catchments manual - canadian beacons project

Creating Catchments for the Boreal Forest of Canada: A Semi‐

automated Procedure using ArcGIS and Arc Hydro Tools

P. Vernier and K. Lisgo

April 13, 2011

Prepared by

The Canadian BEACONs Project

Table of Contents 1. Introduction .............................................................................................................................................. 3

1.1 ‐ Data Sources ..................................................................................................................................... 3

1.2 ‐ GIS‐based Tools ................................................................................................................................. 4

1.3 ‐ Scope and Limitations ....................................................................................................................... 5

2. Acquiring and Projecting Datasets ............................................................................................................ 5

2.1 ‐ Download Data ................................................................................................................................. 5

2.2 ‐ Project Data and Resample DEM ...................................................................................................... 6

3. Step‐by‐Step Procedure ............................................................................................................................ 6

3.1 – Set Target Locations ......................................................................................................................... 8

3.2 – Prepare Datasets .............................................................................................................................. 8

3.3 – Terrain Preprocessing ...................................................................................................................... 9

3.4 – Adjust flow direction in lakes ......................................................................................................... 12

3.5 – Create catchments for Canada ....................................................................................................... 12

3.6 – Extract Boreal catchments ............................................................................................................. 13

3.7 – Add catchment attributes .............................................................................................................. 14

3.8 – Calculate land and water area ....................................................................................................... 15

3.9 – Calculate Intactness ....................................................................................................................... 16

Optional – Extract ocean drainage catchments ...................................................................................... 17

4. Notes and Issues ..................................................................................................................................... 18

4.1 Identifying sinks ................................................................................................................................ 18

4.2 Correcting Problem catchments ....................................................................................................... 18

4.3 Eliminate small catchments tools ..................................................................................................... 20

Eliminate Small Catchments (version 1) ............................................................................................. 20

Eliminate Small Catchments (version 2) ............................................................................................. 21

5. References .............................................................................................................................................. 22

6. Appendix 1 – Troubleshooting ................................................................................................................ 22

6.1 DEM not recognized or does not exist .............................................................................................. 22

1. Introduction

Catchments are the primary spatial units on which the Benchmark Builder operates. Previously, the

Builder used a national catchments map1 that was available from the Geogratis web site. However,

these catchments were not ecologically‐defined drainage areas, and the map had several problems,

including the presence of numerous tiny sliver polygons that caused problems when running the

Benchmark Builder. Consequently, we explored the use of three alternative approaches to create a new

set of catchments for the Boreal region. The first and simplest approach uses a digital elevation model

(DEM) to generate both the stream network and the catchments. The second approach uses an existing

Canada‐wide drainage network along with a DEM to generate catchments. The third approach uses the

same datasets as the second approach but also requires that pour points first be identified2. After

testing the different approaches and to maintain compatibility with the current version of the

Benchmark Builder, we decided to use the second method. More recently, we refined the approach to

deal with clusters of anomalous catchments that were created in certain areas with flat topography and

many lakes. This required the use of an additional dataset describing the distribution of lakes in Canada.

In this report, we describe the data sources, GIS tools, and step‐by‐step procedures that we used to

creating catchments for the Boreal region of Canada.

1.1 Data Sources

Several datasets are required to create a national‐scale catchments coverage using the approach that

we describe, and all of them are freely available over the Internet.

Hydrology data. Currently, the most important data are the Atlas of Canada National Scale Frameworks

Hydrology datasets3 (1:1,000,000 scale), whose attributes are necessary for running the Benchmark

Builder. The hydrology datasets, which are based on 1:250,000 topographic maps, consist of several

themes, three of which are required:

1. The Drainage Network Skeleton data are comprised of linear features: single line rivers and flow

lines within waterbodies and coastlines.

2. The Waterbodies data are comprised of area features: lakes, intermittent waterbodies, islands,

and rivers wide enough to be represented as an area feature.

3. The Drainage Areas data are largely based on the Water Survey of Canada (WSC) drainage areas

boundaries at the sub‐sub‐basin level.

1 Catchments were created by constructing a polygon around each arc (line segment). Technically, the catchment polygons were constructed by creating a Voronoi diagram of the drainage network, producing a set of arcs whose vertices are equidistant from the nearest two drainage network arcs. Because there are often multiple line segments between nodes (junctions between stream orders), this causes the creation of many sliver polygons. 2 Pour points are the locations above which catchments will be determined i.e., for each catchment, they are the lowest elevation point where all water will drain to. 3 http://geogratis.cgdi.gc.ca/geogratis/en/option/select.do?id=27749

An alternative data set for future consideration is the National Hydro Network4 (NHN) which is based on

1:50,000 topographic maps. This data set is more detailed and also contains flow direction, but

modifications to the Builder code would be required.

Digital elevation model. A digital elevation model (DEM) is also required to build catchments, and at

least four different Canada‐wide DEM coverages are available: GTOPO30 data, Canadian Digital

Elevation Data (CDED), Shuttle Radar Topography Mission (SRTM), and Canada3D. GTOPO30 is a global‐

scale DEM with a resolution of approximately 1 km2 and was considered to be too coarse for our

purposes. CDED data is available at 1:50,000 and 1:250,000 scales, although there are several missing

areas (tiles) in the former dataset that limit its usefulness. The SRTM dataset has a 90‐m resolution but

is not available above the 60th parallel, which coincides with the northern boundary of several provinces

(e.g., BC and Alberta). We chose to use the Canada3D DEM5 because it was derived from the CDED

1:250,000 DEM and was available as a seamless coverage of Canada.

Intact Forest Landscapes map. The Global Forest Watch Canada (GFWC) Intact Forest Landscapes (IFL)

map (1:1,000,000 scale) depicts Canada's remaining forest landscape fragments within all eleven forest

ecozones using specified size thresholds of 5,000 ha for Boreal regions and 1,000 ha for temperate

forest regions (Lee et al. 2010).

Fire regions map. The fire regions map stratifies Canada’s forests into 5 or 10 fire regions based on

similarities in fire regime parameters such as frequency, size, and seasonality over a spatial grid of

10,000 km2 hexagons. The dataset was created by BEACONs and is used to inform benchmark size.

Boundary maps. We also used four boundary maps to define the area over which catchments were

generated or extracted: (i) a Canada boundary map that matches the extent of the DEM and drainage

network maps, (ii) a Boreal region map for extracting catchments that comprise the Boreal region, and

(iii) an ocean drainages map for extracting boreal catchments based on ocean drainage affiliation.

1.2 GISbased Tools

The catchment creation procedure was developed using ArcGIS 10 (with an ArcInfo license), the Spatial

Analyst extension, and two additional toolboxes that need to be downloaded and installed:

1. Arc Hydro Tools – An extensive set of tools for facilitating hydrologic and other water resource

analyses. The Terrain Preprocessing functions are used in two of the steps described in the step‐

by‐step procedures section.

HTTP download: http://resources.arcgis.com/content/hydro‐data‐model

FTP download: ftp://RiverHydraulics:river%[email protected] (go to the ArcHydro

directory and select the version of ArcGIS you are using)

4 http://www.geobase.ca/geobase/en/data/nhn/index.html 5 http://ftp2.cits.rncan.gc.ca/pub/canada3D/

2. Catchments Tools – A set of Python/ArcGIS tools created by BEACONs for automating the data

management and analysis steps described in the Step‐by‐Step Procedures section.

User’s Guide: http://www.beaconsproject.ca/catch

The Catchment Tools are available on request ([email protected])

1.3 Scope and Limitations

The catchments we generated, using the procedures described below, were developed specifically for

large‐scale (Boreal or Canada‐wide scale) conservation planning and analysis. Moreover, the coverage is

considered a work‐in‐progress that has already been revised a few times. It is not meant to be used for

identifying precise boundaries of watersheds or protected areas. In addition:

The catchments are generated using a semi‐automated procedure applied to all of Canada. No

specific considerations are given to adapting the procedures to reflect regional variation in

topography e.g., mountainous regions vs. flat and wet regions.

The catchments are not meant to replace “official” watershed boundaries such as those

developed by the Watershed Atlas of BC using 1:20,000 TRIM and topographic maps.

2. Acquiring and Projecting Datasets

Prior to following the catchment creation procedure (§ 3 Step‐by‐Step Procedure), it is necessary to

download and project all of the required datasets.

2.1 Download Data

The first step is to acquire and extract the required data. Although the tool is flexible enough to be used

with other datasets6 and study areas, for our purposes we used the following data:

Canada3D DEM – http://ftp2.cits.rncan.gc.ca/pub/canada3D/

Canada Drainage Network, Atlas of Canada 1,000,000 National Frameworks Data (includes

streams, lakes, and sub‐sub watersheds) –

ftp://ftp.geogratis.gc.ca/frameworkdata/hydrology/analytical/drainage_network/canada/

Intact Forest Landscapes, Global Forest Watch Canada (Lee et al. 2010) –

http://www.globalforestwatch.ca/datawarehouse/datawarehouse.htm

Fire Regions –BEACONs’ dataset, not yet publically available

Canada boundary7, Atlas of Canada 1,000,000 National Frameworks Data –

http://geogratis.cgdi.gc.ca/geogratis/en/download/framework.html

Boreal boundary – request from BEACONs ([email protected]) or download from

databasin.org

6 For example, a different measure of intactness could be used. 7 We removed the Great Lakes from the Canada boundary using the Drainage Network lake coverage.

Water Survey of Canada, Sub Sub Drainage Areas ‐

Ocean drainage boundaries, Atlas of Canada 1,000,000 National Frameworks Data –

ftp://ftp.geogratis.gc.ca/frameworkdata/drainage_areas/atlas_rollup/

For more information on the hydrology datasets: NRCAN. 2009. Atlas of Canada 1,000,000 National

Frameworks Data, Hydrology Version 6.0 ‐ A practical guide to the datasets

2.2 Project Data and Resample DEM

The second step is to project the data to a common system, in our case the Albers Equal Area projection

modified for the Boreal region. We also resampled the Canada3D DEM to a 250m resolution for analysis

at ocean drainage, Boreal, and national scales.

Project drainage network, lakes, and boundary feature classes to the Beacons Albers Equal Area

projection.

Project and resample Canada3D DEM to a 250m resolution.

The Albers Equal Area projection used by BEACONs has the following parameters:

Projection: Albers

False_Easting: 0.000000

False_Northing: 0.000000

Central_Meridian: ‐91.867000

Standard_Parallel_1: 49.000000

Standard_Parallel_2: 77.000000

Latitude_Of_Origin: 63.400000

Linear Unit: Meter (1.000000)

Geographic Coordinate System: GCS_North_American_1983

Angular Unit: Degree (0.017453292519943299)

Prime Meridian: Greenwich (0.000000000000000000)

Datum: D_North_American_1983

Spheroid: GRS_1980

Semimajor Axis: 6378137.000000000000000000

Semiminor Axis: 6356752.314140356100000000

Inverse Flattening: 298.257222101000020000

3. Step‐by‐Step Procedure

Several GIS processing steps are required to generate catchments for use by the Benchmark Builder

(Figure 1). In addition, prior to proceeding with the terrain preprocessing steps, it is necessary to set

target locations for raster and vector data and to copy several files into the Raster Data working

directory. In this section, we describe each step along with the required tools.

Figure1. Overview of the catchment creation process.

DEM grid

StreamsHydroDEM (2)

grid

HydroDEM (1) grid

Hydro DEM (3) grid

Flow direction grid

DEM reconditioning

Fill sinks

Fill sinks (2)

Flow direction grid

Catchment grid

Flow direction

Lakes

Streams

Adjust flow direction in lakes

Canada catchments

Watershed

Catchment polygon processing

Boreal catchments

Boreal catchments

Boreal catchments

Add attributes

Calculate land and water area

Extract boreal catchments

Boreal catchments

Ocean drainage catchments

Boreal catchments

Calculate intactness

Extract ocean drainages

Eliminate small/island catchments

Additional datasets

Lakes

Forest Landscape Fragments

Catchment Creation Process

3.1 – Set Target Locations

Description: The first step involves setting target locations for raster and vector data and ensuring that

those directories exist or are created. This step is very important and must be done prior to using the

Arc Hydro tools. As an example, we used the following locations to create the Boreal catchments:

Map Name: Layers

Raster Data: D:\gis_data\beacons\catchments\

Vector Data: D:\gis_data\beacons\catchments\catchments.gdb

Note: The Map Name (e.g., Layers ‐> D:\gis_data\beacons\catchments\Layers) refers to a workspace

(directory) where all of the data processing takes place. It is important, therefore, to make sure that

new files (e.g., the catchments shapefile) are created in this directory. After all the steps are

completed, the catchments shapefile and any other files can then be safely moved to a permanent

directory.

ArcGIS Procedure:

Right click on the menu bar in ArcMap and select Arc Hydro Tools 9.

Go to the ApUtilities menu and select the Target Locations menu item.

Select HydroConfig and specify the locations where data will be created.

3.2 – Prepare Datasets

Description: Several raster and vector datasets need to be copied to the workspace and manipulated

prior to creating the catchments. The data and procedures include:

Copy and rasterize the Canada boundary shapefile.

Copy the Boreal region boundary shapefile.

Copy the Canada3D raster DEM and overlay it with the Canada raster.

Copy and clip the Atlas of Canada skeleton shapefile with the Canada boundary shapefile;

rasterize the resulting stream network shapefile.

Copy and clip the Atlas of Canada lakes shapefile with the Canada boundary shapefile.

Copy and dissolve (internal boundaries) of the GFWC study area boundary shapefile.

Copy the GFWC intact forest landscapes shapefile.

Catchment Tools – Prepare Datasets: The Prepare Datasets tool copies and manipulates all of the data

that will be required to create catchments for the Boreal region. The user only needs to identify the

location of the workspace, the datasets, and the analysis cellsize.

Run the Prepare Catchments tool located in the Catchments toolset of the Beacons Toolbox.

Verify that all of the parameters are filled properly and click OK.

3.3 – Terrain Preprocessing

Description: The objective of the terrain preprocessing step is to modify the DEM by removing small

errors, imposing existing stream networks, and adjusting flow direction in lakes. Together these steps

ensure the creation of a hydrologically correct DEM that is compatible with existing streams and lakes

coverages. The procedure consists of four broad steps:

1. Fill Sinks. DEMs often have small errors that are known as sinks – areas that are surrounded by

higher elevation values. These areas of internal drainage can cause problems when calculating a

flow direction map and it is often recommended that they be removed. Thus, in the first step,

we fill the sinks in the DEM to remove small imperfections in the data.

2. DEM Reconditioning. DEMs can be modified by imposing linear features onto them i.e., stream

burning. Essentially, streams are pushed into the DEM by a certain depth ensuring better

hydrological flow and connectivity, and more consistency between the two datasets. In the

second step, we use the DEM Reconditioning function to modify the filled DEM by imposing

linear features onto them – in this case the Atlas of Canada drainage network.

3. Fill Sinks (2). New sinks may have been created through the DEM reconditioning process. As a

result, it is necessary to fill new sinks in the reconditioned DEM.

4. Flow Direction. A flow direction map is one of two required inputs for creating catchments. In

the fourth step, we used the filled reconditioned DEM to generate a flow direction map – a map

that identifies the flow direction from each cell to its steepest downslope neighbour.

Catchment Tools – Terrain Preprocessing: The Terrain Preprocessing tool provides a convenient tool for

running the four terrain preprocessing steps.

Select the Terrain Preprocessing tool and fill in the location of the DEM, streams (drainage

network), and lakes. Note that these are all located in the previously defined workspace (see §

3.1). Make sure not to use the original datasets.

Note: Alternatively, the four preceding steps can also be completed one at a time using the Arc Hydro Tools. This may be desirable if one wants to see the results of the intermediate steps, for example, to see what the DEM looks like after stream burning. Instructions for Arc Hydro:

Fill Sinks. Run the Fill tool located under the Preprocessing menu of the Arc Hydro Tools. Select the raw DEM as the Input surface raster. Optionally, specify the Z limit i.e., the maximum depth that depressions should be to be considered sinks.

DEM Reconditioning. Run the DEM Reconditioning tool located under the Preprocessing menu of the Arc Hydro Tools. Select the “filled” DEM and Atlas of Canada drainage network. For the Boreal catchments, we accepted the default values for the other four parameters.

Fill Sinks (2). Run the Fill function once again using the reconditioned DEM as the input surface raster.

Flow Direction. Run the Flow Direction tool, selecting as input the filled and reconditioned DEM.

3.4 – Adjust flow direction in lakes

Description: In some parts of the Boreal, the topography is flat, and there are many lakes that may or

may not be connected to streams (see §4.4 Adjusting flow direction in lakes). This causes problems

which are reflected in the shape of resultant catchments. Adjusting the flow direction in lakes modifies

the input Flow Direction grid to force each cell within each selected lake polygon to flow toward the

closest stream feature within the lake.

Arc Hydro Tools – Adjust Flow Direction in Lakes: This tool is currently not scriptable using Python and

thus needs to be run from the Arc Hydro Tools. When it becomes scriptable, it will be incorporated in

the Beacons’ Catchment Terrain Preprocessing tool.

Run the Adjust Flow Direction in Lakes tool (Arc Hydro Tools / Terrain Preprocessing menu) to

adjust the flow direction of lakes that are connected to streams but have no skeleton segment.

(Note: lakes that do not intersect the drainage skeleton are not considered by this function).

For input, use the flow direction grid created in the previous step, the Atlas of Canada lakes

shapefile, and the Atlas of Canada drainage skeleton network shapefile. Important note: The 3

input files first have to be added to the ArcMap Table of Contents prior to running this tool.

3.5 – Create catchments for Canada

Description: In this step, we use the adjusted flow direction map and the existing rasterized drainage

network map to create catchments for the entire country. This tool uses the Watershed tool (Hydrology

Toolbox in Spatial Analyst) and several other functions (e.g., EucAllocation, Eliminate, Dissolve) to create

the catchments. Briefly, the steps consist of:

Run the Watershed tool to create a catchment grid.

Run the Euclidean Allocation tool to assign GRIDCODEs to rasters with no values – which occur

where watersheds were not created (i.e., too far from streams) and along boundaries (i.e., areas

where DEM and Canada boundaries do not line up).

Use Map Algebra to eliminate small catchments i.e., those less than 100 hectares.

Convert the catchment grid to a polygon feature class.

Clip catchment polygon to boundary of Canada (this removes bits of raster that extend beyond

border of Canada).

Run the Eliminate tool to remove any remaining small polygons.

Use the Dissolve tool to remove any remaining duplicate GRIDCODEs.

Catchment Tools – Create Canada Catchments: The Create Canada Catchments tool requires the user to

specify the locations of the workspace, the flow direction grid, and the drainage network raster (stream

grid).

Run the Create Canada Catchments tool and fill in the five parameters.

3.6 – Extract Boreal catchments

Description: After creating the Canada‐wide catchments coverage, catchments are extracted for the

Boreal region using the Boreal boundary map. Catchments are clipped rather than selected so that they

do not cross the Boreal region boundary. Alternatively, with minor changes made to the tool, it would

be possible to select rather than clip catchments to maintain watershed (hydrological) connectivity.

Catchment Tools – Extract Boreal Catchments:

Run the Extract Boreal Catchments tool and select the workspace, the location of the Canada

catchments, and the boundary of the Boreal region.

3.7 – Add catchment attributes

Description: In this step, several important attributes are calculated (e.g., area and perimeter) or added

from existing maps (e.g., stream network, drainage area, and fire region characteristics) for later use

with the Benchmark Builder:

Area

Perimeter

SKELUID – unique identifier of the stream arc for which the catchment was created; it

corresponds to the UID_v6 attribute of the Atlas of Canada 1,000,000 Drainage Network,

Version 6. The SKELUID is also the same as the GRIDCODE in the catchment dataset.

CATCHNUM – unique identifier for each catchment

ODRAINAGE – Ocean Drainage Areas

WSCSSDA – Water Survey of Canada Subsub Drainage Areas; the subsub drainage areas (SSDA)

are nested within the ocean drainage areas. The SSDA dataset includes an ODRAINAGE

attribute. This dataset was also used to assign the ODRAINAGE to the catchments.

EMFS – estimated maximum fire size from the BEACONs’ fire regionalization dataset

This tool uses the ArcGIS Join tools (spatial and attribute) to add new attributes to the catchment

shapefile. The ODRAINAGE, WSCSSDA, and EMFS are assigned to a catchment based on the location of

the catchment’s centroid.

NOTE: The tool assigns ODRAINAGE and WSCSSDA as described above. However, some catchments are

incorrectly assigned because, in a handful of cases, the location of the catchment’s centroid does not

accurately represent the location of the catchment’s stream arc. We manually corrected these errors so

that ODRAINAGE and WSCSSDA are assigned based on the location of the catchment’s stream arc.

Stream arcs are nested within the ocean drainage areas and subsub drainage areas.

Catchment Tools – Add Catchment Attributes:

Run the Add Catchment Attributes tool in the Catchment Tools and specify the locations of the

workspace, catchment shapefile, WSC sub‐sub drainage areas shapefile, and the fire region

shapefile.

3.8 – Calculate land and water area

Description: One of the criteria used by Builder to inform the construction of benchmarks is minimum

terrestrial area (i.e., the minimum terrestrial area a contiguous set of catchments needs to have to

qualify as a benchmark area). Currently, minimum size is derived from the local estimated maximum fire

size from the fire regions map. Since most catchments include lakes, the area of lakes needs to be

subtracted from the total area of a catchment to derive the catchment’s terrestrial area. In this step, the

total area of each catchment is divided into terrestrial and aquatic area to facilitate calculation of the

minimum size criteria.

NOTE: The Atlas of Canada 1,000,000 Drainage Network lakes polygon (version 6) coverage includes

lakes, double‐line rivers, and wetlands.

Catchment Tools – Calculate Land & Water Area:

Run the Calculate Catchment Land & Water Area tool in the Catchment Tools and specify the

location of the workspace, catchment shapefile, Drainage Network Atlas of Canada lakes

shapefile, and the Boreal region boundary map. Three new attributes are created: Area_water,

Area_land, and Area_total.

3.9 – Calculate Intactness

Description: The Global Forest Watch Canada (GFWC) Intact Forest Landscapes (IFL) map is used to

calculate intactness for each catchment based on the proportion of the catchment area that is 100%

intact. The tool makes use of the Arc/Info Identity tool and currently only works for catchments in the

forest ecozones of Canada. GFWC’s Intact Forest Landscapes coverage does not extend beyond the

forest ecozones.

GFWC’s IFL dataset was created using Statistic’s Canada boundary shapefile of Canada. The catchments

were created using the Atlas of Canada Canada boundary. To account for discrepancies between these

two boundaries, catchments are clipped to the Statistics Canada boundary before calculating the

proportion of area intact for each catchment (IFL_2010). The IFL_2010 attribute is transferred from the

clipped catchment shapefile to the original shapefile using a join based on GRIDCODE.

CatchmentTools – Calculate Boreal Intactness:

Run the Calculate Boreal Intactness tool in Catchment Tools and specify the location of the

output workspace, the Boreal catchments map, and the GFWC intact forest landscape map.

Optional – Extract ocean drainage catchments

In some cases, it may desirable to extract a subregion of the Boreal catchments map based on ocean

drainage boundaries. This tool extracts catchments whose boundaries are defined by the Boreal region

and one of the four ocean drainages intersecting the Boreal region. Each catchment is assigned to an

ocean drainage based on the location of its associated stream segment. No streams cross drainage

boundaries. In all cases, catchments are selected rather than clipped to maintain watershed (hydrologic)

connectivity because catchment boundaries do not necessarily line up with ocean boundaries.

Catchment Tools – Extract OD Catchments:

Run the Extract OD Catchments tool in Catchment Tools. Select the location of the workspace

and the Boreal catchments.

4. Notes and Issues

4.1 Identifying sinks

DEMs often have small errors that are known as sinks – areas that are surrounded by higher elevation

values. These areas of internal drainage can cause problems when calculating a flow direction map and

it is often recommended that they be removed. However, in areas of low relief, sinks may represent true

features such as glacial and karst areas or bogs and lakes – all of which are common in the Boreal region

(Figure 2).

a) Lakes

b) Sinks

Figure 2. Comparison of actual lakes (a) and DEM sinks (b).

4.2 Correcting Problem catchments

In certain areas of the Boreal region, a combination of flat topography and numerous lakes (including

many that are not connected to streams) result in the creation of clusters of narrow parallel catchments

(Figure 3). To fix this problem, we used a combination of DEM reconditioning (see also § 3.3) and

adjusting flow direction in lakes (see also § 3.4). Brief descriptions of both functions are provided here:

DEM reconditioning (stream burning). From the Arc Hydro Tools help file: “The DEM Reconditioning

function (DEM Manipulation menu) modifies Digital Elevation Models (DEMs) by imposing linear

features onto them (burning/fencing). This function is an implementation of the AGREE method

developed by Ferdi Hellweger at the University of Texas at Austin in 1997.”8

8 http://www.ce.utexas.edu/prof/maidment/GISHYDRO/ferdi/research/agree/agree.html

Adjusting flow direction in lakes. From the Arc Hydro Tools help file: “The Adjust Flow Direction in Lakes

function (Terrain Preprocessing menu) allows modifying the input Flow Direction grid to force each cell

within each selected lake polygon to flow toward the closest stream feature within the lake. The

function works on a selected set of Lake Polygon features or on all features if there is no selected set.”

a) Streams and lakes b) Disconnected lakes in light blue

c) “Problem” catchments d) “Fixed” catchments

Figure 3. A sample watershed showing (a) streams and lakes (b) connected and disconnected lakes, (c)

catchments created using unmodified DEM, and (d) catchments created using reconditioned DEM.

4.3 Eliminate small catchments tools

There are two versions of this tool and neither any longer used as a separate step (standalone script) in

the step‐by‐step catchments creation procedure. However, the tools may still be useful in other

situations and are thus described here. Both tools were designed to eliminate small catchments based

on a user defined mininum size, for example, if a minimum catchment size of 100 ha is desired for the

purpose of regional conservation planning. Both versions of the tool use the ArcInfo Eliminate function

one or more times. The second version also considers the stream flow direction so as to merge small

catchments with upstream catchments.

Eliminate Small Catchments (version 1)

Description: This is a utility script that can be used to eliminate small catchments e.g., those less than

100 ha. The number of iterations is set to 2 but can be changed to any value. A minimum of 2 iterations

is recommened in case new small catchments (slivers) are created during the first iteration.

Input: A catchment feature class.

Output: A new catchments feature class with all polygons under a user specified size threshold

eliminated.

Requirements: ArcInfo license.

Instructions: To use the tool, simply click on the Eliminate Small Catchments (version 1) tool to launch

the dialog box. Four parameters need to be entered to run the tool:

Workspace: Specify workspace where new catchments will be created.

Catchments feature class: Specify the location of the catchments feature class.

Minimum Size (ha): Specify the minimum size of polygons in hectares.

Number of iterations: Specify the number of iterations to run (default is 2).

Eliminate Small Catchments (version 2)

Description: This is a utility script that can be used to eliminate small catchments e.g., those less than

100ha. It is similar to version 1 but merges small catchments to neighbouring upstream catchments.

Land and water areas are recalculated for all catchments.

Input: A catchment feature class.

Output: A new catchments feature class with all polygons under a user specified size threshold

eliminated.

Requirements: GPEliminateCatchments.dll9

Instructions: To use the tool, simply click on the Eliminate Small Catchments (version 2) tool to launch

the dialog box. Ten parameters need to be entered to run the tool:

Input features: Specify the name and location of the input catchments feature class.

Output features: Specify the name and location of the output catchments feature class.

Attributes: Seven attributes need to be specified; the defaults should be sufficient.

Eliminate Size: Specify the minimum size of polygons (default = 1000000 m2).

9 To use the DLL, put it in your “C:\Program Files\ArcGIS\Bin” directory. Then run this command to register it: “C:\Windows\Microsoft.NET\Framework\v2.0.50727\regasm.exe C:\Program Files\ArcGIS\Bin\GPEliminateCatchments.dll". Note that if you are installing a newer version of the DLL you will first need to unregister the old DLL using this command: “c:\Windows\Microsoft.NET\Framework\v2.0.50727\regasm.exe /u C:\Program Files\ArcGIS\Bin\GPEliminateCatchments.dll"

5. References

Djokic, D. 2008. Comprehensive Terrain Preprocessing Using Arc Hydro Tools. ESRI.

Lee, P.G., M. Hanneman, J.D. Gysbers, R. Cheng, and W. Smith. 2010. Atlas of Canada’s Intact Forest Landscapes. Edmonton, Alberta: Global Forest Watch Canada 10th Anniversary Publication #1. 74 pp.

6. Appendix 1 – Troubleshooting

6.1 DEM not recognized or does not exist

Problem: This problem was documented when using the Terrain Preprocessing tool (step 3.3). An error

was generated stating that the selected DEM is not recognized or does not exist. Presumably, this

problem could also occur when using other tools that use input grids.

Solution: Go to the Customize menu and make sure the Spatial Analyst extension is selected.

catchments manual - canadian beacons project

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