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Step 1 - Establishing the Domain

This page described the initial steps for establishing a study domain for the VIC model. This is the very first step that must be completed in setting up a new VIC model simulation.  Once this process is completed, the other required setup steps can be complete in any order, however, all input files must exist before the model can be run.

This document concludes with links to specifics for completing setup of all major inputs to the VIC model.  Steps here assume that the user is working with ArcGIS and has the ArcHydro toolset installed.  Most of the steps involved should translate directly to geoprocessing algorithms available in QGIS, SAGA and other commonly available, non-proprietary GIS packages.

The VIC model can function on gridded domains both unprojected (geographic) latitude and longitude grids and equal area grids (e.g., Albers equal area).  Because there are no horizontal exchanges between VIC model grid cells, the model can also be set up on individual points and on irregular shapes (e.g., HUCs).  The instructions in this document should apply to all setups over a spatial domain whether gridded or irregular.

  1. Define the projection for your domain. 

    • No projection (geographic) means that the area of grid cells changes as you move away from the equator.
    • Equal area projections preserve the area of the grid cell at the expense of other factors.
    • In general, matching your setup to existing data projections or other project constraints can substantially reduce the setup time and reduce the need for changes later.
    • In ArcGIS you can add the selected projection to your list of favorites, which makes it much easier to find and reuse it.
    • One thing to note is that many of the pre-processing steps (delineation of the watershed, calculation of Dsmax, setting up the GIS routing model) will require analysis in an equal area projection, even if you have chosen to use no projection for the model setup.

  2. Define the extent and resolution of your domain.

    • The extent and resolution should be defined in the projection selected for the model simulation.

      • The extent should include the entire drainage area and be defined to the outside edge of the grid cells. 
      • Keeping the domain as small as possible will help minimize setup time, but making it too small will require redoing many of the early steps to expand the extent.  Reducing the extent can be done at any later time by clipping the spatial domain, a relatively simple process.
      • If working with irregular VIC elements rather than a grid the resolution of the simulation is not important at this stage of setup.

    • If using a geographic grid, you can create a rectangle polygon layer in ArcGIS using the following steps

      • Create an Excel workbook (.xls not .xlsx) with at least three columns: ID, Longitude, and Latitude.
      • This file should have coordinates for all four corners of the rectangular domain.
      • Save the file and close Excel (ArcGIS and Excel will not open the file if it is open in the other program)
      • Find the XLS file in ArcCatalog and open it to find the internal worksheets.
      • Drag the worksheet with your polgon coordinates to an ArcMap project that is in in the correct reference frame - if the file provides coordinates in latitude and longitude then the ArcMap project MUST be in a geographic/unprojected state.
      • Right-click on the file and select "Display XY Data...'.
      • Make sure that the 'X-field' and 'Y-field' boxes are filled in with the correct columns (should be automatic if you use the column headers given above). I often get an error about not having an OBJECT-ID, even when I label a column in the file, so just click the 'OK' button and move on.
      • Export the contents of the file into a geodatabase (gdb) (right-click -> Data -> Export Data) using the same projection as the ArcMap data frame.
      • Convert the points to a polygon, which will be called the Analysis Region in the rest of the documentation.
      • The feature can now be exported as a shape file, if desired (right-click -> export)
      • This feature / shapefile can now be used to clip all other GIS data sets to the full domain.  By using a consistent clipping mask, you minimize the risk of clipped layers having slightly offset corners and thus grid cells not lining up directly which results in unexpected problems during the rest of the analysis.

    • If using an equal area projection, you can follow the same steps outlined above, but determining the latitude and longitude of the corners of the rectangle is more difficult.  Some suggestions for working around this:

      • You can create a polygon bounding box using ArcEditor tools.
      • You can use the 'i' information/investigate tool to get coordinates from a correctly projected raster file.  Make sure that you get those coordinates in latitude and longitude, and that you get the edges of the pixels at the correct resolution.

  3. Reproject and clip all data sources to match the Analysis Region

    • Yes, ArcMap will reproject data on the fly to make it 'look' like the other data layers, but you need to actually project the data into the correct projection for your analysis to be correct.

    • Data file should include:

      • Digital elevation map (DEM) at a higher resolution than the VIC model
      • Land use cover map at the same resolution as the DEM
      • RIver network (optional but recommended) used to burn the stream network into the DEM before delineating the watershed (see ArcHydro analysis)
      • Soils data
      • Political boundaries (optional but very useful for clarifying the location of your study domain)

    • This task is best to do within a new ArcGIS workspace (folder) so that all of data for your study domain is in one location and is easily archived.

    • I suggest that you add  '_AR' to the file to indicate that it has been clipped tot he Analysis Region, and that you add a short name for the project, e.g. = '_GCS' for geographic, to the file name to indicate that it has been projected.  This will simplify the task of keeping track of files throughout the processing, especially if you have to switch projections or modify the study domain.

Developing the Cell Number File

The cell number file is critical to all of the pre-precessing steps, so make sure it is right before your done with this step.  The cell number file must be defined so that each VIC solution element (grid cell, HUC, etc) has a unique number.  This number will be used throughout the pre-processing and by the VIC model to element parameters to their spatial location without having to keep track of latitude and longitude.

Note

Note that cell numbers do not need to be continuous within the model domain, just unique. Therefore you can use any available data layer that meets that criteria to serve as the cell number file.  For example, if you are using a subset of domain established by someone else their numbered cells can be used. If you are using HUCs to define your simulation domain, you can also use the HUCs themselves as your cell numbers, as long as you include enough digits for them to be unique.

If you need to create your own cell number file, here are some suggestions:

  1. If you have a grid file at the correct resolution and extent, then you can export the grid to an ASCII raster file and write a script to replace each value with an integer counter.
  2. These instructions generate a numbered grid within ArcGIS, if you use the correct grid cell size and extent when generating the fishnet the resulting grid will be correct.
  3. Within ArcGIS you can make use of Python, this page includes an example script for creating an attribute based on the FID of a vector layer.

Be sure to check that the grid cell size and extent are EXACTLY what you wanted them to be for your simulation before moving on to the next step. If this layer is off, then all other steps will be compromised.

Delineating the Watershed

This is best done using the ArcHydro toolset, since it will manage the files for you and guide you through the process of delineating the watershed.  Documentation for using ArcHydro is available on-line from the ESRI Support center and is attached here.  There is also a simpler version written by Dr. Merwade that focuses on the minimum steps required for delineating a watershed that is posted here. Most of these steps can also be completed outside of ArcGIS and without the ArcHydro tools, but you should complete them in the order described.

To complete the setup of the routing network, you should complete the Terrain Processing document{.external-link rel="nofollow"} through Batch Watershed Delineation, you do not need to complete the last two tasks in the document.

A few notes on processing:

  • Watershed delineation is best done in an equal area projection, or UTM.  If you are working with an unprojected (geographic) simulation grid, then you should identify an appropriate equal area projections for your hydrologic analysis and add it to your favorites list in ArcGIS so that it is easy to find.
  • Complete steps until you have the watershed and its subcatchments defined.

Continuing the Pre-processing

Now that you have established your domain and have the watershed delineated the order of the next steps is for the most part not important, except that the soil parameter file requires some information about the regional climate so it is useful to have the climate data processing complete before moving to the soil file. All of these steps must be completed before you can start your VIC model simulation. Links to all pre-processing steps can be found at the top of this page.

Exporting a feature class / vector layer attribute to a file http://resources.arcgis.com/en/help/main/10.1/index.html#//005p0000003v000000