Formerly merged with Water Pipit. Note the reduced ventral streaking and buff underparts suggesting that this individual is starting to molt into breeding plumage. 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. Alström & Mild merged the race A. r. 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 in this area is A. r. alticola which breeds above tree line in the Sierra Nevada of California. It has much reduced ventral streaking.
Reference: Alström, P. & Mild, K. (2003) Pipits & Wagtails of Europe, Asia and North America. Identification and Systematics. Christopher Helm, London.