Modelling P from OWTS to lake

[anish katwal]

Hi all,

I am working on modeling septic systems (OWTS) in SWAT (ArcSWAT 2012) using the biozone approach, and I would appreciate some clarification on how septic parameters should be assigned when multiple systems fall within a single HRU.

In my case, I have mapped individual septic systems spatially and linked them to HRUs. As a result, some HRUs contain multiple septic systems (houses), each with attributes such as year built and distance to stream.

My questions are:

1. SEP_CAP (persons):
   SWAT documentation defines this as the number of residents per septic system.
   If an HRU contains multiple septic systems, should SEP_CAP be:

   • the average household size (e.g., 2.5 persons), or

   • the total population served by all septic systems within that HRU?

2. ISEP_IYR (year):
   For multiple systems in one HRU, is it appropriate to use:

   • the median year built,

   • the mean year,

   • or another approach?

3. SEP_DEN (systems/km²):
   Should this be calculated at:

   • HRU level (systems per HRU area), or

   • subbasin level?
     Also, is SEP_DEN actually used in SWAT 2012 simulations?

4. SEP_STRM_DIST (distance to stream):
   For multiple systems in one HRU, is it better to use:

   • average distance,

   • minimum distance (worst-case),

   • or is this parameter not active in SWAT 2012?

Overall, I am trying to determine the best way to represent multiple septic systems within a single HRU, while remaining consistent with SWAT’s conceptual framework.

Any guidance or references would be greatly appreciated.

Thank you!