function mymodule_form_alter(&$form, &$form_state, $form_id) {
  $tid = get_tid_from_termalias (arg(1)); // here I get tid
  $form_state['input']['field_sity1_tid'] = (!empty($tid)) ? $tid : 'All';
  $form_state['field_sity1_tid']['#value'] = (!empty($tid)) ? $tid : 'All';
  $form['field_sity1_tid']['#default_value'] = (!empty($tid)) ? $tid : 'All';
  $form['field_sity1_tid']['#value'] = (!empty($tid)) ? $tid : 'All';
}