available to fall as rain

[paracompact]

Trees take in a great deal of water through their roots and release this by transpiration back into the air. This helps to keep the air moist. When forests are removed the air becomes drier, less moisture is available to fall as rain and therefore agriculture suffers.

Question: Is to fall as rain adjectival or adverbial?

[Alan]

Adverbial (it specifies the sense in which the moisture is 'available', not naturally deducible from the context).