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Using the GIS Routing Model
The model itself is relatively simple and faster to run than the original Lohman routing model. This is because most of the computational power is diverted to the initial GIS processing of the input files. Note that the GIS routing model input files are specific to the topography and land use used to create them. Additionally, the unit hydrograph files built on GIS input files are specific to the time step used to generate them. The GIS routing model is capable of routing hourly to daily output from the VIC model, but changing the time step being processed requires rebuilding the routing model input files.
Before working on the GIS routing model input files, you must delineate the watershed. It is suggested that you make use of the Arc Hydro toolset (http://resources.arcgis.com/en/communities/hydro/01vn0000000s000000.htm), as the GIS routing model toolset is setup to use the output layer names from Arc Hydro by default. If you use another method to delineate the watershed, then you will need to change the defaults in each GIS routing tool to complete processing.
Notes on process:
- Delineation should be completed with a projected DEM. If horizontal units are not in meters (preferred) or feet then the routing model will not be able to compute discharge in correct units.
- Unless you are running on a system with ArcGIS installed locally and with significant local hard drive and RAM storage, it is suggested that you use a moderate resolution DEM. DEM resolution should be finer that the VIC model simulation resolution to capture within cell flow paths, but using too high a resolution DEM can result in long processing times and failures due to memory or network problems. For example, if running the VIC model at 1/16th degree (~6 km) resolution, a 30 meter DEM may prove too difficult to process, while a 300 meter DEM will provide suitable results and resolution with substantially fewer processing issues.
- Consider the resolution of your land use data when picking the DEM resolution. The GIS routing model uses land use data in determining the roughness (Manning N) of overland flow. if the DEM is finer resolution than the land use data, then all you gain is additional granularity in flow path time, you will not gain additional information on land use impacts.
This toolbox helps walk you through the steps required to develop the input files required for the GIS routing model. In order to complete this processing, you will need to have the following layers in an ArcMap Project file.
- High-Resolution, Projected Data Layers
- Layers must be projected to the same datum and projection with horizontal units of meters (preferred) or feet
- Raster layers must be finer resolution than VIC simulation cells, and all raster layers must have the same cell size
- ArcHydro / Watershed delineation output
- Flow Direction (fdr)
- Flow Accumulation (fac)
- Stream network raster (str)
- Watershed polygons (Watersheds)
- Land use / land cover map
- Feature (polygon) file of VIC simulation cells, where one attribute is unique for each simulation cell (typically created by converting VIC cell number raster into a feature class, and then projecting to match this project.
- VIC Model Resolution Data Layers
- Layer projection must match the desired VIC model simulation (unprojected/geographic, or projected) as they will be used as a template for creating the GIS model input files for the specific simulation.
- For output of grided routing model input files
- Raster cell number layer
- For output of non-gridded routing model input files
- NOTE: This method is not currently supported
- Feature (polygon) layer of cell numbers
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Compute the Shreve stream order number for the watershed. This step computes the Shreve streamorder for every grid cell in the raster layer. The stream order is used to scale the Manning's number for the channels, so DO NOT use the Strahler stream order as it will change the channel roughness results.
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Compute the slope of land and channel surfaces. This step calculates a slopes for the land surface, except along stream channels where it computes the slope of the stream channel. All output is to a single raster layer filled with slopes values for the land surface or channel based on the channels defined in the ArcHydro stream network raster file (default: Str). This is NOT the same as output from the ArcHydro slope calculation. The ArcHydro algorithms does not compute slopes along stream channels, only for the land surface. Use this tool to produce the correct slope raster for the rest of the routing model setup.
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Compute the Hydraulic Radius. In this step you will use the flow accumulation raster to estimate the hydraulic radius of (channel) flow across the watershed. The hydraulic radius can be estimated based on default parameters for 2 year, 10 year or 100 year return flows. Generally, a flood frequency with 2-year return period is chosen for normal floods. Additionally, the hydraulic radius distribution can be set by using the depth of flow, channel width and drainage area at two locations along a stream channel within the delineated area. Resources from the USGS and River Forecast Centers, along with direct observation, can be used to establish these relationships.
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Compute Manning's N for Land Surface and Channel. In this step you will calculate Manning's N for all pixels in the high-resolution DEM. Mannings N is computed first for all hillslopes using the local land use class to determine surface roughness. Then Mannings N is computed for the stream network. For the stream channel Mannings N values are scaled between the maximum and minimum Shreve stream order, where maximum Shreve order (farthest downstream channel) will have the lowest Mannings N, and lowest Shreve stream order (1 = headwater streams) will be assigned the highest Mannings N.
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Compute Flow Velocity. In this step the tool uses the previously calculated slope, hydraulic radius and the Mannings N roughness to compute the flow velocity for the entire analysis region. Because slopes, radius and roughnesses are calculated differently for hillslopes and channels, the calculated flow velocities will reflect the differences in flow regime.
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Estimate Travel Time in Hours. This step uses the velocity and flow direction raster files to compute the flow from each point in the raw DEM to the outlet(s) in hours. Travel time is computed for all grid cells, with minimum travel times (0 hours) set for the points that flow out of the set of clipped raster files.
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Extract Routing Model Parameters for Each Watershed to be Routed. Here the parameters required for the routing model are extracted for each delineated watershed in the provided watersheds layer (default: Watersheds from Arc Hydro). The feature layer of VIC simulation cell number area is used to produce zonal statistics from the projected dataset, and tables are written relating the VIC simulation cell number with the metrics. Metrics include the mean and standard deviation of travel time to the outlet from each VIC model simulation cell, and fractional contribution of each VIC model simulation cell to discharge at the selected outlet.
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Produce Routing Model input Files for Gridded Model Application This is the method required for the original Fortran routing model code. It exports all required input files for the GIS routing model as ASCII raster files.
The following steps must be completed before the routing model can be run.
Convert the Contributing Fraction File. Convert the ASCII raster file containing the contributing fraction for each grid cell into a list as required by the original Fortran model. Note that the first two columns are not used by the original model. Third columns is the name of the flux file for each VIC simulation cell, and the fourth column is the fraction of the simulation cell that contributes to discharge for the current watershed.
# Sample File Contents
1 1 fluxes_38.65625_-86.21875 0.046000
2 2 fluxes_38.65625_-86.15625 0.315790
3 3 fluxes_38.65625_-86.09375 0.596795
4 4 fluxes_38.65625_-86.03125 0.527950
5 5 fluxes_38.65625_-85.96875 0.302845
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The ArcGIS toolbox currently develops an ASII raster file, so a simple script such as the one provided here can be used to convert that file into the format required by the routing model.
# Sample script for converting the ASCII raster file to the required list file
arcgrid_to_xyz.script <watershed>_fract.txt | awk '{ if ( $3 > 1 ) $3 = 1; printf( "%i\t%i\tfluxes_%.5f_%.5f\t%f\n", NR, NR, $2, $1, $3 ) }' > fract_<watershed>.list
Generate Unit Hydrographs. The program iuhcell is applied to the GIS routing model input files for the current watershed and builds a unit hydrograph file specific to the setup and the selected routing time step. Time step should match the VIC model output files.
**# Command line requirements for the iuhcell program**
iuhcell <mask grid> <mean time grid> <std time grid> <out file> <routing time step (hrs)>
Currently there are two versions of the routing model, conv3h and convdaily. The former will route VIC model output files with a 3 hour time step, the latter will route daily data from VIC model output files. The routing model can be run any number of times for the same watershed, using any number of VIC model output file sets - as long as the output file time step does not change. To change the time step of routing, you must rerun the step to build the unit hydrograph. To change the effects of land use or channel you must redo the ArcGIS pre-processing steps.
# Command line requirements for running the routing model
convdaily <Flux Path> <watershed>_mask.txt <watershed>_iuh.txt fract_<watershed>.list <watershed>_daily_flow.asc