SWAT2012 – Adding point source discharge reduces outlet flow even though FLODAY is positive

[Ashwin Ghimire]

Hello everyone,

I am working with SWAT2012 via QSWAT. My watershed model runs correctly and produces reasonable discharge at outlet reaches when no point sources are assigned.

However, when I assign observed daily point-source files (recday) to several subbasins using SWAT Editor, the simulated flow at downstream outlet reaches decreases significantly, even though the point-source files contain positive FLODAY values.

Some details:

• The point-source files follow the standard SWAT daily format (DAY YEAR FLODAY ... MINPDAY)

• FLODAY values are positive and range roughly from 10³–10⁵ m³/day

• Nutrient loads such as MINPDAY appear correctly in the outputs

• The routing in fig.fig uses the structure:

recday
Pnt.Source 8p.dat
add
route

• The same model without the point-source assignments produces correct flows

So SWAT is clearly reading the files, but the outlet flow becomes smaller after the point sources are added, which seems counterintuitive.

My questions are:

1. Can adding point-source discharge reduce downstream flow due to routing structure in .fig?

2. Could the add / route sequence in the .fig file alter how flow is combined from subbasins and point sources?

3. Is there a known issue when adding point sources to multiple subbasins using QSWAT's “Add point source to each subbasin” option?

Any guidance would be appreciated.

Thanks!