Installing the topographic wetness index python script. Kelly, i have a arctoolbox in alphatest that has cti implemented. Topographic wetness index urban flooding awareness act. The indices read the metadata from the image to check the band names. Topographic exposure, also known as topex, is a measure of surrounding landforms and how they effect wind. Topographic wetness indices twis computed from digital elevation models dems are means to forecast the. Twi is calculated by evaluating the flow direction, flow accumulation, slope, and various. For those running topographic index cti float is a steady state wetness index. Since its introduction, the twi concept has been integrated into many popular hydrologic models e.
I am trying to create a topographic wetness index twi raster in r from a dem that i have. How to calculate topographic wetness index using arcgis duration. I have restructured the raster calculator expression as below and got rid of negative values. They are provided free of charge, but they require that you have access to arcgis software. On the calculation of the topographic wetness index. In arctoolbox, spatial analyst tools toolbox, map algebra toolset, open the raster calculator geoprocessing tool 74. A new method for the determination of flow directions and upslope areas in grid digital elevation models, water ressources research, vol. The index is a function of both the slope and the upstream contributing area per unit width orthogonal to the flow direction. Topographic wetness index geonet, the esri community. Calculate the topographic wetness index in qgis youtube.
The topographic wetness index twi, also called compound topographic index cti, is a steadystate wetness index. Identification of surface water storing sites using. How to calculate topographic ruggedness index in arcgis desktop other resources. Use radians as slope unit as the twi module expects the slope expressed in radians. 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. Topographic wetness index abstract the topographic wetness index, coined by beven and kirkby in 1979, aims to quantify the influence of topography on hydrological processes within a watershed. It is commonly used to quantify topographic control on hydrological processes. The training data are coupled with the merged predictor variables to train a random trees breiman, 2001 model. Thus it can provide indirect information on land cover and agricultural potential. Topographic wetness index urban flooding awareness act action. How to calculate topographic ruggedness index in arcgis. The units of upslope contributing area are m2m or m in fact a is as or specific contributing area, contributing area per unit contour width so thats why you multiply by cell resolution 30 m and not by cell area 3030 a more correct way to calculate this would be to take into account. I think this post in the saga gis forum might prove useful in answering your question about how slope is calculated.
Topographic wetness index that is purely based on topography is a function of the upstream contributing area and slope 1719. What is the typical topographic wetness index range of. I have derived topographic wetness index twicti with arcgis 10. When they find a match, the index will be automatically applied. Compute simple topographic roughness in arcmap youtube. How to deal with no data values in topographic wetness index. Be certain that the input image is from a sensor that has the proper bands wavelengths and range to support the index of choice. This python script can be imported to create a twi tool. You can use this procedure for catchments, subcatchments or parcels. Esri could add this as a function to a mosaic dataset or raster. Training data are derived from the ground truth data. I know there are a lot of different way to estimate it. 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. Inputs are the workspace and the input dem, output is the twi layer.
The code ran very quick and produced an output that looked correct. A new topographic index to quantify downslope controls on local drainage. As such, in this paper, the steadystate topographic wetness index was used to represent the spatial distribution of water flow and water stagnating across the country. 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. This video shows how to calculate the topographic wetness index twi from a dtm using qgis. Arcgis or qgis, then using several tools within the program to calculate the slope, flow direction, flow accumulation, tan of slope, and ultimately twi using simple. Twi topographic wetness index spi stream power index.
Download the pamap program tile index northsouth shapefiles here. What is the best way to calculate topographic wetness. 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. The one caveat is that it is written around arcpy so is only available for arcgis 10. Implement topographic position index as a tool or toolset in spatial analyst tools. Description the topographic wetness index was computed using upstream accumulated areas or flow accumulation a and local slope b as twi lnatanb. Requires production mapping and arcgis spatial analyst extension. Topographic wetness index the twi is a physically based index or indicator of the effect of local topography on runoff flow direction and accumulation. Therefore, from user point of view, the choice of the best software in. Gis topographic wetness index twi susquehanna river basin. Which is the correct way to calculate the topographic wetness index. Watershed delineation using taudem tutorial and video. We chose to fill all sinks due to some small sinks in a lidar redived dem. Symbology used for forsythe ii project area topographic wetness index maps in arcgis.
Twi is calculated by using a digital elevation model raster layer cell size 9m2 in a geographical. I tried with saga, but, whenever i tried to import my. The formula used for calculating twi at a point is. I found a site with 12 different types of ways to do ruggedness. The computation of twi is performed using both geographic information systems gis and python, a programing software. This script was written to be used as a tool in arcgis. Requires defense mapping and arcgis spatial analyst extension. Thus, the index may not capture conditions in areas where there are other more dominant processes governing the flow. Calculation of the slope and specific catchment area sca based topographic wetness index twi references. 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. I expected that the resulting raster should look like and display index values like the following image in the pdf pdf, page 14.
The topographic wetness index twi, also known as the compound topographic index cti, is a steady state wetness index. The index is a function of both the slope and the upstream contributing area. Tool topographic wetness index twi sagagis tool library. Topographic wetness index is calculated as ti lnupslope contributing area tanof slope angle the upslope area is the flow accumulation map, you can calculate this map using arcgis hydrology. Add the following natural logarithm ln equation in the expression window. How to calculate topographic wetness index twi, in arcgis. The cti is a function of both the slope and the upstream contributing area per unit width orthigonal to the flow. Jenness enterprises has created a number of free, custombuilt applications for the esri arcgis software. You can calulate such a slope grid with the terrain analysismorphometry slope, aspect, curvature module. Which is the correct way to calculate the topographic. These tools are built to work in all three license levels arcview, arceditor and arcinfo. Compute simple topographic roughness in arcmap ramon arrowsmith. A physicallybased variable contributing area model of basin hydrology.
A physically based variable contributing area model of basin hydrology. How to calculate topographic wetness index using arcgis. Earlier, it was propagating negative values which were mainly confined to ridges. How to calculate topographic wetness index using arcgis youtube. Module topographic wetness index twi sagagis module. Here is a nice explanation provided by tom vanwalleghem to a similar thread on the gis stack exchange. The preprocessed dem is used to calculate the predictor variables. If you do not have access to arc 10 let me know and i can send you an aml, and directions, for calculating cti in workstation arcinfo. Calculating topographic wetness index choosing from different. However, it generally correctly predicts the accumulation of flow in topological. The free and extremely powerful open source gis system. I have filled and projected my digital terrain model to obtain with the hydrology tool. This post shows how to generate a topex model based on a digital elevation model. Introduction soil moisture is a key variable controlling hydrological and biogeochemical processes buchanan et al.
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