Fastscape_Set_BCsubroutine (https://fastscape.org/fastscapelib-fortran/#_fastscape_set_bc). Note that a bug has been fixed recently regarding this (https://github.com/fastscape-lem/fastscapelib-fortran/pull/34). It should work as expected using the last release 2.8.2.
g(bedrock dimensionless deposition/transport coefficient)? For
g > 1the number of iterations may grow rapidly, which is a limitation of the implicit numerical scheme used to solve the equation. Setting lower precipitation values (which thus impact the values of "drainage area") may amplify this effect (because of the G / A ratio in the PDE solved by fastscapelib-fortran).