uda is substituted for udaka when the compound is a Name.,
Thus उदमेघः 'a person called Udamegha', उदवाहः \a person named Udavahah.\ The well-known Patronymics from these are औदमेघिः and औदवाहिः ॥ Why do we say 'when it is a Name'? Observe उदकगिरिः ॥
Vart:- उद is the substitute for उदक when it stands as the second member of a compound and denotes a Name: as, लोहितोदः, नीलोदः, क्षीरोदः ॥,