@Rassster to be honest, I don’t know how to express this in German because I didn’t really understand what you mean by saying “I am to have” if this means that you have to have 100 litres of water then it would be; ich muss 100 Liter (von) Wasser haben.
Your English text sounds a bit strange to me. Is this Australian grammar? Anyway, here are some suggestions: I have 100 l of water. - Ich habe 100 l Wasser. I‘m going to have 100 l of water. - Ich werde 100 l Wasser haben. I should have had 100 l of water. - Ich hätte 100 l Wasser haben sollen. I will have had 100 l of water. - Ich werde 100 l Wasser gehabt haben.
"I am to have" sounds completely fine to me. I would translate it as "Ich sollte 100 Liter Wasser bekommen" (if you did not recieve the water you were ment to get and are e.g. complaining to s.o. about it) or "Ich soll 100 Liter Wasser bekommen" (if you are for example telling a third party about the expected water delivery).
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