This is a quote from the SINMAP 2.0 website: "SINMAP has its theoretical basis in the infinite plane slope stability model with wetness (pore pressures) obtained from a topographically based steady state model of hydrology."  Rainfall intensity would not be a part of a steady state model of hydrology. 

I think the part of the SINMAP model that you are trying to work with is:

"4.8 Calibration Methods

"4.8.1 Adjusting parameters in the DEM view

"The bottom group on the SINMAP Stability Analysis menu has two items ‘Calibration Parameter Adjust’ and ‘Update Grids and Lines.’  These two menu items allow the user to change the values of soil parameters in the calibration regions table and then update the Stability Index and Saturation grids, and SA plot lines based upon these changes."  (from users manual http://hydrology.usu.edu/sinmap2/sinmap2.PDF )

This is where you would adjust the parameters of your "calibration regions" to match the climate and soils of sub-regions of your study area.  Implicit in these calibration parameters are variations in the soil and climate properties that drive the relative stability assessments that SINMAP produces.  You will be entering values that reflect a range of variation for parameters which in the real-world will be driven by climate factors, such as precipitation.  In SINMAP's model of the world, there are no actual precipitation amounts or intensities, just the "relative wetness" that is produced by real-world precipitation.

Thus, you will be informing SINMAP about real-world precipitation parameters, such as rainfall intensity, by adjusting the calibration region parameters.  Because of the nature of the SINMAP model, precipitation is not directly used in the way your original question implies.

I hope this helps.

Dear Professor Matthew Mabey,

Thanks for your answer. 

This is a good question.