The correct answer is: A. Average daily rate of flow.
Dry weather flow is the average daily rate of flow in a stream or river during periods of low precipitation. It is used to calculate the amount of water that is available for human use, such as for drinking, irrigation, and industrial purposes.
Option B, average monthly rate of flow, is not as accurate as average daily rate of flow because it does not take into account the variability of stream flow from day to day. Option C, average annual rate of flow, is even less accurate because it does not take into account the seasonal variation in stream flow. Option D, water supply allowance per capita, is not a measure of stream flow at all. It is a measure of the amount of water that is available for each person in a population.
Dry weather flow is an important concept in water resources management. It is used to determine the amount of water that is available for human use, and to design and operate water supply systems.