Ja'far bin Muhammad (may Allah have mercy on him) narrates from his father that the Messenger of Allah (peace and blessings be upon him) prescribed "a tenth ('ushr) on what the sky waters, or what is the produce of floods, streams or rain-fed land, and prescribed half a tenth (nisf al-'ushr) on what is watered by water-wheels and the like."
Hatim said: Al-ghayl is what is watered by flowing channels, and al-ba'l is the same as al-'adhy, which the rainwater waters. Yahya bin Adam said: I asked Abu Iyas al-Asadi, and he said: Al-ba'l, al-'athari and al-'adhy are what is watered by water from the sky. Yahya said: Al-'athari is what is sown specifically for the clouds, that is, for rain; it is watered only by the rain that falls on it. Al-ba'l refers to grapevines whose roots have reached down into the earth to the water, so they do not need watering for five or six years and can bear the absence of watering. Al-sayl is the water of the valley when it flows; and al-ghayl is a flow less than the great flood: when a little clear water flows, that is al-ghayl. And al-'adhy is rainwater.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7494