Measure water flow accurately?

I want to measure the amount of water in my spring in the mountain using simple means but as accurately as possible.

I have a shaft with a flat base measuring approximately 60 x 40 cm. The water flows from a 100 mm pipe onto the flat surface.

I want to back up as little as possible, because otherwise the cleansing effect of the flowing water is lost and in the long run the pipes could become clogged with deposits.

Are there any special methods besides a water wheel? This would only be 1-2 cm below the water level. I would be concerned that the number of revolutions corresponds to the water speed, and that this is not proportional to the volume, since with higher flow rates, not only the speed but also the water level increases, though presumably not proportionally.

Another problem is that, roughly speaking, I'll always have the same flow rate. The spring has been supplying water very consistently for as long as I can remember. This means I need a high level of accuracy so that any minimal fluctuations/trends are more easily detectable, not just when I calculate the average over four weeks to reduce the random error.

For example, I want to capture the 800-900l/h range as accurately as possible.

Furthermore, it should be as energy-efficient as possible, since I don't like to use 5W 24/7 for years.