Problem with Urban HRUs reducing phosphorus

[Jim Almendinger]

All --
I'm trying to test some simple scenarios to reduce phosphorus yields from my urban HRUs.  According to the Driver&Tasker (1988) USGS regression equations given in the SWAT2009 Theory manual (p. 385), loads are proportional to the FIMP (fraction of impervious area).  Reducing FIMP should result in reduced phosphorus loads (& yields).  The equation as given does just that -- if I hold everything else constant and reduce only FIMP, indeed the phosphorus load is reduced (I did these simple calculations in Excel.)

However, when I reduce FIMP in my model in my URHD HRUs, I actually get a slight increase in phosphorus yields (and loads).  In looking at the hru output table, I see that for my test URHD HRUs --
SEDPkg_h and ORGPkg_ha do decrease, but
SOLPkg_ha and P_GWkg_ha increase, enough so to overwhelm the above decreases, resulting in a net increase in total P yield from the HRU.  The big increase is in SOLPkg_ha. 

Surface Q is reduced when FIMP is reduced, as you'd expect.  Presumably this leads to greater infiltration, and consequently greater P_GWka_ha, so I can explain that increase.  But the other components, including the problem-causing SOLPkg_ha, are all part of the overland transport path contributing to the P load that should be subject to the USGS regression equations. 

I further note that when I reduce the fraction of connected impervious cover (FCIMP), I do actually get a reduction in total P yield, but the effect is very small.  I'm not sure why this should affect the regression equation calculations, since FCIMP is not a variable in the equation. 

Evidently SWAT doing something more than the simple application of the USGS regression equations.  I haven't dug through the code to figure this out -- has anybody else?  Does anyone out there have an explanation for my results? 

Cheers,
-- Jim

--

Dr. James E. Almendinger
St. Croix Watershed Research Station
Science Museum of Minnesota
16910 152nd St N
Marine on St. Croix, MN  55047
tel: 651-433-5953 ext 19

[I. Morin]

Hi Jim,

 

I know it's been a while, but did you ever get a response to your question (if so, can't see it online) or figure it out on your own?

 

We've observed similar non-intuitive behaviors in some HRUs when increasing the amount of infiltration -- but not in all HRUs where we applied the changes. I figured I'd check with you before we spend a lot of time tracking this out.

 

Isabelle

Abt Associates Inc., Cambridge, MA

tel: 651-433-5953 ext 19