Resources: York River Water Budget - Hydrologic Methods
 The hydrologic method relies on an analysis of historic flow measures, making it the fastest and cheapest of the three methods.
Assumptions: lower than natural flows are harmful to the stream ecosystem and that keeping the minimum flow within natural, historic bounds will sustain aquatic life.
Advantages: maintains the hydraulic conditions so a fast moving river will remain a fast moving river, while a slow river will still have a slow flow. Maintains the character of the river, and may help preserve diversity.
Disadvantage: does not specifically take into account the aquatic biota or recreational water uses.
There are five types of hydrological methods that are commonly used:
1 . The Tennant method is based on a percentage of mean annual flow, which varies seasonally, and is intended to be optimized for faunal and recreational usages,
2. 25% of mean annual flow,
3. The ABF Q50 (Aquatic Base Flow calculated from the median flow in August),
4. Q90 (monthly flow equaled or exceeded 90% of the time), and
5. 7Q10 (the statistical 7-day low flow that occurs on average every 10 years).
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