Alms Tax: Supplementary Irrigation That Does Not Increase the Crop

Practical Laws of Islam as per the teachings of Ayatullah Sistani

Ruling 1895

If wheat, barley, dates, or grapes are watered by rain and by a stream, and if they do not need buckets of water and suchlike but are also irrigated by means of buckets of water nevertheless, and if the buckets of water do not help produce an increase in crop, then the zakat on them is one-tenth [10%]. And if they are irrigated by buckets of water and suchlike and do not need stream or rainwater, but they are also watered by a stream and rainwater nevertheless, and if the stream and rainwater do not help produce an increase in crop, then the zakat on them is one-twentieth [5%].

-Ayatullah Sistani, Practical Laws of Islam, Alms Tax (Zakat)