QGIS...25.02.2019 Introduction QGIS Catchment Delineation FM/BTU 11 QGIS Flow Direction r.fill.dir...

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QGIS

Introduction QGIS Catchment Delineation25.02.2019 FM/BTU 2

QGIS - Geospatial Information System

QGIS (QuantumGIS)

• initiated in 2002, version 1.0 available 2009

• free and open-source cross-platform GISlicense: GNU GPLv2

• integration of other GIS initiatives such asGRASS (Geographic Resources Analysis Support System)

initiated in 1984, GRASS GIS 7 released in 2015

• similar functionality to other GIS (e.g. ArcGIS)

• several extensions (plugins) via API available



QGIS: Geospatial Data

• vector data (all relevant formats, e.g. shp, gml)

• raster data (all relevant formats, e.g. GeoTiff)

• Web GIS services (WMS, WFS, WCS, CSW,…)

• (geo-)databases (PostGIS, Spatialite, Oracle Spatial, …)



QGIS: Standard Functionality

• QGIS > 110 geo-algorithm

• SAGA > 350 geo-algorithm(System for Automated Geoscientific Analyses)

• GDAL > 45 geo-algorithm(Geospatial Data Abstraction Library)

• GRASS GIS 7 > 310 geo-algorithm(Geographic Resources Analysis Support System)



QGIS: Examples for additional plugins

• PostTelemac pre- and postprocessing Telemac

• Crayfish visualization HD model results

• FREEWAT environment for hydrological modelling

• GEarthView interface to Google Earth

• Ghydraulics interface to EPANET

QEPANET

• RiverGIS support for HEC-RAS models

• TUFLOW support for TUFLOW models


QGIS - Catchment Delineation

QGIS: DEM Catchment Delineation

• DEM described by raster data

• support for different raster formats and partial direct access to sources such as SRTM

• several library and plugins supporting DEM analysis for catchment delineation

• well documented in QGIS and related documents as well as scientific papers



General Workflow

• creation of a new project

• specification of the project CRS

• import of the DEM as raster data

• fill sinks towards a depressionless DEM

• watershed analysis:flow/drainage direction, flow accumulation, stream identification, watershed identification

• additional analysis (e.g. area, slope, flow length)



QGIS User Interface with DEM

• DEMtopo300.asc

• CRSNTF (Paris) / Lambert zone IIEPSG: 27572



QGIS User Interface with GRASS 7 Toolbox

geo-algorithm for catchment delineation used

• r.fill.dir

• r.watershed

• r.lfp (GRASS add-on extension)



QGIS Fill Sinks

r.fill.dir toolThe tool generates a depressionless DEM and a flow direction map from a given elevation raster map.

Each cell in a depressionless DEM has always a flow direction to a neighbourhood cell. Assuming an ongoing rainfall, fill sink tool is simulating filling up sinks with water up to an overflow to the lowest neighbourhood cell for a continuous flow path towards a depressionless terrain.

Sinks might appear in a DEM as natural sinks (lakes, ponds, ripples, …)or due to local effects of measurement resolution, data quality, processing inaccuracy and other reasons.

Reference: https://sagatutorials.wordpress.com/


QGIS Flow Direction

r.fill.dir toolAs 2nd result the flow directionis determined based on the depression less DEM in degree.

The range 0 – 360°, 0° is North,90° is East, 180° is South and270° is West.

Each cell has eight neighbours,Flow direction is the direction to the neighbour cell with the highest slope, so eight directions in 45° steps are possible (D8 algorithm).



QGIS Flow Accumulation

r.watershedFlow accumulation is the accumulated flow to (drain through) each cell based on the flow direction by summing up the cells which do have a flow path connection to the cell.A mountain peak has a flow accumulation value of 0,the outflow cell of a catchment has the highest valueof all cells in this catchment.



QGIS Watershed Identification

r.watershedWatershed (sub-catchment) identification is based on the flow accumulation. Cells with high flow accumulation identify streams (stream segmentation).Connection of streams identify watershed outflow points andrelated watershed basins / subcatchments. A threshold for theminimum size of the watershedas impact to the scale of sub-catchment resolution.




QGIS Longest Flow Path

r.lfp (addon in GRASS 7)The longest flow path is the possible longest distance for water drop within a sub-catchment to the outlet.The outlet point might be defined based on the streamsegmentation as part of the watershed identification. The streams to the outlet points are converted to polyline vector data. The length of theseare the longest flow path.


QGIS Rainfall Stations Thiessen Polygons

Vector data6 stations as point data

v.voronoi

Thiessen polygons (or Voronoi regions) for a set of points include all locations with the closest distance to one point in comparison to thedistance to all other points.

Each point is representative tothe related Thiessen polygon.

QGIS - Rainfall Interpolation


QGIS Rainfall Station / Sub-Catchment Intersection

Vector Data Intersection5 sub-catchment & 6 Thiessen polygons are intersected and the area of the resulting 30 polygons calculated.

polygon area as geometry property



QGIS Landuse

Raster Data300 m cell size

44 classes

Landuse will be reclassified by 6 classes and converted to vector data (polygons).




QGIS Landuse / Sub-Catchment Intersection

Vector Data Intersection5 sub-catchment & 6 landusepolygons are intersected and the area of the resulting 30 polygons calculated.



QGIS Soil Type

Raster Data2000 m cell size

6 classes1 Loam2 SandyLoam3 SiltyClayLoam4 SiltLoam5 SandyClayLoam6 ClayLoam

Soil Type will be converted to vector data (polygons).



QGIS Soil Type / Sub-Catchment Intersection

Vector Data Intersection5 sub-catchment & 6 soil type polygons are intersected and the area of the resulting 30 polygons calculated.



QGIS...25.02.2019 Introduction QGIS Catchment Delineation FM/BTU 11 QGIS Flow Direction r.fill.dir...

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