DOC: clarify default depo_veloc values in soiling.hsu Notes

Author: weixuan-song

docs/sphinx/source/whatsnew/v0.16.0.rst

@@ -75,6 +75,9 @@ Enhancements
 Documentation
 ~~~~~~~~~~~~~
 * Remove `g_poa_effective` from the :ref:`nomenclature` page (:pull:`2704`)
+* Clarify in the ``Notes`` section of :py:func:`pvlib.soiling.hsu` that the
+  default ``depo_veloc`` values correspond to the settling velocity of
+  particulates. (:issue:`2535`)
 
 
 Testing

pvlib/soiling.py

@@ -67,6 +67,14 @@ def hsu(rainfall, cleaning_threshold, surface_tilt, pm2_5, pm10,
     corresponding to a soiling ratio of approximately 0.6875.
     See [1]_ for details.
 
+    The default values of ``depo_veloc`` correspond to the settling
+    velocity of particulates, i.e., only the gravitational movement of
+    particles. The model also accepts deposition velocities, which
+    comprise other types of particle-fluid interaction; however, the
+    authors of [1]_ found that deposition velocity considerably
+    overestimates soiling, while settling velocity performed well in
+    their case study.
+
     References
     -----------
     .. [1] M. Coello and L. Boyle, "Simple Model For Predicting Time Series