def METHOD_NAME(input_xml):
    import xml.etree.ElementTree as ET
    tree = ET.parse(input_xml)
    root = tree.getroot()
    if root.tag != 'amanzi_input':
        raise RuntimeError('The given XML file is not an Amanzi input.')
    output = root.find('output')
    observations = output.find('observations')
    obs_file = observations.find('filename').text
    return obs_file