Slow sand filter flow rate

It is now March 8 and the flow rate test has been done. I am feeling much better, and physical work is getting easier. The 4 litre container is used now instead of the 12 litre. It is a “gallon” jug, but the 4 liter mark is right at the very top where the lid goes on, so accuracy can now be down to tenths of a second and the results are more indicative of instantaneous rate of flow. I guess I could generate the formula and then take the derivative, but it is more fun watching the water flow and using a stopwatch to time it!! All the filters have slowed down. The interesting thing is that the two filters with float valves controlling the flow essentially decide how much water will flow through based on the condition of the schmutzdecke. These filters are self-regulating and do change from month to month. I am wondering if the barometric pressure has anything to do with the change? I imagine it does because the water has to push up in the output pipe to flow out of the filter. The variation does not seem to be enough to significantly influence the water quality, although this would be something to check out . . . . when I get lots more time and lots more money!!