Which factor is crucial in determining the amount of water that can be diverted under the doctrine of prior appropriation?

In the context of Arizona water law and the doctrine of prior appropriation, the crucial factor in determining the amount of water that can be diverted is historical use and need for the water. This principle is foundational in prior appropriation, which operates on the idea of "first in time, first in right." This means that water rights are allocated based on the chronological order in which water was put to beneficial use.

Therefore, historical use establishes the priority of rights among those who divert water from the same source. This serves to ensure that users who have historically relied on a certain amount of water for their needs are not negatively impacted by later appropriators. The need for water, closely tied to historical use, reflects the critical importance of demonstrating ongoing beneficial use in order to maintain those rights.

In contrast, while proximity to the water source, the type of crops needing irrigation, and rainfall patterns are relevant considerations in water management, they do not serve as the primary determinants under the doctrine of prior appropriation. The doctrine emphasizes previously established use and the rights that flow from it, making historical usage paramount in any discussion of water diversion rights.