Updating Townsends snow model references

pvlib/snow.py

@@ -278,16 +278,19 @@ def loss_townsend(snow_total, snow_events, surface_tilt, relative_humidity,
     axis to the module edge.
 
     The parameter `string_factor` is an enhancement added to the model after
-    publication of [1]_ per private communication with the model's author. The
-    definition for snow events documented above is also based on private
-    communication with the model's author.
+    publication of [1]_, as described in [2]_,
+    The defination for snow events documented above is based on [3]_.
 
     References
     ----------
     .. [1] Townsend, Tim & Powers, Loren. (2011). Photovoltaics and snow: An
        update from two winters of measurements in the SIERRA. 37th IEEE
        Photovoltaic Specialists Conference, Seattle, WA, USA.
        :doi:`10.1109/PVSC.2011.6186627`
+    .. [2] Townsend, T. (2025). Snow Loss Model Enhancements.
+       Available at: https://www.nrel.gov/docs/fy25osti/90585.pdf
+    .. [3] Townsend, T. (2025). Snow Events Definition.
+       DOI: 10.13140/RG.2.2.14299.68647
     '''
 
     # unit conversions from cm and m to in, from C to K, and from % to fraction