There are several solutions I would suggest:
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A simple extension to the weather plugin including one or more algorithms to calculate a correction factor for irrigation duration similar to OpenSprinkler
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A separate plugin for this calculation, maybe also as an extension of the Pimatic Filter Plugin
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A complete new plugin with UI for irrigation control which can control several types of irrigation actuators e.g. mySensors.
See also
https://github.com/rszimm/sprinklers_pi/wiki/Weather-adjustments
https://www.mysensors.org/build/irrigation
https://opensprinkler.com/
https://forum.pimatic.org/topic/523/weather-history-ie-rain