17 votes
Accepted

Getting Negative Topographic Wetness Index (TWI) values in SAGA GIS?

This is a good question, and one that I tend to get asked from time to time. First, as you've pointed out, the equation for TWI = ln(a / tan(B)), where a is the 'specific' catchment area (i.e. the ...
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  • 10.8k
13 votes
Accepted

Generate channel network (rainwater run off) - which algorithm is the best?

Choosing the right algorithm for hydrologically correcting a DEM really depends on your particular application. DEM size is certainly an issue, as you discovered. If you have a massive DEM, there are ...
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  • 10.8k
13 votes

How to cartographically represent water flow?

While Jacub is correct that a stream ordering technique is your best bet for being able to represent stream magnitude or discharge, since the position in the network is a surrogate for these two ...
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  • 10.8k
12 votes
Accepted

Methodology for creating accurate drainage networks (and catchments) from high resolution LiDAR DEM?

Have you consider to use GRASS GIS analysis? I have expirience that GRASS algorithms have very good accurance on hydrology analysis. For example, I want to generate something like drainage network on ...
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  • 1,743
11 votes

In hydrologic terrain analysis is there a difference between "pits", "depressions", and "sinks"?

There is a difference, and I recommend the typology presented by Lindsay (2015) be used. Lindsay (2015) presents a typology which defines a pit as a single cell in a DEM whose elevation is below that ...
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  • 1,761
10 votes
Accepted

Account for bridge overpasses in a ground DEM to allow for water flow under the overpass

Martin is correct that while your workflow will do well for a specific user case, it doesn't account for many of the issues that road embankments create for flowpath modelling using fine-resolution ...
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  • 10.8k
10 votes
Accepted

How to cartographically represent water flow?

You can classify the streams using, among others, the Strahler Stream Ordering method. In the Strahler method, all links without any tributaries are assigned an order of 1 and are referred to as ...
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10 votes
Accepted

Calculating width of unusually shaped polygon in QGIS

Find the centerline using v.voronoi.skeleton Create points along the centerline (Points along geometry) Create perpendicular lines to the points with Geometry by expression: extend( make_line( $...
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  • 51.2k
9 votes
Accepted

How to calculate "water accumulated in each cell" using QGis?

In spatial hydrology, DEM-based flow accumulation operations are typically static. That is, they represent a steady-state condition of the discharge of surface and near surface water passing through a ...
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  • 10.8k
9 votes
Accepted

How to run a tool from Whitebox GAT in ArcGIS?

I'm afraid that when I wrote Whitebox GAT, I never really anticipated that there would be a demand to be able to run the Whitebox plugin tools from outside the user interface. I knew that there would ...
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  • 10.8k
9 votes
Accepted

Calculating average stream slope at each location along stream using ArcGIS Desktop?

As @Hornbydd pointed it is network searching problem. I suggest the following workflow: find source points of the streams sort them in descending order by flow length In the picture below 139 green ...
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  • 21.4k
9 votes
Accepted

Lowering density of GRASS r.stream.extract result?

You need to adjust the "threshold" parameter, which is Minimum flow accumulation for streams in the Processing GUI. With this parameter you determine what is the minimum drainage area for each stream ...
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  • 14.1k
8 votes
Accepted

Extract river polygons from DEM

One approach that our amazing intern recently used, which turned out really well for us: Fill the DEM Calculate flow direction on the filled raster Accumulate the flow of the flow direction output Do ...
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  • 4,068
7 votes

Account for bridge overpasses in a ground DEM to allow for water flow under the overpass

Ok here is the image Blue are the bridges from OSM. The DEM is in grey-scale with buildings (I want buildings) I selected the intersect roads that cross the bridges. Buffered the selected roads by ...
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7 votes

Account for bridge overpasses in a ground DEM to allow for water flow under the overpass

This looks like a valid workflow for this task. However, depending on the level of accuracy that you work with, you will miss out on things like pipes under roads allowing for ditches and small creeks ...
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  • 4,892
7 votes
Accepted

Delineating large scale watershed?

I have a tremendous respect for GRASS and the r.tarraflow algorithm and I'm sure that given enough effort, you would be able to make it work for this application. But as an alternative, I develop a ...
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  • 10.8k
7 votes
Accepted

Does this ArcGIS flow accumulation example have an error?

It is an error. Your link and presumably image source is the 10.0 help. The error is still present in the 10.1 help. However in the 10.2 help, it has been corrected to show only a 1. It's also worth ...
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  • 15.5k
7 votes

Flow Direction created from depressionless DEM inaccurate in flat areas

These are common problems, typically resolved by stream burning, but that relies on you having a vector network to enforce drainage. I recommend you check out the free GIS software Whitebox GAT it has ...
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  • 38.4k
6 votes

Calculating cells elevation (height) above nearest stream cell using ArcGIS Spatial Analyst

I think that the original poster may want to calculate for each cell the height above the first stream cell that would be reached by water flowing from the cell. So the 'nearest stream' is calculated ...
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  • 293
6 votes

What satellite imagery would help to show spring locations?

As someone who did feature capture from imagery for a while, I would caution you against expecting a pool at a spring. The majority of the ones I've encountered (both in capture and on the ground in ...
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  • 15.5k
6 votes

Is there hydrologically correct water-course data for Africa

You can download hydrology shapefiles from here for a number of countries around the world. Select your country of choice and Inland water as your subject. You should receive 2 types of shapefiles: ...
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  • 72.2k
6 votes

Ways to determine winding topographic characteristics of a river

In the world of hydrology and geomorphology, there is indeed a metric that we use to classify/quantify the "curviness" of a river......sinuosity. Sinuosity is simply a measure of the actual path ...
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6 votes
Accepted

Delineating multiple nested catchments in R whitebox; prevent primary catchment from being cut by subcatchment

This scenario is precisely what the WhiteboxTools UnnestBasins tool is used for. The tool documentation states: In some applications it is necessary to relate a measured variable for a group of ...
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  • 10.8k
5 votes
Accepted

How to assign correct flow direction to stream network polylines?

I can think of a few ways to do this: Symbolize the lines so they have direction indicated, and manually Flip those in the wrong direction. Use an attribute if available or a calculation (such as end ...
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  • 15.5k
5 votes
Accepted

Calculating flowpaths downstream of a point

There is a tool in the free and open-source (GNU GPL licensed) GIS Whitebox Geospatial Analysis Tools that can identify the flowpath from any point or collection of points specified either as a ...
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  • 10.8k
5 votes

Identifying highs on DEM using ArcGIS Spatial Analyst?

One easy way of doing this would be to inverse your DEM by multiplying it by negative one (Raster Calculator) then running the Fill tool on the inverted DEM. Finally, subtract the filled DEM from the ...
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  • 10.8k
5 votes
Accepted

Isochrone along direction map of watershed

Here how "Isochrone Variable Speed" from SAGA works: First you need to load your filled DTM into SAGA Then you need to run the "Slope, Aspect, Curvature" tool as shown in the two ...
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  • 1,406
5 votes

Hydrological Enforcement of culverts in QGIS

I am in doubt this plugins have the clear cut answer for you. but for this kind of analysis there are three plugins which can be useful if you like to tamper. Prerequisites To open these plugins you ...
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4 votes
Accepted

Hydrology toolbox versus Arc Hydro toolbox?

I found an answer to this on the ArcGIS Discussion Forums where Crystal Dorn back in 2007 says more or less the same as @GetSpatial: The core Hydrology tools serve as a foundation for the ArcHydro ...
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4 votes
Accepted

ToolBar icon images missing

Assuming the basic directory structure of the uninstalled plugin is: GraphicGroundwater icon.png icons addBase.png ... resources.qrc ... You will want something like this in ...
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  • 7,586

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