On the calculation of the topographic wetness index. Originally developed in the topmodel framework beven et al. Kernel density uses a cell size equal to the default value, divided by four. How to install the twi python script into an arcgis toolbox. Calculations can be performed using either python or vbscript. The focus is on discussing digital elevation models dems that are the primary data source for the analysis. I am trying to use a flow accumulation fac raster and a slope slope raster to calculate the topographic wetness index twi in raster calculator as. Note that another available script, calculate compound topographic wetness index, is written with avenue. Watershed delineation using taudem tutorial and video.
Topographic wetness index explains soil moisture better than. Cub river example data referenced in arcgis toolbox quick start guide logan river example data referenced in command line functions guide and watershed delineation tutorial eno river example data referenced in arcgis toolbox quick start guide learning material. Pydem is a package for topographic terrain analysis written in python 2. The sections below include examples of using the field calculator. Python is the recommended scripting language for arcgis. It is commonly used to quantify topographic control on hydrological processes. Jenness enterprises has created a number of free, custombuilt applications for the esri arcgis software. By nrcs soil scientists tom davello, joe brennan, and lynn loomis. To calculate the topographic wetness index, tools available from the arcgis spatial analyst license for arcgis pro were used. This may be useful when attempting to model ancient environments. The twi has been used to study spatial scale effects on hydrological processes. When you analyze these weekly vegetation indices you would expect to see a brightening through the growing season. Using this tpi at different scales, plus slope, users can classify the landscape into both slope position i.
In arctoolbox, spatial analyst tools toolbox, map algebra toolset, open the raster calculator geoprocessing tool 74. I know there are a lot of different way to estimate it. Installing the topographic wetness index python script. Twi has been frequently tested with measured sm and other soil properties, revealing high correlations mckenzie and ryan, 1999, guntner et al. Dec 22, 2009 as an important topographic attribute widelyused in precision agriculture, topographic wetness index twi is designed to quantify the effect of local topography on hydrological processes and for modeling the spatial distribution of soil moisture and surface saturation. The topographic wetness index twi was developed by beven and kirkby 1979 within the runoff model topmodel. If the input features are points, then the distances will be measured using the geodesic method. Dec 17, 2011 compound topographic index cti moore et al. I have filled and projected my digital terrain model to obtain. Add the following natural logarithm ln equation in the expression window. The computation of twi is performed using both geographic information systems gis and python, a programing software.
It makes use of an index of hydrological similarity based on topographic data. Id like and easy way to classify the landscape into both slope position i. Installing the topographic wetness index python script into. Hi everybody, i need to calculate the wetness index with arcgis.
Modifying digital elevation models to develop more realistic wetness index layers for soil survey applications. In its basic form, it works by deriving slope and accumulation distribution parameters from a topographic model sorensen et al. One has described the genesis and definition, main types, data sources and available free global dems. I convert to specific catchment area because this is what the original formulation by beven and kirkby called for. Then i run the twi option, in terrain analysis hydrology topographic indices topographic wetness index twi, with the options to convert area to 1cell size and use standard calculation. If digital topographic maps are available in your area, they can provide a good means of evaluating the potential accuracies of your output wetness index images. Topographic wetness indices twis computed from digital elevation models dems are. For those running calculate the topographic wetness index twi 73. Calculating topographic wetness index gis stack exchange. The most frequently used index is the topographic wetness index twi proposed by beven and kirkby 1979. How to calculate topographic wetness index using arcgis. How to calculate topographic wetness index twi, in arcgis.
The resultant filled dem is used to generate a slope raster with the slope tool. Compound topographic indexcatchment wetness index from dtm data. Is there any option to saga to calculate the topographic wetness index. Topographic index maps are grids derived from digital elevation models dems. Feb 16, 2020 twi topographic wetness index spi stream power index. The topographic wetness index, coined by beven and kirkby in 1979, aims to quantify the influence of topography on hydrological processes within a watershed. Conference describing the concept of topographic position index tpi and how it could be calculated weiss 2001. The saga wetness index is, as the name says, similar to the topographic wetness index twi, but it is based on a modified catchment area calculation modified catchment area, which does not think of the flow as very thin film. Majid hojati tehran university, department of gis and remote sensing, tehran, iran majid. Which is the correct way to calculate the topographic wetness. To calculate in arcgis first calculate the slope, flow direction then flow accumulation from dem for your area srtm, 1 arc, is better than aster. Which is the correct way to calculate the topographic wetness index with arcgis.
The file that you download to your working directory will have the name of as11863. Gis topographic wetness index twi susquehanna river basin. Apr 08, 2017 how to install the twi python script into an arcgis toolbox. Digital elevation models in geomorphology intechopen. The vegetation index would be computed for each image. This chapter presents place of geomorphometry in contemporary geomorphology. If nothing happens, download github desktop and try again. You can contribute your data to this service and have it served by esri. Superimpose the wetness index image with the transparency of 60% over the topographic map. Compute simple topographic roughness in arcmap youtube.
For details on the coverage in this map and the users who contributed data for this map via the community maps program, view the list of contributors for the world topographic map. Creating map of topographic index at hubbard brook. I have a big area and i tried many times with saga, the last time i thought the results was ok before saga would crash or g. Then when senescence begins in the fall, you would see the index diminish until the plant is harvested or the leaves are dead at the end of the season.
What is the typical topographic wetness index range of values. Module topographic wetness index twi sagagis module. Compound topographic index anthropology analytical. Jul 29, 2019 the topographic wetness index one step tool is a tool chain, in which the dem is preprocessed with the sink removal tool and the slope is computed using the slope, aspect, curvature tool, then the twi is computed using the following tools in sequence. Topographic wetness index for jemez raster dataset tags topographic wetness index, twi, compound topographic index, cti summary this dataset represents the steady state wetness index computed by d8 and dinfinity flow models and is intended for the visualization and support topographic and geospatial analysis. Modifying digital elevation models to develop more. The dem is corrected using the fill tool to remove sinks i. Topographic wetness index the twi is a physically based index or indicator of the effect of local topography on runoff flow direction and accumulation. Twi values are calculated using the saga gis software university of hamburg, hamburg, germany. Calculate the topographic wetness index in qgis youtube. A terrain ruggedness index that quantifies topographic. Shuttle radar topography mission srtm dem data were downloaded from.
A new topographic index to quantify downslope controls on. One approach to quantify this control is the topographic lnatanb index. A revised version of the topographic wetness index written for arcpython. It takes in digital elevation model dem rasters, and it outputs quantities like slope, aspect, upstream area, and topographic wetness index. The index is a function of both the slope and the upstream contributing area. Evaluating topographic wetness indices across central new york. The terrain analysis process involves combining primary attributes to form secondary attributes. This python script can be imported to create a twi tool. I think this post in the saga gis forum might prove useful in answering your question about how slope is calculated. Twi topographic wetness index spi stream power index. Inputs are the workspace and the input dem, output is the twi layer we chose to fill all sinks due to some small sinks in a lidar redived dem.
Tehran university, department of gis and remote sensing, tehran, iran. In addition, you can calculate area, length, perimeter, and other geometric properties on fields in attribute tables. Calculate field examples help documentation arcgis. Generating wetness indices for watersheds in grass cuosg. Lnfactanslope the resulting output values do not make sense relative to hand calculations. Modifying digital elevation models to develop more realistic. When i launched the compound topographic index function it seems that was running but at the end i couldn t get any output raster. How to calculate topographic wetness index using arcgis youtube. A terrain ruggedness index that quantifies topographic heterogeneity. Flow accumulation topdown, flow width and specific catchment area, topographic wetness. An approach to computing topographic wetness index based on. The topographic wetness index twi, also known as the compound topographic index cti, is a steady state wetness index.
Thus it can provide indirect information on land cover and agricultural potential. They are provided free of charge, but they require that you have access to arcgis software. Which is the correct way to calculate the topographic wetness index. The data for the world topographic map is provided by the gis community. Miscalculated topographic wetness index geonet, the esri. The index is a function of both the slope and the upstream contributing area per unit width orthogonal to the flow direction. Then we focus on landform parameters, starting with primary morphometric parameters, then morphometric. The core primary attributes used for this terrain analysis include flow direction, flow accumulation, and slope. This video shows how to calculate the topographic wetness index twi from a dtm using qgis. This script was written to be used as a tool in arcgis.
Density values are calculated using the algorithm from the kernel density tool in arcgis pro. Title topographic wetness index summary calculates the ratio of the natural log of the specific catchment area contributing area to slope, lnas, or lnatan beta. You can use this procedure for catchments, subcatchments or parcels. Background performing additional processes on digital elevation models is a common practice. A new topographic index to quantify downslope controls on local drainage k. These tools are built to work in all three license levels arcview, arceditor and arcinfo. Which is the correct way to calculate the topographic. The dem of the test area has been processed with grass gis. I have filled and projected my digital terrain model to obtain with the hydrology tool. Topographic wetness index for jemez university of arizona.
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