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=40.00&lon=-99.00&zoom=4&dataElement=SRVO¶meter=PCTAVG&frequency=MONTHLY&duration=last&customDuration=1&dayPart=E&monthPart=E .