Asim bin Damrah (may Allah have mercy on him) said that Sayyiduna Ali (may Allah be pleased with him) said: On what the sky waters and what is watered by channels and the like, a tenth is due; and on what is watered with buckets and the like, half a tenth is due.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7492