3 Downloading and compiling TRIGRS

3.1 Download the source code

Download the official 2.1.0 release from the USGS GitLab: https://code.usgs.gov/usgs/landslides-trigrs/-/releases/v2.1.0

Extract the .zip into a working folder, for example C:\TRIGRS\landslides-trigrs-2.1.0\. The src\ folder contains four programs:

Program	Purpose
`TRIGRS`	Main model: transient infiltration + factor of safety
`TopoIndex`	Topographic preprocessor: runoff routing and array sizing
`GridMatch`	Checks whether the input grids are congruent with each other
`UnitConvert`	Unit conversion of the grids

3.2 Compile

Open PowerShell in the src\ folder of the extracted source code and run the four blocks below. The order of the files matters (modules first) — copy and paste them exactly:

# TopoIndex
gfortran -O2 -w -static -o topoindex.exe TopoIndex\ssizgrd.f95 TopoIndex\tpindx.f90 `
  TopoIndex\nxtcel.f90 TopoIndex\isvgrd.f TopoIndex\mpfldr.f TopoIndex\rdflodir.f `
  TopoIndex\sindex.f TopoIndex\slofac.f TopoIndex\srdgrd1.f

# TRIGRS (serial version — handles small and medium watersheds in seconds)
gfortran -O2 -w -static -o trigrs.exe TRIGRS\grids.f95 TRIGRS\input_file_defs.f95 `
  TRIGRS\input_vars.f95 TRIGRS\model_vars.f95 TRIGRS\dsimps.f95 TRIGRS\iverson.f95 `
  TRIGRS\pstpi.f95 TRIGRS\satfin.f95 TRIGRS\savage.f95 TRIGRS\steady.f95 TRIGRS\trini.f95 `
  TRIGRS\unsinf.f95 TRIGRS\ivestp.f95 TRIGRS\pstpf.f95 TRIGRS\rnoff.f95 TRIGRS\satinf.f95 `
  TRIGRS\smallt.f95 TRIGRS\svgstp.f95 TRIGRS\unsfin.f95 TRIGRS\unsth.f95 TRIGRS\ssizgrd.f95 `
  TRIGRS\svlist.f95 TRIGRS\trigrs.f90 TRIGRS\flux.f90 TRIGRS\prpijz.f90 TRIGRS\svxmdv.f90 `
  TRIGRS\svijz.f90 TRIGRS\dzero_brac.f90 TRIGRS\calerf.f TRIGRS\dbsct.f TRIGRS\derfc.f `
  TRIGRS\irdgrd.f TRIGRS\irdswm.f TRIGRS\isvgrd.f TRIGRS\roots.f TRIGRS\srdgrd.f `
  TRIGRS\srdswm.f TRIGRS\ssvgrd.f

# GridMatch
gfortran -O2 -w -static -o gridmatch.exe GridMatch\ssizgrd.f95 GridMatch\gridmatch.f90 `
  GridMatch\srdgrd.f GridMatch\ssvgrd.f

# UnitConvert (this one needs the -fdec flag because of a legacy Fortran format in the code)
gfortran -O2 -w -static -fdec -o unitconvert.exe UnitConvert\unitconvert.f90 `
  UnitConvert\srdgrd.f UnitConvert\ssvgrd.f

At the end, the src\ folder will contain topoindex.exe, trigrs.exe, gridmatch.exe, and unitconvert.exe. Keep them in a bin\ folder — they will be copied into the folder of each project/run.

About the parallel version

The Makefile that comes with the code uses mpif90 (MPI) and also compiles a parallel version (trigrs_p), useful for very large areas. It is not needed for this short course — the serial version above is sufficient.

3.3 What the compiler options mean

Option	Effect
`-O2`	Optimization (faster code)
`-w`	Suppresses style warnings from the legacy code (harmless)
`-static`	Embeds the libraries in the executable — this is what avoids the DLL problem
`-fdec`	Accepts legacy DEC extensions (needed only for UnitConvert)