Exploring Hydrological Analyses using ILWIS

Purpose

This Wiki tutorial has been produced for the purpose of exploring and describing the methods used for hydrological analyses in ILWIS (a FOSS4G program). The objective of this tutorial is to provide users with a better understanding of how the hydrological analyses within ILWIS works, in addition to providing the instructions necessary to reach the desired output for their study region. Additionally, this tutorial will allow for comparison of ILWIS and SAGA GIS, using the same data for analysis as the Exploring Hydrological Analyses using SAGA GIS tutorial from 2013.

Introduction to ILWIS and the Project Tutorial

ILWIS stands for "Integrated Land and Water Information System". It is a open source Geographic Information System software program that was designed to be a user-friendly integrated software that contains both raster and vector processing capabilities, allowing for both analyses on remotely sensed images, vector maps and numerous spatial modeling abilities. ILWIS was developed by the International Institute for Aerospace Survey and Earth Sciences (ITC) in The Netherlands. This tutorial will focus on its ability to use Digital Elevation Models (DEMs)to run hydrological analyses of a study region. More information about this product can be found on the ILWIS Website

This project will focus on hydrological analysis of Okanagan Lake, a popular tourist destination in British Columbia, Canada. The tutorial will cover topics such as Aspect mapping, Slope mapping, Watershed Delineation mapping and Wetness Index mapping through the use of ILWIS hydrological analysis tools.

Methods

Data and Programs

In order to carry out this exercise, a DEM of the area of interest is needed. For this project a open source DEM was obtained using the Geospatial Data Extractiontool created by Natural Resources Canada.

For DEMs located outside of Canada open source DEMs can be obtained from ASTER and SRTM Global Digital Elevation Model - USGS

This turtorial explores the hydrological toolset for ILWIS, but the use of open-source program QGIS is needed as well. This can be downloaded from the QGIS website

Getting Started

Before starting any analysis, create your working folder and ensure all data needed for the exercise are in this folder. Keeping all your data in one folder will make analysis easier.

Starting ILWIS

ILWIS 3.08.03 can be downloaded for free from 52 North.The newer version of ILWIS 3.08.04 is available but do not download this one, as a bug does not allow you to import DEMs into this version.

• Ensure that ILWIS 3.08.03 is properly installed on your computer
• Locate ILWIS from the Program list and double click on the icon to launch program.
• ILWIS main page will open
• Use the ILWIS Navigator Tab to locate your working folder. The Navigator lists all drivers and directories.

Figure ??: Screenshot of ILWIS Interface

Projecting a DEM in QGIS

Projecting in ILWIS did not work, so QGIS (another open source GIS software) was used instead. These steps explain how to load a DEM into QGIS, assign a projection, and export the DEM to be used in other software (e.g. ILWIS)

1) Download QGIS and open the program.

2) Add your DEM using the Add Raster Layer tool (Figure ??). You will have to navigate to the working folder and select the DEM.

3) To reproject your DEM click Raster -> Projection -> Assign Projection

4) A new window will open (Figure ??)called Assign Projection, within this window the DEM to projected is selected for input file

5) Click Select.. under desired srs in the Assign Projection window and a new Select Desired SRS window will open (Figure ???).

6) Within this window select the desired coordinate system. Click OK.

   (note: For this hydrological analyses, the coordinate system chosen should measure in metres and not in degrees)

7) A new DEM will be added to the QGIS interface with the desired projection

8) The new DEM can be exported by right-clicking it and selecting Save as...

9) A window called Save raster layer as.. will open. Under Save as browse to the desired folder location for your projected DEM and rename it.

10) Click Save. A projected DEM for the region of interest is now saved and can be loaded into ILWIS.

Importing DEMs into ILWIS

Before any analysis can be completed, a DEM of teh region of interest must be imported into ILWIS.

1) Click Operations --> Import/Export --> GDAL (Geospatial Data Abstraction Library)

Figure ?? - Operations to find import tool in ILWIS

2) An Import window will open

Figure ?? - ILWIS import window

3) Click to the DEM you want to import and ensure that the import format is Use GDAL.

5) Give your Output DEM a name and click OK.

      * The new DEM (compatible in ILWIS) will save in your working directory 

6) To view your DEM right-click on the DEM and select Open

Figure ?? - Projected DEM of Lake Okanagan in ILWIS software

Fill Sinks in a DEM

Fills should be run on DEMs to remove any local depressions, which can cause inaccuracies in following hydrological anlyses.

Within ILWIS there are two Fill Methods:

• Fill Sinks – removes depressions that consist of a single pixel and depressions that consist of multiple pixels. These pixels will then be given the smallest value of adjacent pixels.
• Cut terrain – removes the depressions, does not re-assign values to the pixels but removes them entirely from the DEM

Steps

1) Under Operations select Flow Determination then Fill Sinks

Figure ?? - Path to find Fill Sink operation

2) A Fill Sinks window will open

Figure ?? - Fill Sinks window

3) Input the DEM and decide which Fill method to use (Fill Sinks method was used for the purpose of this tutorial), finally name the output DEM.

4) A new DEM will depression filled will be created and added to the working folder

Figure ?? - DEM with Fill Sink applied

Create Flow Direction Map

Flow direction depicts a main direction of flow (run-off). The flow direction operation determines into which neighbouring pixel any water from a given pixel would flow. This is determined by a moving 3x3 window for every single pixel, based on its neighbouring pixels height values

Flow direction can be calculated by two methods:

• Steepest Slope: finds steepest slope of a central pixel to one of its 8 neighbour pixels
• Lowest height: finds the neighbouring pixel with the smallest height value

More information on the methods can be found at thisILWIS Flow Direction webpage

Steps

1) Operations -> Flow Determination -> Flow Direction

Figure ?? - Path to Flow Direction

Figure ?? - Flow Direction window

2) Input the Fill DEM

3) Choose a flow direction method

4) Click Show

5) The Flow Direction map will save in the working folder and open in a new window

Figure ?? - Flow Direction Map for Lake Okanagan

Flow Accumulation Map

Flow Accumulation performs a cumulative count of the number of pixels that would flow into any given cell on the way to an outlet. This operation is commonly used to determine potential drainage patterns of terrain.

Flow Accumulation is calculated from the map created in Flow Direction, counting the number of cells that would drain into outlets based on the flow direciton.

More information on the method can be found at this ILWIS Flow Accumulation webpage

Conclusion

This tutorial showed the steps required to run a hydrological analyses using a DEM in ILWIS software. GQIS was used to reproject the DEM, and ILWIS software was used to run the hydrological analysis on the landscape.

References

• 52 North

http://52north.org/communities/ilwis