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DOC: clarify hsu soiling ratio minimum value in docstring #2743

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d1aecc4
DOC: clarify hsu soiling ratio minimum value in docstring
Omesh37 Apr 26, 2026
ff99459
DOC: add PR number to whatsnew
Omesh37 Apr 26, 2026
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5 changes: 4 additions & 1 deletion docs/sphinx/source/whatsnew/v0.15.2.rst
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Original file line number Diff line number Diff line change
Expand Up @@ -22,6 +22,9 @@ Enhancements

Documentation
~~~~~~~~~~~~~
* Clarified that :py:func:`pvlib.soiling.hsu` has an implicit minimum
soiling ratio of approximately 0.6563 due to the mathematical form
of the model. (:issue:`2534`, :pull:`2743`)


Testing
Expand All @@ -42,4 +45,4 @@ Maintenance

Contributors
~~~~~~~~~~~~

Omesh37
18 changes: 14 additions & 4 deletions pvlib/soiling.py
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Expand Up @@ -16,9 +16,11 @@ def hsu(rainfall, cleaning_threshold, surface_tilt, pm2_5, pm10,
Calculates soiling ratio given particulate and rain data using the
Fixed Velocity model from Humboldt State University (HSU).

The HSU soiling model [1]_ returns the soiling ratio, a value between zero
and one which is equivalent to (1 - transmission loss). Therefore a soiling
ratio of 1.0 is equivalent to zero transmission loss.
The HSU soiling model [1]_ returns the soiling ratio, a value between
zero and one which is equivalent to (1 - transmission loss).
Therefore a soiling ratio of 1.0 is equivalent to zero transmission loss.
Due to the mathematical form of the model, the soiling ratio has an
implicit minimum of approximately 0.6563. See ``Returns`` for details.

Parameters
----------
Expand Down Expand Up @@ -49,10 +51,18 @@ def hsu(rainfall, cleaning_threshold, surface_tilt, pm2_5, pm10,
It is recommended that `rain_accum_period` be between 1 hour and
24 hours.

Returns
Returns
-------
soiling_ratio : Series
Values between 0 and 1. Equal to 1 - transmission loss.
Due to the mathematical form of the model
(``SR = 1 - 0.3437 * erf(0.17 * ω^0.8473)``),
the soiling ratio has an implicit minimum value of approximately
0.6563 (i.e., maximum transmission loss of ~34.37%), regardless
of the accumulated particulate mass. Note that the valid range
of the underlying equation is further limited to accumulated
mass densities up to 10 g/m², corresponding to a soiling ratio
of approximately 0.6875. See [1]_ and [2]_ for details.

References
-----------
Expand Down