il.li describes the upper member of a paired set of water sources in the two cited accounts.

In both occurrences, the adjective identifies the upper water source in a contrastive pair with a lower one, whether the APB calls them bowls or springs.

1. Upper member of a water-source pair · dominant

   In Joshua 15:19, the feminine plural form occurs in the gift of upper and lower bowls of water; the APB renders the pair as “the upper bowls and the lower bowls.” In Judges 1:15, the feminine singular form occurs in the corresponding gift of upper and lower springs; the APB renders the pair as “the upper springs and the lower springs.”

   Joshua 15:19Judges 1:15

In the Anselm Project Bible: The APB presents the term within contrasted upper/lower water-source pairs, using “upper bowls” in Joshua 15:19 and “upper springs” in Judges 1:15. These renderings show the clauses' presentation rather than a fixed English equivalent.