I have 8 SIPs out side with 6 strawberries each. I filled them up once when I put them up on their racks. Have the mix mounded up and they are cover with black plastic with "X" cut for the plants to poke through. Every few days I check them and water will pour out the drain holes by barely tipping the container. SIPs may be very efficient at conserving water, but I don't think they are that efficient.
We had had an almost "what used to be normal" spring as far as rain goes. My guess is that my SIPs are getting refilled by the rain. My concern about heavy rain (2 1/2" in the past 36 hours) is that it may wash the fertilizer out of my SIPs. Enough water getting in through the "X"s for the plants, finding its way through the fertilized mix and then out the drain holes has the potential of taking all of the water dissolved fertilizer with it. I use non-slow release, granular fertilizer and think that stuff dissolves when exposed to water.
So, should I be concerned about this and cover up my SIPs when heavy rain is due in?