# How to determine irrigable acres from a single source/point?

I am trying to graphically depict the exact amount of irrigable acres from a single point, in this case a headgate from a drainage ditch. Basically, I have the original water right decrees (i.e. confusing legal documents) from the late 1800's and am interested in accurately illustrating the legally allowable area to which they can irrigate.

Example: From the stream a drainage ditch runs west for 1 mile (on contour) before it opens into a small pasture, irrigating 50 acres below its outlet.

Is there a way to show this acreage below this specific outlet point? Perhaps a sort of reverse Snap Pour Point operation? Some function that fills in a specified area below a certain point, following topography lines?

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• What GIS software and version are you planning to use for this? – PolyGeo Oct 3 '13 at 22:24
• Your example is confusing, can you upload an image sketching what you are after? – Hornbydd Oct 3 '13 at 23:32
• I am using Arc v.10. (with limited extensions, whatever comes with the student version) – Adam B. Oct 11 '13 at 17:54
• Nice diagram (+1). But it's unrealistic, because it shows no information concerning where the irrigation water stops flowing. Any calculation based only on the contours and the information you describe would show the irrigation area extending far below where it is delimited in the diagram. So: what additional information do you have that would allow those 50 acres to be accurately delineated? – whuber Oct 11 '13 at 18:42
• Your description is the crux of my situation. There isn't a point at which the irrigation stops flowing. Sometimes it ends in a small basin below the ditch's outfall, sometimes it ends in an expansive field which spans for thousands of acres. The main limitation is the legal obligation of the water right; if there are 50 irrigable acres allowed by law/historical precedent (despite them being at the top of a 100 acre field) then all you get to irrigate is 50 of those acres that seem best for the user. It's both complex and hazy, perhaps trying to make sense of it is futile, but alas... – Adam B. Oct 15 '13 at 17:51