Well, if the top of your bed in the draw is lower than the bottom of your irrigation row leaving the draw the rows are going to mix and your irrigation is do at the point in the field. 

You are going to have to fill in that draw. 

I just shoot the field every 100 ft or if you have to every 50 ft and the numbers will tell you how much drop you have and how constant the drop is.  It will tell you where you have to cut and where you have to fill to get a constant drop. 

Here they have a bunch of wooden lath.  Every time they take a shot they pound in a lath. Later they come back with numbers on metal that go over the top of each lath. The number tell the man how much to fill or cut. Example, a red number 5 means they need to fill 5/10 of a foot and a black number 3 means they need to cut 3/10 of a foot from that area.