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From Joalah Designs:
With one simple height measurement of the water flowing over the top of a V-Notch weir, NotchFlow calculates the flow-rate and the population that could be supported given a set per capita water usage.NotchFlow can also calculate the expected height and supported population for a given flow, or the required flow and expected height for a given population.These types of calculations are useful in both developing nations and agriculture in general where an estimate of a water courses flow-rate has to be quickly and easily made in the field.