uda is optionally substituted for udaka before a word beginning with a single consonant and which expresses that which is filled with water.,
Thus उदकुम्भः, or उदककुम्भः, उदपात्रम् or उदकपात्रम् ॥ The word एकहलादि means 'a word beginning with a single simple consonant'. The rule does not apply to उदकस्थालम् as the second member begins with a conjunct consonant: nor to उदकपर्वतः, as the पर्वतः is not a vessel which is to be filled.,