Clarification needed on HRU Impoundment Parameters — vol_dp_a and vol_dp_b

[Swetasree Nag]

Hi,
Please give insight on the following matter.

While setting up HRU impoundments in SWAT+, I found limited documentation explaining each parameter, their units, and how they should be derived. One example is the vol_dp_a and vol_dp_b coefficients, which represent the volume–depth relationship using a power law:

V=vol_dp_a×dvol_dp_b

Here:

• V = Storage volume (m³)

• d = Depth (m)

• vol_dp_a = Scaling coefficient (unit depends on exponent)

• vol_dp_b = Shape exponent (dimensionless)

Example from my study:

• Volume = 100,000,000 m3

• Max depth = 5.0 m

• vol_dp_b=1.8

Then:

vol_dp_a=5.01.8100,000,000​ vol_dp_a=11.18100,000,000​≈8,944,272 m(3−1.8)=m1.2

This results in a large value (~8.94 million) with a unit of m1.2, which is very different from the default value of 1 suggested in some examples.

My questions to the community:

1. Can SWAT+ handle such large, unit-bearing coefficients for vol_dp_a directly, or is it recommended to normalise these values?

2. Is there a standard reference or default formulation for setting vol_dp_a and vol_dp_b when using actual measured volume–depth data?

3. How do others ensure these parameters accurately reflect the storage–depth capacity without introducing scaling inconsistencies?

Any official documentation or user experience on this would be greatly appreciated.

The information contained in this electronic communication is intended solely for the individual(s) or entity to which it is addressed. It may contain proprietary, confidential and/or legally privileged information. Any review, retransmission, dissemination, printing, copying or other use of, or taking any action in reliance on the contents of this information by person(s) or entities other than the intended recipient is strictly prohibited and may be unlawful. If you have received this communication in error, please notify us by responding to this email or telephone and immediately and permanently delete all copies of this message and any attachments from your system(s). The contents of this message do not necessarily represent the views or policies of BITS Pilani.