For SPI and TWI published work on line available.

SPI is a measure of the erosive power of flowing water calculated as:

SPI = As×tan(β)

TWI gives the spatial distribution and zone of saturation sources for runoff generation calculated as:

TWI = ln(As/tan(β))

where, As is the specific contributing area and β is the gradient or slope.

SPI and TWI, when used together, are good predictors of the location of ephemeral gullies in small agricultural watersheds (Moore et al., 1988).

Details of the theoretical background about SPI and TWI are available at the links below:

https://dl.sciencesocieties.org/publications/sssaj/abstracts/50/5/SS0500051294?access=0&view=pdf

https://onlinelibrary.wiley.com/doi/epdf/10.1002/hyp.3360050103

http://onlinelibrary.wiley.com/doi/10.1002/hyp.5730/epdf

https://www.hydrol-earth-syst-sci.net/10/101/2006/

You may check the link below on how to calculate TWI in a GIS environment and similar procedure can be applied to calculate SPI:

https://www.researchgate.net/post/What_is_the_typical_Topographic_Wetness_Index_range_of_values

Thank you everyone.

Ilwis software:

SPI = (flowacc_rasmap * PIXSIZE(%1) * PIXSIZE(%1)) / (TAN(DEGRAD(temp_slopemap_deg)))

you can use a SAGA GIS software (is open source)

SPI = catchment area * tan slope angle

TWI = ln (catchment area / slope angle)

You can use the SAGA GIS Software.

In ArcGIS

SPI= (Ln(flowaccumulation+0.001)*((slope/100)+0.001

TWI= (Ln(flowaccumulation+0.001)/((slope/100)+0.001

Use Whitebox GAT to produce SPI and TWI. To calculate these indices use Specific catchment area (sca) without log transformation instead of simple Flow Accumulation. The SCA is also available in Whitebox GAT. A manual calculation of TWI using the raster calculator may generate too many no data value pixels. Whitebox GAT has the capability to avoid this.

Use slope in radians.

Use specific catchment area

You may check SAGA documentation..