groundwater hydraulic gradient confusion

[engineer123]

Hey all I'm getting really hung up on what length (distance) to use in the hydraulic gradient equation. In the solution below, they use the "diagonal" distance between the wells. I would have just used 175 ft. The example from this link also used the "horizontal" distance https://www.quora.com/What-is-hydraulic-gradient

Which one is the correct value to use?!

 

[PowerStroke79_PE]

I would say its negligible. In problems where slope or length is within the equation, mannings or friction loss (darcy/hazen), I would really look at the datum and the difference of head elevations . If the difference is considerable, to the point  the length changes substantially due to the large head difference, then I would consider it. In this case if they are giving you the angle then just use it. But I doubt your answers will be that close as to account for approximately 4 ft difference in length.