Exploring the Hydrological Tools in QGIS

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Purpose

The purpose of this Wiki tutorial is to demonstrate and explore various hydrology tools used for hydrological analyses in QGIS. The objective of this tutorial is to provide instructions on how to use digital elevation models (DEM) for extracting channel networks, delineating watersheds and defining a catchment area. Additionally, this tutorial will allow for comparison between open source softwares of QGIS, ILWIS and SAGA GIS, tutorials created for hydrological analyses in 2013, Exploring Hydrological Analyses using SAGA GIS and 2014, Exploring Hydrological Analyses using ILWIS.

Introduction

Hydrological analysis tools are important resources and are used in a variety of programs to define how the geographic range of an area interacts with water. Hydrological analyses can aid researchers and emergency personnel in identifying the source and pathway of groundwater, delineating watersheds and identifying communities prone to flooding conditions (Sui et. al., 2011, Mani et al., 2014). A variety of programs can be used to perform hydrological analyses including ARCGIS, ILWIS and SAGA GIS. This tutorial will give instructions on using QGIS, a Free and Open Source Software for Geospatial Data (FOSS4G), for hydrogeological analyses. QGIS is an open source geographic information system that runs on various platforms including Windows, Mac OS and Linux (QGIS, 2015). Using a digital elevation model of the West Ottawa region, we will demonstrate and explore hydrogeology toolsets offered by QGIS.

Materials and methods

Data and available platforms

QGIS is a free and open source geographic information system. The current version of QGIS is 3.14.16 ‘Pi’ that was released on September 11th, 2020. When downloading the software, it is usually best to download the long term release, as it is the most stable, this would be version 3.10. QGIS is available for a variety of platforms including Windows, Mac OS, Linux and Android. Various releases for download can be found at the QGIS website. The case study is using a previous version of QGIS on Mac OSX, but instructions will be given for the current version; images of windows and menus may differ relative to your version of QGIS.

Digital elevation models (DEM) are used for hydrological analyses. DEMs can be generated using point data sets from elevation data or from aerial imagery using remotely sensed data (NRCAN, 2013). Downloading a point data set would generally require interpolation of the data to create a DEM. Interpolation of a DEM will not be discussed in this tutorial. Instead, we will explore the hydrological toolset using a DEM that has previously been interpolated.

A DEM of the southeastern quadrant of the outskirts of Ottawa, Ontario was used for the purpose of this tutorial. Data were obtained from the GIS Library of Carleton University using a 3-dimensional raster data set from the Ontario Ministry of Natural Resources. The data capture terrain elevations and cover the province of Ontario with a cell resolution of 30 cm.

Additional data sources can be found from Natural Resources Canada, Geogratis and from the Ontario Ministry of Natural Resources. Macodrum Library is also a valuable tool for finding elevation data, they provide links to many resources, which Carleton students and faculty have access to.

Getting started

It is very important to create a working directory where you will keep all your data and progress files. After opening QGIS, create a new project, and save it to your working directory.

Import your DEM dataset by clicking on the “Layer" tab at the top of the menu > add layer > add raster layer > click on the '...' button next to the empty field next to 'raster dataset(s)' > select your raster file > add

Note: The file must be in a raster format; for example, .GRID, .tiff, .rst.

Set projection information for your DEM

Assigning a projection to your DEM enables it to be properly displayed and analyzed. To set the projection for your DEM in QGIS, follow the instructions below.

1. Click Raster > Projection > Assign Projection (Figure 2)

2. The Assign Projection window will appear > Click select, Desired SRS > Choose desired coordinate system and click > OK (Figure 3)

3. For the purpose of this tutorial we will be using NAD83, UTM zone 18N

4. Right click over your DEM layer > Properties > General > Coordinate reference system. Here is where you can verify the projected coordinate system applied to your DEM layer.

5. Reclassify your DEM > Right click over your DEM layer > Properties > Symbology > Band Rendering > Render Type > Choose Single Band Pseudocolor > Classify > Click OK (Figure 4). Reclassifying the DEM may help you have a better understanding of the data you are working with.

Projecting1.png

Figure 2. Assigning a projection to your DEM layer.

Projecting2.png

Figure 3. Choosing a desired coordinate system for your DEM layer.

Projecting3.png

Figure 4. Reclassification of a DEM.

The hydrology toolboxes

QGIS has an extensive set of tools available for hydrological analyses located in the hydrology toolbox. To access the toolbox, click on:

  • Processing Tab > Toolbox > SAGA > Terrain Analysis: Hydrology.

When opening Terrain Analysis - Hydrology, you will see a long list of different tools, some are very similar and it may be hard to know which to use and for what. Accessing the help tab within each hydrology tool results in no description being available. Therefore, we have created short descriptions for each tool assessed during this tutorial for additional information.

Hydro.png

Figure 5. Example of no available help for selected tools.


TOOLBOX A. Catchment Area

Description

  • A catchment area is otherwise considered a drainage basin. The basin is where water flows over topographic terrain and consists of water runoff into surrounding rivers, streams and lakes. (Wagener et al. 2007).

Parameters

  • Elevation > Insert DEM data layer
  • Method > Choose one model to apply to the DEM data layer

There are five methods to choose from when delineating a catchment area from a DEM.

  • [0] Deterministic 8 (D8)

This is the classical method in which water flow moves from the center of one cell to the center of one of the cells surrounding the first cell. This restricts the flow direction to multiples of 45 using 8 flow directions. (O’Callaghan et al. 1984).

  • [1] Rho 8

This method uses an algorithm to randomly assign the direction of flow to downslope neighbouring cells depending on the degree of slope of the surrounding neighboring cells. Cells surrounding the central cell are weighted according to the degree of slope and the flow is assigned depending on the calculated weighting. (Wainwright et al. 2013)

  • [2] Braunschweiger Reliefmodell

This method uses an algorithm to determine multiple flow directions. This model allows for flow to disperse to multiple, adjacent cells downslope from the central cell. The model restricts the flow to three cells, thus limiting the total flow dispersion of the DEM. (Buchanan et al., 2014).

  • [3] Deterministic Infinity

This model delineates water flow from one cell to two surrounding cells. The model considered a bi-dimensional flow pattern and is the recommended model for reducing the limitations of the D8 model. (StackExchange, 2011).

  • [4] Multiple Flow Direction

This model assumes that flow occurs in all directions downslope from any given point. (Wolock et al. 1995).

  • [5] Multiple Triangular Flow Direction

This method avoids unrealistic flow dispersion on planar or concave slopes. This model allows for flow to disperse into one or two downslope cells, thus allowing for multiple flow directions. (Seibert et. al., 2007).

  • Catchment Area > Specify the name for the output layer


TOOLBOX B. Channel Network

Description

  • A channel network is otherwise considered a drainage network of water flow over terrain. The resulting map will demonstrate pixels with a TRUE value where flow drains with all pixels having no drainage will result in the pixels having a FALSE value.

Parameters

  • Elevation > Insert DEM data layer
  • Flow Direction (optional) > Insert optional flow direction layer
  • Initiation Grid > Insert Catchment Area layer or desired input layer of your choice
  • Initiation Type > Choose from [0] Less than, [1] Equals, or [2] Greater than
  • Initiation Threshold > Insert desired threshold value

The threshold value is the cell value required for flow to be considered TRUE in the drainage network. Adjusting the threshold value will alter the resulting drainage network because all other cells that do not meet the minimum threshold value will be considered FALSE. Increasing the threshold will result in a sparse channel network and decreasing the threshold will result in a dense channel network.

  • Divergence (optional) > Insert optional divergence layer
  • The divergence layer is a layer that determines if flow diverges from the specified channel.
  • Tracing: Max. Divergence > default = 10, adjust accordingly
  • Tracing: Weight (optional) > Insert optional weighting layer for tracing network
  • Min. Segment Length > default = 10, adjust accordingly
  • Channel Network > Specify the name for the output layer
  • Channel Direction > Specify the name for the output layer


TOOLBOX C. Watershed Basins

Description

  • Watershed basins are sub-basins determined for water flow using the channel network of the DEM being analyzed (Trenhaile, 2010).

Parameters

  • Elevation > Insert DEM data layer
  • Channel Network > Insert channel network layer
  • Sink Route (optional) > Insert optional sink route layer
  • Min. Size > default = 0, adjust accordingly
  • Watershed basin > Specify the name for the output layer

The hydrology SAGA Toolbox

To access the Hydrology SAGA Toolbox in QGIS follow the below instructions.

  • Processing > Toolbox > SAGA > Terrain Analysis > Hydrology

Filling sinks in a DEM

In order to extract a channel network from a DEM layer, sinks need to be filled and catchment areas need to be determined. Filling sinks allows for the removal of any local depressions from your DEM layer that can cause inaccuracy when determining channel networks. The catchment area is otherwise considered flow accumulation and can be used as the threshold layer for extracting subsequent channel network.

1. Processing > Toolbox > SAGA > Terrain Analysis: Hydrology > Fill Sinks XXL (Wang & Liu)

2. Fill in the specified parameters (Figure 6)

  • DEM > Insert DEM data layer
  • Minimum Slope > Adjust slope as necessary
  • Filled DEM > Specify name for output layer

3. Reclassify DEM (Figure 7)

  • Right click on DEM layer > Properties > Symbology > Band Rendering > Select Render Type > Single band Pseudocolour > Classify > OK

Sinks1.png

Figure 6. Filling sinks toolbox location for DEM.

Sinks2.png

Figure 7. Filled DEM of the southeastern quadrant of Ottawa.

Flow directions and watershed basins

Flow direction is the main direction of water run-off over the geographic area of interest. Flow is determined with the algorithm differentiating where a given pixel would go depending on elevation and cell height values. Watershed basins are sub-basins (polygons) depicting where water would accumulate in different areas of the DEM.

Using the Fill Sinks (Wang & Liu) tool, flow direction and watershed basins can be created while filling the sinks within the DEM.

1. Click on Processing > Toolbox > SAGA > Terrain Analysis: Hydrology > Fill Sinks (Wang & Liu, Figure 8)

2. Fill in the specified parameters

  • DEM > Insert DEM Data Layer
  • Min. Slope > Specify minimum slope requirement, adjust as necessary
  • Filled DEM > Specify name for output layer
  • Flow Directions > Specify name for output layer
  • Watershed Basins > Specify name for output layer

3. Reclassify DEM

  • Right click on DEM layer > Properties > Symbology > Band Rendering > Select Render Type > Single band Pseudocolour > Classify > OK

Flow1.png

Figure 8. Fill sinks (Wang & Liu) toolbox for flow directions and watershed basins.

Flow2.png

Figure 9. Resulting flow direction map of the Southeastern quadrant of Ottawa.

Flow3.png

Figure 10. Resulting watershed basin for the Southeastern quadrant of Ottawa.

Defining the catchment area

A catchment area is otherwise considered a drainage basin. The basin is the where water flows over topographic terrain and consists of water runoff into surrounding rivers, streams and lakes. (Wagener et al. 2007). There are five models to choose from: Deterministic 8, Rho 8, Braunschweiger Reliefmodell, Deterministic Infinity, Multiple Flow Direction and Multiple Triangular Flow Direction. A description of each model can be reviewed under 3.4 The Hydrology Toolbox.

1. Click on Processing > Toolbox > SAGA > Terrain Analysis: Hydrology > Catchment Area

2. Fill in the specified parameters

  • Elevation > Insert DEM layer (filled DEM)
  • Method > Choose appropriate method
  • Catchment Area > Specify name for output layer

Catch1.png

Figure 11. Catchment area tool in the Hydrology Toolbox of QGIS.

Catch2.png

Figure 12. Catchment area of the Southeastern Quadrant of Ottawa using the Deterministic 8 method.

Catch3.png

Figure 13. Catchment area of the Southeastern Quadrant of Ottawa using the Rho 8 method.

Catch4.png

Figure 14. Catchment area of the Southeastern Quadrant of Ottawa using the Braunschweiger Reliefmodell method.

Catch5.png

Figure 15. Catchment area of the Southeastern Quadrant of Ottawa using the Deterministic infinity method.

Catch6.png

Figure 16. Catchment area of the Southeastern Quadrant of Ottawa using the Multiple Flow Direction method.

Catch7.png

Figure 17. Catchment area of the Southeastern Quadrant of Ottawa using the Multiple Triangular Flow Direction method.

Channel network of a DEM

A channel network is otherwise considered a drainage network of water flow over terrain. The resulting map will demonstrate pixels with a TRUE value where flow drains with all pixels having no drainage results in pixels having a FALSE value. Here we will demonstrate how to extract a channel network from the DEM of Ottawa using the SAGA Toolbox in the advanced processing function in QGIS. This enables us to create multiple maps using one tool.

1. Processing > Toolbox > SAGA > Terrain Analysis: Hydrology > Channels > Channel Network and Drainage Basins

2. Fill in the specified parameters

  • Elevations > Insert DEM layer (filled DEM)
  • Threshold > default = 5, Insert desired threshold limit
  • Flow Direction > Specify name for output layer
  • Flow Connectivity > Specify name for output layer
  • Strahler Order > Specify name for output layer
  • Drainage Basins > Specify name for output layer
  • Channels (shapefile) > Specify name for output layer
  • Drainage Basins (shapefile) > Specify name for output layer
  • Junctions (shapefile) > Specify name for output layer

Channel1.png

Figure 18. Channel Network and Drainage Basins tool in the advanced interface processing SAGA tool box in QGIS.

Channel2.png

Figure 19. Drainage Basins and Channels of Southeastern Quadrant of Ottawa using the Channels and Basins SAGA tool in QGIS.

Channel3.png

Figure 21. Channels of Southeastern Quadrant of Ottawa using the Channels and Basins SAGA tool in QGIS.

Conclusion

In conclusion, we have successfully demonstrated how to use various hydrological tools offered in the free and open source software of QGIS using a DEM of the West Ottawa region. Hydrological analyses are important resources for determining how the geography of any area interacts with water, and thus, have important applicability to safety, engineering and ecology perspectives.

Literature Cited

Buchanan, BP., Fleming, M., Schneider, RI., Richards, BK., Archibald, J., Qui, Z., Walter, M.T., 2014. Evaluating topographic wetness indices across central New York agricultural landscapes. Hydrology Earth System Sciences, 18: 3279-3299.[1]

Maidment, D., Djokic, D., 2000. Hydrologic and Hydraulic Modeling Support with Geographic Information Systems. Environmental Systems Research Institute Inc., Redlands, California. 213pp. [2]

O’Callaghan, JF., Mark, DM., 1984. The extraction of drainage networks from digital elevation data. Computer Vision, Graphics, and Image Processing, 28(3): 323-344. [3]

Seibert, J., McGlynn, BL., 2007. A new triangular multiple flow direction algorithm for computing upslope areas from gridded digital elevation models. Water Resources Research, 43(4): doi: 10.1029/2006WR005128.[4]

Shamsi, UM., 2005. GIS Applications for Water, Wastewater, and Stormwater Systems. CRC Press Inc., Boca Raton, Florida. 398pp. [5]

StackExchange, 2011. Geographic information systems: Given a terrain, how to determine stream flow path? Retrieved on Dec 1st 2015 from [6]

Trenhaile, AS., 2010. Geomorphology: A Canadian Perspective. Fourth Edition, Oxford University Press. Ontario, Canada.

Wainwright, J., Mulligan, M., 2013. Environmental Modeling: Finding Simplicity in Complexity. John Wiley & Sons, Ltd. West Sussex, UK. 496pp. [7]

Wagener, T., Sivapalan, M., Troch, P., Woods, R., 2007. Catchment classification and hydrologic similarity.Geography Compass, 1(4): 901-931. [8]

Wolock, DM., McCabe Jr., GJ., 1995. Comparison of single and multiple flow direction algorithms for computing topographic parameters in TOPMODEL. Water Resources Research, 31(5): 1315-1324 [9]

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