url<-"https://www.wcc.nrcs.usda.gov/ftpref/data/climate/table/temperature/history/california/"
###下载文件
doc <- htmlParse(url,encoding='UTF-8')
Warning message:
XML content does not seem to be XML: 'https://www.wcc.nrcs.usda.gov/ftpref/data/climate/table/temperature/history/california/' 
hrefs<-xpathSApply(rootNode,"//a/@href") 
names<-xpathSApply(rootNode,"//tbody/tr/td",xmlValue) # /td:td层的内容xmlValue