In this context I am referring to the point at which the amount of rainwater building up on a leaf compromises its integrity and causes it to collapse and spill said water.
First thought was that the water has accumulated to the point of critical load.
Critical defined as:
- Relating to or denoting a point of transition from one state to another.
Proof load is used in construction and engineering as a term for maximum load.
the greatest load that can be applied to a piece, as a beam, column, etc., without straining the piece beyond the elastic limit.
Maximum load sounds equally good.
That is the very literal example of the idiom The straw that broke the camel's back.
The maximum amount that something can contain.