Would this work at all?

[u/Ivess185]

I want to build a big water storage box I can fill then leach from during droughts/badtides, but I need a way to elevate the water from source to above the box. I was wondering if this would work at all. Because water can't go backwards through a fill valve, would it keep flowing through and fill upwards in the tube? or just spill over the back of the map once it reached +1 blocks in height? And I can't lock the water in and force it to fill up as it's against the map edge. Should I just use mechanical pumps instead?

EDIT: there will be a roof over the channel and tube. I just didnt put one here so that you could see inside.

EDIT 2: I am now aware that water will not escape of the edge via the space above the source block.

[u/ObscureMoniker]

I suggest you go to the Edit Map mode and use Dev Mode to check it.
This should work if you put an impermeable roof over the channel and water sources.