During the day, the salt collects in the capillary material. If it was working 24/7, then it would continuously increase in salinity until you had to do somethi

During the day, the salt collects in the capillary material. If it was working 24/7, then it would continuously increase in salinity until you had to do something about it. However he sun goes down, which means that osmosis works in its favour. The less salty water wicks into the matting, the matting over saturates and leaks back down, equalising the salinity.