanareas in the analysis. As shownby the DEM in this exercise, bydesignating the cells in theocean area as null you can auto-matically exclude them fromprocessing.

You can use the Mask Editor toedit the null mask to designatenull areas for a DEM or anyother raster object. You can ac-cess the Mask Editor from theTNTmips menu (Image / Utili-ties / Edit Mask) or from theLayer Manager in the DisplayProcess (by right-clicking onthe DEM’s layer entry and se-lecting Mask Editor from thedropdown menu).

Modeling Watersheds

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Mask Parts of the DEMSTEPS

press the Openicon button, then[No] in the Questionwindowuse the Select Objectdialog to select DEM_W3from the WATERSHD

Project Fileon the General panel,turn Off the Basinstoggleon the Flowpath andBasin panel, set theBranch value to 128on the Options tabbedpanel, press [Mask...]in the Select Objectwindow, select theMASK_W3 object from theWATERSHD Project Fileturn on the toggle next tothe Mask buttonnote the Crater Lakearea is masked out andis transparent in theViewpress the Runicon button

Another way to limit processing to particular partsof a DEM is to use the masking capability of theWatershed process. A mask is a binary raster objectthat contains a value of 1 for each cell that will beprocessed and a value of 0 for cells that will be ex-cluded. You can use a mask to exclude a particulararea that spans a range of elevation values. In theCrater Lake (Oregon, USA) example used in thisexercise, we might be interested in drainage patternsin the area surrounding the crater and want to ex-clude the lake surface and inner crater walls. Thelake occupies a crater at the summit of an extinctvolcano.

You can load a mask raster that you have preparedoutside the Watershed process by pressing the Maskbutton on the Options panel and turning on the adja-cent toggle button to apply the mask in the View andfor processing. This panel also includes a menu forsetting compression options for the raster objectscreated by the Watershed process.

The mask usedhere wasproduced usinga region objectcreated with theBasin regiongenerationmethod.

Flowpaths computed for the terrainoutside the rim of Crater Lake; the craterwalls and lake surface were excludedfrom the analysis using the mask.

DEMwithoutmask

Modeling Watersheds

page 21

STEPSon the Flowpath andBasin tabbed panel, setthe Parameter menuunder FlowpathSegmentation toElevationset the Interval menu forsegmentation to Equaland enter 500 in the fieldbelow the menupress the Runicon buttonin the Layer Manager,turn on the Show/Hidetoggle for the SEGMENTED

FLOW PATHS layer and left-click on its LayerControls icon buttonon the Linestabbed panel ofthe Vector LayerControls window,choose New - ByAttribute from the Stylemenu and press theadjacent Specify buttonin the Select Table/Fieldwindow that appears,choose RANGE from theTable list and Minimumfrom the Field list, thenpress [OK]on the Automatic tabbedpanel of the AssignStyles window, underStyle Options chooseRGB from the ColorPath menuuse the Start Color andEnd Color buttons tochoose cyan andyellow colors,respectively as shown

Segment Flow Paths by ElevationThe Watershed process provides the option to createa segmented flowpath vector object in addition tothe standard flowpath vector. A segmented flowpathvector has additional nodes inserted to divide lineelements based on values of either elevation or flowaccumulation. All attributes of the flowpath linesare recomputed for lines that have been split by thesegmentation procedure.

In this exercise we segment flowpaths based on el-evation, using an equal elevation interval of 500meters. The flowpath lines extend from an eleva-tion of 1068 meters to maximum of 2406 meters.Our segmentation results in line elements in threeelevation ranges: 1068 to 1500 m, 1500 to 2000 m,and 2000 to 2406 m. The segmentation procedureproduces a Range table with a record for each range,and each record is attached to all corresponding

flowpath lines.The steps inthis exerciseshow how youcan use therange valuesas the basis forstyling thes e g m e n t e dlines By At-tribute.

press [OK] on theAssign Styles,New Table, andLayer Controlswindows

Modeling Watersheds

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Segment by Flow AccumulationSegmenting flow paths by flow accumulation val-ues relates the segments to the upstream area thatcontributes flow to each segment. Since flow accu-mulation values increase rapidly downstream alongflow lines, it is appropriateto use the Exponential inter-val selection for this type ofsegmentation. With this op-tion the interval value youenter is used to define thesize of the first flowpath interval; in this example aninterval value of 2000 results in an initial value rangeof 0 to 1999. But each successive flow accumula-tion interval is twice as large as the previous one, asshown by the illustration below of the Range tablecreated for this exercise. The later steps in this exer-cise show how you can set up styling By Attributebased on the Range table with differing widths (ratherthan colors) set for the different flow accumulationranges.

STEPSpress the Openicon button, then[No] in the Questionwindowselect DEM_W1 from theWATERSHD Project Fileon the Flowpath andBasin tabbed panel, setthe Parameter menuunder FlowpathSegmentation to FlowAccumulationset the Interval menu forsegmentation toExponential and enter2000 in the field belowthe menupress the Runicon buttonin the Layer Managerturn on the SEGMENTED

FLOW PATHS layer andopen its Layer Controlson the Lines tabbedpanel of the VectorLayer Controlswindow, choose New- By Attribute from theStyle menu and pressthe adjacent Specifybuttonin the Select Table/Fieldwindow that appears,choose RANGE fromthe Table list andMinimum from the Fieldlist, then press [OK]on the Automatic tabbedpanel of the AssignStyles window, chooseAll Same from the StyleBy menuunder Size, set thesecond Width value to1.50press [OK] on the AssignStyles, New Table, andLayer Controls windows

Range tablecreated in thisexercise.

Segmented flow pathsstyled with differingline widths based onflow accumulationrange.

Modeling Watersheds

page 23

Process Depressions SequentiallyWhen you run the Watershed process with the FillDepressions option turned off, the process takes asequential approach to filling depressions and find-ing watersheds. In the initial run the process findsdepressions in the original DEM and creates water-shed polygons. If you press the Run button again,the process attempts to fill all remaining depressions.In many cases, however, small depressions may benested inside larger ones. As these small depres-sions are filled they merge into a residual largerdepression. These complex relationships may notbe resolved in a single processing pass, so it maytake several runs to fill all depressions. Only whenall depressions are filled does the process computeand display flow paths and basins.

If all depressions are not filled at the end of a run, aset of watershed polygons are displayed automati-cally in the View window. Some of these watershedsdrain to the edges of the raster (or to a null bound-ary), while others drain into a remaining depressionin the adjusted DEM. Yellow point symbols in thewatershed vector object indicate the lo-cations of pour points along theboundaries of watersheds containing adepression. A pour point is the pointthrough which water would spill overinto the downstream watershed if thedepression were completely filled.

By running the Watershed process se-quentially, you can investigate thecharacteristics of watersheds and de-pressions at each stage and identifydepressions that are natural features ofthe landscape rather than DEM arti-facts.

STEPSpress the Openicon button, then[No] in the Questionwindowselect DEM_W4 from theWATERSHD Project Fileturn off the FillDepressions togglebutton on the Generalpanel

The DEM used in this seriesof exercises has a cell sizeof 30 meters and shows amountainous terrain that hasbeen modified greatly byglacial erosion. Naturaldepressions and lakes arecommon in glacially-erodedvalleys such as these.

pour points

press the Runicon button

Modeling Watersheds

page 24

STEPSpress the SelectWatershed iconbutton at the top of theView windowleft-click with the mouseinside the largewatershed polygon at thecenter of the DEMpress theWatershedAttributes iconbutton on the WatershedAnalysis window

Watershed RelationshipsWhen depressions remain in the adjusted elevationraster, a single large watershed may be divided intoa number of individual watershed polygons. A par-ticular watershed polygon may have one or moreupstream watersheds that would drain into it if theirdepressions were filled. The same watershed mayalso drain into a downstream watershed (or less com-monly more than one) if its depression is filled. Theserelationships are displayed graphically when you usethe Select Watershed tool to select a watershed poly-gon. The active watershed and its upper and lowerneighbors are shown in different highlight colors.

One neighboring upper watershed(if any) is automatically designatedthe selected upper watershed andshown in a different highlight colorthan the remaining upper water-sheds; the same system is used formultiple lower watersheds.

You can use the icon buttons on theWatershed Attributes window (illus-trated below) to use these hydrologicrelationships to change the activewatershed selection.

Next Upper Pour

Previous Upper Pour

Move ThroughUpper Pour

Move ThroughLower Pour

Previous Lower Pour

Next Lower Pour

selected lowerwatershed andpour point(bright green)

upper watershed andpour point (purple)

selected upperwatershed and pourpoint (magenta)

activewatershed

Modeling Watersheds

page 25

Depression and Pour Point AttributesSTEPS

press theDepressionAttributes icon button onthe Watershed Analysiswindowscroll down in theDepression Attributeswindow to showwatershed 19

The Depression Attributes window lists the ID num-bers of all of the watersheds containing depressions,along with each depression’s area and volume. If awatershed containing a depression is selected in theView, its record is highlighted in the list in red. Forexample, the watershed selected in the previous ex-ercise is number 19, and its included depressioncovers 10,800 square meters (a fraction of the wa-tershed area). The input DEM has a cell size of 30meters, so the area of a single cell is 900 squaremeters. Dividing the depression area by the cell areashows that this depression covers 12 raster cells.

Identification numbers of the upper and lower pourpoints for the active watershed are highlighted in thePour Attributes window in the appropriate colors.The columns Left and Right list the numbers of thewatersheds that lie on the respective sides of the vec-tor line separating them (relative to the arbitrarydirection of that line in the vector topology). Thearrows indicate the directionof potential flow through thepour point. In this example,pour point 10 is the selectedlower pour point for watershed19, which drains to watershed18. The upper pour for water-shed 19 is number 13, whichdrains into watershed 19 fromwatershed 23.

press the PourAttributes iconbuttonscroll down in the PourAttributes window toshow the two pour pointswhose entries arehighlighted in color

Modeling Watersheds

page 26

When you have selected a watershed polygon withthe Select Watershed tool, you can run the water-shed process again to fill only the depression in thatwatershed or those in watersheds hydrologically re-lated to it as well. These options are controlled bythe Manual Filling toggle buttons on the Depressionpanel.

STEPSclose the Pour Attributes,Depression Attributes,and WatershedAttributes windows bypressing their respectiveClose buttonsin the Manual Fillingsection of theDepression panel in theWatershed Analysiswindow, turn on theLower toggle buttonpress the Runicon button, then[No] in the Questionwindow

Fill Depressions Selectively

With the Upper manual filling option on, the pro-cess attempts to fill depressions in the selectedwatershed polygon and in all watersheds upstreamfrom it. The Lower manual filling option fills de-pressions in the selected watershed polygon and inall watersheds downstream from it. If neither of thesetoggle buttons is turned on, then only the depression

in the selected polygon is filled.(If no watershed polygons areselected, all watershed polygonsare processed). In any of thesecases, depression-filling stops ifthe process encounters a doubledepression (see next page).

New watershed polygoncreated by filling depressionsin the selected watershedpolygon and in thosedownstream from it (until adouble depression wasencountered). Otherwatersheds were unaffected.

Modeling Watersheds

page 27

Double Depressions and Pour PointsSTEPS

select the watershedpolygon with ID 81 nearthe center bottom of theDEM, as shown in theillustration below

press the PourAttributes iconbuttonin the Pour Attributeswindow, scroll down toshow pour point 53on the Depressionstabbed panel of theWatershed Analysiswindow, turn off theLower manual fillingtoggle and turn on theDouble toggle buttonpress the Run iconbutton, then [No] inthe Question window

If two adjacent depressions share a pour point, andneither has another pour point that is lower in eleva-tion than the shared one, then the two depressionsform a double depression. The pour point that theyshare is termed a double pour point. If either of thesedepressions is filled, it will spill over through thedouble pour into the other member of the doubledepression. Only when both members are filled willflow continue to downstream watersheds.

The watershed and depression selected in this exer-cise form part of a double depression. Pour point53 is the double pour point linking the two mem-bers. Double pourpoints are indi-cated in the PourAttributes windowby a double-headedarrow in the Flowcolumn.

Final flow pathsfor the DEM

Filling merges the doubledepressions. For thisexample the mergedwatershed drains northeastthrough a pour point to themajor valley.

You can choose to fill only double depressions byturning on the Double manual filling toggle buttonand then running the process. This options fills alldouble depression pairs within the DEM withoutregard to any set of watershed polygons you mayhave selected (unlike the other manual filling toggle

buttons).

turn off the Doublemanual filling togglepress the Run iconbutton, then [No] inthe Question window;this run should produceflow paths

Modeling Watersheds

page 28

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Indexadjusted elevation.................................16basin

from seed point..................................9parameter. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8polygons..............................5,8,9,12,13

branch parameter........................................7depression....................................4,16,23-27

attributes............................................25double................................................27draining. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .18filling...................................4,16,23-27sequential filling....................23-27

flow accumulation.........................7,8,14,22flow directions..........................................14flow path....................................5,7,8,19

from seed point...................................9

hydrologic connectivity...................13inlet parameter...........................................7mask raster.......................................20null cells......................................18,19outlet parameter.........................................7pour point............................................23-27

attributes.......................................25,27double.........................................27

ridges.........................................................14seed points, watershed...............................9segment flow paths

by elevation...........................21by flow accumulation.................22

stream order...............................................11watershed polygons..........5,6,12,17-18,23-27

attributes.......................................6,12selecting........................................6,24

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MicroImages, Inc.