Name for the difficulty of finding information you have no knowledge of

The exact thing you are looking for is called the Relevance Paradox. The wiki actually includes your irrigation example.

When searching for knowledge - of which there is really quite a lot - one doesn't search for things that do not seem to apply to the situation. So, when searching for information, you first try to establish what items would be relevant to the matter at hand. But, starting from the point of having no information, you are likely to be missing information that would make the relevance of another piece of information obvious.

Not sure if I understand your question 100 percent, but here goes my take.

One sometimes distinguishes between known unknowns and unknown unknowns. For example, if you're planning a mission to Mars, you may not know the speed of wind on your landing spot, that would be a known unknown. But there will also be circumstances/phenomena which you simply cannot foresee. Those would be unknown unknowns.

Perhaps you could somehow use the latter?

The learner's paradox is that in order to learn about something, you must first know that thing. As Socrates puts it:
[A] man cannot search either for what he knows or for what he does not know[.] He cannot search for what he knows--since he knows it, there is no need to search--nor for what he does not know, for he does not know what to look for. –Quora.com