[Custom Concerning the Runoff of Plots and Plantings]

Answer—and Allah is the One who grants success: It is appropriate to affirm them upon that custom and to judge accordingly, because their consent to it—absent any opponent—permits the judge to rule by it.
If it is said: Trees and land are among the public rights in which people share; therefore none should have exclusive claim to them.
We say: Those who divide that are from the people possessing the public right; each one of them has precedence to a right. It is mutually acknowledged that whoever precedes to a right has the stronger claim to it: the one with trees has the stronger claim to the trees; the one with pasture has the stronger claim to the pasture; the one with water has the stronger claim to the water.
Accordingly, those we have mentioned have priority over other people. There is no contradiction between what we have said and the ḥadīth, “People are partners in three …,” for the upshot of this ḥadīth is that each person may take his need from those three.
Source: Min Thimār al-ʿIlm wa al-Ḥikmah vol.2