It has also been said that this ruling is not universally applicable in every time and place. Rather, it is determined by need, which may vary in five respects. First, lands differ: some are irrigated by a small amount of water, while others cannot be irrigated except with a large amount. Second, what grows on them differs: crops require a certain amount of irrigation, while date palms and other trees require another. Third, summer and winter differ, for each season has its own measure. Fourth, there is a difference between the planting season and the period preceding it, for each of these two periods has its own measure. Fifth, the condition of the water differs according to whether it remains available or becomes interrupted: interrupted water is drawn in an amount that can be stored, whereas continuously available water is drawn only in the amount that can be used.
Because it varies in these five respects, the matter cannot be fixed on the basis of what the Messenger of Allah (Allah bless him and grant him peace) ruled in one particular case. Rather, it is determined according to custom and established practice when the need for it arises.
If a man irrigates his land or causes water to flow through it, and some of that water flows onto his neighbor’s land and floods it, he is not liable, because he exercised control over his property by means of something permitted. Aḥmad expressly stated an analogous ruling in the narration of al-Barzāṭī: “If he burns a field belonging to him, and the fire spreads to another person’s crops and burns them, he is not liable.”
If fish happen to be in that water, the second party has a greater right to catch them than the first, because they are on his property. Aḥmad indicated this in the narration of Mūsā ibn Abī Mūsā: “Regarding a man who bought a plot of broad beans—or some other crop—and the water receded from it, leaving fish in it: the fish belong to the owner of the land.” Thus, he awarded them to the landowner rather than to the purchaser of the broad beans.
The third category of rivers consists of those dug by human beings through the land. The river is then jointly owned by them, like a common water allotment shared among its users; none of them has exclusive ownership of it. If the river is in Basra, where tidal water enters it, it serves all its users, and they do not compete over it because its water is abundant. They do not need to hold it back, since the tide raises it to a level sufficient to irrigate all the lands; then, after they have been irrigated, the water recedes among the islands.
If, however, it is in a city other than Basra, in lands where there is neither a tide nor natural flow, the river belongs to whichever of the landowners dug it. No one else has the right to draw irrigation water from it or to make a drainage outlet into it, nor is it permissible