A good way to address this requirement is to add a rain sensor which automatically extends its ‘rain sensed’ state beyond the period of rain fall depending on the amount of rain and the current weather conditions (temperature, humidity).
I have installed the inexpensive Irritrol RS500 Rain Sensor on my roof gutter. I can set a ‘rain sensed’ trigger level to between 3 and 19 mm of rain. Inside the sensor, it has leather disks that swell up when it rains (triggering the preset ‘rain sensed’ micro-switch). After it has stopped raining, the leather disks start to dry out and shrink back to their original size, eventually turning off ‘rain sensed’. The rate of shrinkage depends on the day’s temperature and humidity – a simple and cleaver way to automatically adjust watering operation for the amount of rain and current weather conditions.
Mark