So just to add a bit to this conversation. The new approach using OWM (OWM graph lines) is very different to the old Weather Underground approach (PWS graph lines). I’ve attached a side by side comparison of the two approaches in the attached image. You can see that OWM is a pretty reasonable comparison to the current conditions reported by my PWS (as it happens I am publishing to OWM so your mileage may vary on this). But when you look at the top graph you can see very different Water Level calculations. The PWS line uses the old Weather Underground historical method using data from the last 48 hours. Whereas the OWM line uses forecast data for the next day to calculate the watering level.
Ignoring the regions where OWM water level is showing zero for the moment, you can see the PWS line is trending down over the last two days but the OWM line is trending up. That’s because in London we have been forecasting rain the last two days but are now heading into some fairer weather. So the PWS/WU water level says reduce watering as we have had some rain, whereas, the OWM approach says increase watering as we are predicting a dryer spell. This can appear a bit unintuitive, for example, you could now have the sprinklers coming on immediately after a heavy downpour if the forecast for the next day is sunny and dry. But then again WU approach had the lawns being watered immediately before the rain came along. So swings-and-roundabouts.
The periods where the OWM water level drops sharply to zero happens when OWN starts predicting rain in the next three hours. When that happens the logic says don’t water for an hour and check again later. This is somewhat analogous with the old Weather Underground approach of setting water level to zero when it is currently raining. Looking at the graph, my concern is that a “forecast of rain in the next three hours” isn’t quite as certain as “rain happened in the last hour” so this could be too sensitive and we might need to fine tune.
Be interested in peoples observations and thoughts on the logic. I’d also be interested to hear from those that experienced the water level reducing into the evening or rising during the morning. The OWM logic uses the next ten 3-hour forecasts to calculate the average forecast weather. Since this is 30 rather than 24 hours, I could see that this might bias the numbers a bit, For example, as you move into the evening, the preceding afternoon forecast (warmer) will roll off and be replaced with an overnight forecasts (cooler) from 30 hours out.
So the change from historical based calculations to forecast based calculations is different. Not sure which is better for the plants but I guess time will tell.