[GIS] Calculate Topographic wetness index in SAGA

hydrographyhydrologysagatopographic-wetness-indextopography

i am currently working on a project which involves the quantification of the wetness in certain areas. However it produces quite different results from what i expected. I'll describe the steps i have taken.
To do so i used the ASTER global DEMv2 and clipped it to the parts to be examined. I have to clip the raster, because using the original DEM results in various errors saying "memory allocation failed".
ASTER DEM
Then i calculated the slope and Catchement area via the SAGA function Catchment Area (Mass-Flux-Method, parameter: flow-spit method area cell). I also tried the Parallel and recursive method, however the results are mostly the same.
Then i calculated the Topographic Wetness Index, but the results look very mixed (somewhere between a rainbow and random fuzzy bits) and values are always negative.
example of the resulting TWI
I expected that the resulting raster should look like and display index values like the following image in the pdf PDF, page 14.

Has someone here experience with the quantification of topographic wetness in an area and could provide a tutorial how to do it right?
I would appreciate using SAGA or QGIS, but i also have ArcGis 9 at Hand.

By the way:
If i just use the calculate the SAGA wetness index with the clipped elevation data above, SAGA always keeps crashing (i believe because of the large no-data areas, because smaller smarts work)

Best Answer

First, you need to calculate at least the slope. F.ex I have the following data: enter image description here

Then put the correct data as variables to the module: enter image description here

And at last you should get the result: enter image description here

UPDATE

With Catchment Area as input

enter image description here

the results are:

enter image description here

Related Solutions

Understanding Negative Values in Compound Topographic Index (CTI) / Topographic Wetness Index (TWI)

You should not be seeing negative values in the CTI. Since you did not provide a reproducible example I cannot speculate as to why you are getting incorrect results. The expected range is not limited 1-10. The range will be defined by flow accumulation which is influenced by the size of the basins that are accumulating flow. The index does not rely on washed boundaries so the extent of the DEM should not matter.

The common implementation of CTI can be shown as: CTI = ln(a / (tan (beta)) where; a=Upland contributing area [(flow accumulation + 1 ) * (cellsize)]and beta=slope in radians.

Here are the raster algebra steps to calculate CTI (you will have to translate syntax to your version of ArcGIS):

cellsize=30
fd = flowdirection(dem)
sca = flowaccumulation(fd)
slope = ( slope(dem) * 1.570796 ) / 90
tan_slp = con( slope > 0, tan(slope), 0.001 )
sca_scaled = ( sca + 1 ) * cellsize
cti = ln ( sca_scaled / tan_slp )

Calculating Topographic Wetness Index – Choosing the Right Algorithm

I think this post in the SAGA GIS forum might prove useful in answering your question about how slope is calculated:

https://sourceforge.net/p/saga-gis/discussion/354013/thread/27ecdc6b/

Also, based on my understanding of TWI (as a PhD hydrology student involved in hydrologic modeling), the D-Inf (Tarboton), MFD-md (Qin), DEMON (Costa-Cabral), and MFD (Quinn) with the exponent p=1.1 (Freemann) are the best options for determining the accumulation area 'a' in the TWI calculation.

I think Sorensen's work, and Qin's work, lends credence to my own semi-professional opinion. However, Qin's improved algorithm (MFD-md) hasn't been as throughly tested and used as much as the others have.

When I have used SAGA to calculate TWI, I first calculate the slope using the slope commend in the Terrain Analysis / Morphometry / Slope, Aspect, Curvature module, using the default algorithm mentioned in the forum post. Then I calculate catchment area using the Terrain Analysis / Hydrology / Catchment Area / Catchment Area (Parallel) choice, using either the MFD algorithm or the D-INF algorithm with 1.1 as the convergence factor (p=1.1, from Freeman).

Then I run the TWI option, in Terrain Analysis / Hydrology / Topographic Indices / Topographic Wetness Index (TWI), with the options to convert area to "1/cell size" and use standard calculation. I convert to specific catchment area because this is what the original formulation by Beven and Kirkby called for. As for the difference between the "Standard" and "TOPMODEL", I am not sure what it is - looking into that myself right now.

Reference page 106 onward of the linked pdf for more specific help: http://sourceforge.net/projects/saga-gis/files/SAGA%20-%20Documentation/SAGA% 20Documents/SagaManual.pdf/download

I forgot to add, this all assumes that the DEM has been preprocessed by filling sinks. That is another complicated topic, with two (main) separate options.

I hope this helps!

Tom

P.S. Edits for @reima:

This is something I have only recently dug into, and I can admit I don't think I've reached the bottom yet! I prefer the method of Lindsay and Creed, the minimum impact approach that chooses breeching or filling based on minimizing total topographical impact (officially named "Impact Reduction Algorithm" - IRA) which I thought was implemented in his Terrain Analysis Software tool (formerly TAS, now WhiteBox GAT - link: http://www.uoguelph.ca/~hydrogeo/Whitebox/).

However, even his tool seems to implement other filling schemes:

The sink/depression filling algorithm (basic, but insanely fast) by Wang and Liu (2006) - which I don't believe operates in the IRA manner, but similar to the way ArcMap fills sinks/depressions, just straight up without any breeching.
And the sink/depression filling of Planchon and Darboux (2001), which floods a DEM and then removes the water bit by bit - it can enforce a slope on the filed area, which I think might improve TI calculations.

ArcMap has a new "de-pitting" add-on (http://blogs.esri.com/esri/arcgis/2013/03/05/optimized-tool-for-dem-pit-removal-now-available/) that seems similar to Lindsay and Creeds IRA, but I haven't read the cited paper yet to determine how similar. This method might be worth a look.

I'm also interested in scrutinizing my assumption that TI calculations need filled DEMs. I have three different sized watershed DEMs (<100 sq km, 100-1000 sq km, >1000 sq km), clipped using a shape file from 10 m NED data. These are not filled, since the shape file already provided the watershed delineation. I am going to run the SAGA GIS TI calculation (MFD, p=1.1) on all three watersheds, on both filled and unfilled DEMs, using ArcMaps filling scheme (old and new), and the Wang and Liu algorithm (in Whitebox, maybe in SAGA), and the Planchon and Darboux algorithm (in Whitebox, maybe in SAGA). I will also be calculating the TI values using the TI calculation embedded in my hydrological model.

If you want, I can share these results with you. I might not have them for a month or so though, as I have other more pertinent research that my focus is currently on, but I need to refine my TI calculation process by mid May at the latest.

Best Answer

Related Solutions

Understanding Negative Values in Compound Topographic Index (CTI) / Topographic Wetness Index (TWI)

Calculating Topographic Wetness Index – Choosing the Right Algorithm

Related Question