Please follow http://www.wcc.nrcs.usda.gov/webmap/index.html#elements=&networks=!&states=!&counties=!&hucs=&minElevation=&maxElevation=&elementSelectType=all&activeOnly=true&hucLabels=false&stationLabels=&overlays=&hucOverlays=&mode=data&openSections=dataElement,parameter,date,elements,location,networks&controlsOpen=true&popup=&base=esriNgwm&lat=39.98&lon=-98.96&zoom=4&dataElement=SRVO¶meter=PERCENTILE&frequency=MONTHLY&duration=wytd&customDuration=&dayPart=E.