What are the valid methods for dividing shared river water through time-sharing arrangements (al-muhāya'ah)?

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Dividing the shared river water through mutual agreement (al-muhāya'ah) is permissible, provided all parties consent and each share is clearly defined. This can involve allocating specific time periods, such as day and night, or any defined duration, whether longer or shorter. Division based on fixed periods like from sunrise to noon for one and noon to sunset for the other is also valid. Furthermore, division by quantifiable hours is permissible if a measurable standard is used, such as a perforated cup with markings, an hourglass, a sun dial to determine daylight hours, or lunar mansions at night.