[Division of Runoff Waters]

Answer—and Allah is the One who grants success: The madhhab holds that a right, on its own, may validly be divided, because division is not, in every respect, like a sale. On that basis, the refuser is compelled—unless the division would cause harm to one of the partners, in which case compulsion is not permissible.
We said that division is obligatory here because they [the jurists] allowed its division as a follower of the (underlying) property; therefore we allow its division when it stands alone, by analogy with the division of property; and because the interest requires that, so long as no partner is harmed.
Source: Min Thimār al-ʿIlm wa al-Ḥikmah vol.2