Kind of Flat land with mounds: how to do swales

Hello,

Since I have a piece of land that is mostly flat, with small mounds or indentations due to it being plowed two years ago, although it’s obviously not perfectly flat since the land which is of 3.3 hectares goes from 101m of altitude to 99m of altitude to the other longest side. I would like to understand how to create swales here. The contour lines are definitely there, since it cannot be perfectly flat, but they are difficult to identify because the fact that is flat, and because of these surface irregularities since it hasn’t been leveled, so I’m not sure how to identify the contour lines. Even thinking about using an A-frame, I’m not sure how to go about it.

I can identify a keyline, which is a line that cuts through some parts of the land where water runoff is clearly visible. What should I do there?

Is there anyone who can explain this to me and give me some guidance?

Thanks