This species goes under the name Buff-bellied Pipit in European literature but in winter they lack the buff-colored underparts which this species gets in the breeding season. At that time the underparts may be largely unstreaked. Alström & Mild merged pacificus as a junior synonym of nominate. However pacificus is still recognized as valid by Clements/eBird and H&M4 but not by IOC. If recognized, pacificus is supposedly more heavily streaked and more yellow (less buff) below. Also possible is A. r. alticola which breeds in the Sierra but which has much reduced ventral streaking. Canon PowerShot SX50 HS.
References:
Alström, P. & Mild, K. (2003) Pipits & Wagtails of Europe, Asia and North America. Identification and Systematics. Christopher Helm, London.
Hendricks, Paul and N. A. Verbeek.(2012).American Pipit (Anthus rubescens), The Birds of North America (P. G. Rodewald, Ed.). Ithaca: Cornell Lab of Ornithology; Retrieved from the Birds of North America: https://birdsna.org/Species-Account/bna/species/amepip DOI: 10.2173/bna.95
Tyler, S. & Kirwan, G.M. (2017). Buff-bellied Pipit (Anthus rubescens). In: del Hoyo, J., Elliott, A., Sargatal, J., Christie, D.A. & de Juana, E. (eds.). Handbook of the Birds of the World Alive. Lynx Edicions, Barcelona. (retrieved from http://www.hbw.com/node/57806 on 29 June 2017).