Zimmerman calculation is incorrect and changes throughout the day, which should only occur if there’s rain. The calculation is supposed to be based on the previous day’s average temperature, average humidity, and total precipitation, plus any precipitation today. If there’s no rain, then the water level should not change from midnight to 11:59 p.m. It’s actually changing with each weather call, which makes it useless in a climate where the night time temps differ radically from the day time temps. Zimmerman is intended to approximate evotranspiration, so that the amount of water applied is reasonably constant across varying weather conditions. The current calculation does not do that, particularly when watering at night. Furthermore, there’s no way that the current weather data source can provide the necessary data unless OpenSprinkler accumulates the data from the hourly weather calls and calculates the averages.