Watershed Delineation Tool
Watershed delineation tool in eRAMS processes and analyzes Digital Elevation Model (DEM) and DEM-driven rasters to delineate watershed using TauDEM5 (Tarboton, 2013) and GDAL/OGR in CSIP (David et al. 2015) platform. Watershed delineation tool provides not only watershed boundary but also useful hydrologic information for a defined boundary to users. There are three options such as ‘advanced’, ‘basic’ and ‘watershed extraction’. In an advanced option, user can execute any individual steps related to watershed delineation. If users want to use any particular tool or customize their inputs in any individual step, advanced option is recommended. Basic option allows users to run individual procedures at once. If users want to execute every step automatically using default inputs, basic option is recommended. Watershed extract option implements only ‘Move Outlets’ and ‘Gage Watershed’ procedures for rapid watershed delineation. If users want to create only watershed boundary rapidly, watershed extract option is recommended.
Using basic option, user can acquire same outputs from the advanced at once. The input data are DEM and outlet point layer. To fully automate the procedures, two conditions have been given. At first, the outlet point moves to the modified Peuker Douglas stream pixels automatically. Next, 10 pixel is used as threshold for ‘stream raster – threshold’ if drop analysis can’t find threshold value (Example: watershed is too small). The merit of this option is the automated execution of every procedure related to the watershed delineation.
Watershed Extraction Option
The output of this option is the watershed polygon vector. This option only runs ‘move outlets’ and ‘gage watershed’ procedures using NHDPlusV2 (US EPA, 2015) flow direction raster. The inputs are “outlet points” and “the area of analysis”, which can be defined by the bounding box. This tool has a great advantage of rapid delineation of watershed using only essential modules for watershed delineation.
The individual procedures related to watershed delineation are 1) Fill Sinks, 2) Compute Flow Direction, 3) Compute Flow Accumulation, 4) Grid Analysis, 5) Stream Network – Modified Peuker Douglas, 6) Move Outlets, 7) Weighted Flow Accumulated Area, 8) Stream Network – Drop analysis, 9) Stream Raster – Threshold, 10) Extract Streams and Channel Network, and 11) Gage Watershed, which are described as follows.
Fill Sinks – The output of this tool is a DEM, whose pits are removed. The input for this tool is Digital Elevation Model (DEM). Users can upload their own DEM or use NHD+ v2.0 DEM (US EPA, 2015) provided in eRAMS. This tool removes pits to create hydrologically reasonable DEM for watershed delineation
Compute Flow Direction – The output of this tool is flow direction raster. Using pits-removed DEM as an input DEM, flow direction is calculated at each pixel. The value of 1 in flow direction raster means stream flows to the east direction. The direction increases by 45 degrees counter clockwise as direction value increases by 1. Also, slope raster is created at the same time.
Compute Flow Accumulation – This tool calculates flow accumulation output raster, which represents the total contributing numbers of stream pixels summed at the target pixel based on flow direction raster. Also, this raster is recommended to be used as a ground truth for outlets selection in large scale watershed for later step – ‘Move Outlets’
Grid Analysis – This tool creates three output raster files such as the longest flow path raster, total flow path raster, and grid network order raster. The pixel value of the longest flow path raster represents the length of the longest upstream flow path from the cell. The pixel value of the total flow paths raster represents the total length of the upstream paths. Also, total flow path raster is recommended as a ground truth for outlets selection for mid – large scale watershed in later step – ‘Move Outlets’. The pixel value of the grid network order raster shows the Strahler order (Strahler, 1952, 1957) number.
Stream Network – This tool creates modified Peuker Douglas stream raster. The original ‘peukerdouglas’ module is based on Peucker Douglas (Peucker and Douglas, 1975) algorithm. However, the original algorithm turns off flat pixels during stream pixels calculation. This seems to underestimate stream pixels in flat area (Example: the location where river meets lake). Underestimated number of stream pixels can 1) fail to move outlets to stream pixels 2) move outlets to stream pixel located in unreasonably far distance. Therefore, Taudem 5’s ‘peukerdouglas’ module has been edited to add the pixels in flat area with the flow accumulation values larger than the defined threshold to the original Peuker Douglas stream pixels. The threshold has decided as the average of flow accumulation on the original Peuker Douglas stream pixels. It has been found that modified Peuker Douglas tool properly includes additional stream information and has improved the results of moving outlets. The modified Peuker Douglas stream raster can be considered as initial approximation of streams. Rigorous stream raster will be created in step i) Stream Raster – Threshold.
Move Outlets – The output of this tool is the vector of outlet points, which has been moved to the stream pixels of input stream raster. Outlet points have to be located on stream pixels for successful watershed delineation. Therefore, this tool moves outlet points on non-stream pixels to the first stream pixel which outlet points meet when they move along flow direction
Weighted Flow Accumulation Area – The output of this tool is the weighted flow accumulation raster. This tool sums up the number of pixels of weight grid (modified Peuker Douglas). The boundary of this raster is consistent with the boundary of watershed because one of conditions is that streams on contributing pixels should eventually flows into the outlet. The weighted flow accumulation raster will be used as input data for rigorous stream raster.
Stream Network: Drop Analysis – The output of this tool is a table including the optimal value for rigorous stream calculation. The threshold value is found by t-test, which evaluates the significant difference between the first and higher order streams created based on the threshold assumption. Also, output table shows related statistics such as number, mean, standard deviation of drops, the ratio of 1st order stream and t values.
Stream Raster: Threshold – This tool calculates rigorous stream output raster by choosing pixels, whose values are larger than the optimal threshold from weighted flow accumulation raster. In addition, user can choose other threshold values by clicking another row in drop analysis output table if more rigorous or lenient criteria is required.
Extract Stream and Channel Network – The outputs of this tool are stream network and sub-watersheds vectors. Also, the table of stream network has been updated for the attributes to be consistent with NHDPlusV2’s file format – plusflowlinevaa.dbf (US EPA, 2015). Sub-watershed attributes includes id of the stream segments to which each sub-watershed drains.
Gage Watershed – The output of this tool is a watershed polygon vector for a defined outlet. Multiple watershed polygons will be created in an output vector if more than one outlet is defined.