SWAT: Soil Data for non-US Study Areas

This blog post addresses:

a. Acquiring soil data & soil map for modeling a non-US study area using ArcSWAT

b. Basic interpretation of the imported soil data using freely available online documents

a. Soil Data

Default ArcSWAT database (SWAT2012.mdb) does include the US soil data in usersoil table.

World soil data can be imported from MWSWAT database (mwswat2012.mdb). Therefore, you need to install MapWindow and MWSWAT if you want to have access to mwswat2012.mdb.

After you complete the installation, you can find mwswat2012.mdb under the MWSWAT2012 subdirectory of MapWindow directory (e.g., C:\MapWindow\Plugins\MWSWAT2012).

If you want to bypass this procedure, feel free to download custom SWAT2012.mdb I use in my project that includes the worldwide soil data in user table.

Once you download the custom SWAT2012.mdb, you can simply replace the original file with the newly downloaded custom file.

Soil Map

You can practically download soil maps in raster format for any place in the world from the download section of MWSWAT site. You may need to pre-process the raster file and clip it in a GIS software, which is beyond the scope of this post. You may refer to MWSWAT documents for pre-processing tips.

You will notice that every cell, which is also called as soil mapping unit, in the downloaded raster file has a unique value. The values refer to the specific soils listed in usersoil table of the custom SWAT2012.mdb.

For instance, if the value of a cell is 1736, that cell or soil mapping unit has Vp19-3a-1736 soil. I will explain how we can interpret the abbreviations used in the soil name below.

In ArcSWAT, you will need to link the values in your soil map with the corresponding soil data. The instructions are available in pp. 17-18 of ArcSWAT 2012 User's Guide. I share the simple soil lookup table I wrote for my project as an example.

b. Soil Data Interpretation 

Information on the soil taxonomy is typically required in the technical document preparation phase. Therefore, SWAT user needs to know what the SNAM field abbreviation in usersoil table of SWAT2012.mdb stand for.

The soil data in the custom SWAT2012.mdb is compiled from 1:5000000 scale World Soil Map (version 3.6) of the United Nations Food and Agriculture Organization (FAO).

MWSWAT site includes two documents that can be used for the data interpretation, namely readme.pdf and notes.txt.

FAO has its own web page dedicated to the World Soil Map. Two Excel files (BasicFilesSC.xls and WORLD764.xls) acquired from this web page are particularly useful for the data interpretation purposes.

Below I give an example from my own SWAT project where I extracted the taxonomic information from the abbreviations

Example:

Unique abbreviation for the soil mapping unit: Ao111-2bc-3003

Ao stands for Orthic Acrisols (Reference Used: readme.pdf)

Summary Information (Reference Used: BasicFilesSC.xls)  : Orthic Acrisols as the `dominant soil` constitutes 40% of the map unit (MU). 100% of the map unit has medium texture. 75% of the MU has rolling topography and 25% of the MU has mountainous topography.

Detailed Information (Reference Used: WORLD764.xls): Map unit consists of

20% Orthic Acrisols-2b texture-slope class
20% Orthic Acrisols-2c texture-slope class

30% Dystric Cambisols (Bd)-2b texture-slope class

10% Calcic Cambisols (Bk)-2b texture-slope class

10% RENDZINAS (E)-2b texture-slope class

10% LITHOSOLS (I) 10%  of the map unit

• 5% LITHOSOLS (I)-2b texture-slope class
• 5% LITHOSOLS (I)-2c texture-slope class

Texture, topography/slope classifications can be found in readme.pdf