I thought about leaving it open to collect rain water but then i figured that evaporation would quickly defeat the purpose,at the time of year where you'll be needing the drip system the most it will be hot as hell & evaporation will take most of the water,also if it rains the plants will allready have what they need,i decided i'd be better off just planning on visiting them every weekend & bringing water with me & knowing that the drip system worked for the 3 days i tested it at.
If the drip system remains closed & was clean when it was filled then simply adding a bit of peroxide will keep the water clean,it will kill any bacteria thats in the water & the closed system will not allow new bacteria in.
I bring my nute's with me each time i visit,i pre mix them in a cup with a lid then add them on site when i water.
Ive thought about using 5 gallon buckets with air tight lids & rubber gaskets(home depot $5 each) instead of milk jugs if we end up with a drought,a 5 gallon bucket full will easily last a week & supply a heavier drip rate,i had the valve on the one gallon milk jug set to where if i closed it any more the water would not drip.
Ive been thinking alot about this,at the extremely slow drip rate needed to make the water supply last 3 days, if its very hot outside the water will evaporate from the top of the soil before it reaches the roots.
In order to get past this i plan on digging a small hole at the base of the plant,then inserting a large plastic tumbler cup upside down so the bottom of the cup is facing upward,then drilling a small hole in the bottom of the cup to accept the water line,after that i'll be back filling the hole with soil so the cup is burried,this should keep the line free & allow water to reach the root system in extremely hot dry weather.