What is the PERCO parameter?

[Gopal Penny]

I've been trying to figure out why I'm not getting sufficient percolation in my SWAT+ model, and I've recompiled the sourcecode (in particular, v59.3) so that I can debug the program interactively using Visual Studio. I've determined that all of the percolation values from the bottom soil layer are attenuated by hru(ihru)%hyd%perco_lim for the HRU, which is calculated in topohyd_init.f90 as:

perc_ln_func = 1.0052 * log(-log(hru(ihru)%hyd%perco - 1.e-6)) + 5.6862

hru(ihru)%hyd%perco_lim = exp(-perc_ln_func)

Note that the PERCO parameter (i.e., in hydrology.hyd) is hru(ihru)%hyd%perco in the equation above. In my watershed, where PERCO is 0.5 for most HRU's (and never higher than 0.5), this means that perco_lim is typically 0.0049 -- meaning that percolation from the bottom layer is reduced by about 95.5%. I've been using the Theoretical Documentation for Version 2009 to understand SWAT calculations, but because the PERCO parameter was introduced with SWAT+, I can't find the theory behind the equations above. The SWAT+ Input document says PERCO is the "Percolation  coefficient   -  adjusts  soil  moisture  for  perc  to occur (1.0 =  fc)". I'm still confused about this parameter and wondering how it should be used.

The PERCO parameter has been mentioned in other threads, but never explained. For example, see: Percolation SWAT2012 vs. SWAT+, Spiky flow_out hydrograph, and Spiky Hydrograph with low Baseflow

Thanks in advance for any assistance.

[Gopal Penny]

The above equation is highly nonlinear, so I'm attaching the plot below to show the relationship between PERCO and perco_lim (noting that 1 - perco_lim is the reduction in percolation from the bottom soil layer).