Ibn Jurayj (may Allah have mercy on him) narrates from Ata (may Allah have mercy on him) that he was asked: How is zakah to be paid on the produce of land that was watered by flowing water and then watered by water-wheels and the like, or first by water-wheels and the like and then by river water? He said: By whichever of the two it was watered with more. Yahya bin Adam (may Allah have mercy on him) said: Zakah is to be collected in proportion to each share.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7495