# Determining inflow outflow of polygon based on flow accumulation?

I'm working with ArcGIS 10.2 to try to determine if a given site has some sort of outflow for the water coming into the site. I'm using the ArcHydro tools and have calculated the flow direction and flow accumulation for every watershed in the state. In addition, I have a shapefile that has ~25 polygons (self-created) that depict each of my sites.

I have approximately four broad types of polygons: (1) Type I polygons have stream channels running through them and thus drain well. (2) Type II polygons have small-scale streams coming in (they don't show up because I've set my flow accumulation threshold > 250 pixels). However these polygons DO show a larger-scale stream flowing out of the polygon, and can also be considered to drain. (3) Type III polygons have no apparent streams coming into or out of them, and do not drain. Lastly, (4) Type IV polygons have streams draining in, but do no have streams draining out.

I'm trying to find a way to:

A) calculate the inflow vs outflow to show that Type I and II polygons have more water flowing out than they do flowing in, and thus drain. Versus Type III and IV polygons that have a greater inflow than outflow. B) develop an inflow/outflow model that can then be used on any random polygon in the state to determine which type of polygon it would fall into.

• A flow accumulation grid, calculated from a hydrologically corrected DEM (i.e. depressions and flat areas removed) is a monotonically increasing function. This means that all of your polygon types will have greater outflow than inflow if you are using the upslope area as a surrogate for discharge, which is implicit in your method. – WhiteboxDev Sep 16 '14 at 17:47
• That's true for Type I, II, and IV. But for Type III that doesn't have an outflow, there shouldn't be a greater outflow. Basically I'm trying to develop a model/mathematical way to differentiate these 25 polygons by flow-type so that I can apply it at a state-wide scale, to any randomly selected polygon. If you have any thoughts on a way to use the flow accumulation raster (or another calculated layer) I would love any ideas. – KKL234 Sep 16 '14 at 17:53
• If you've calculated your flow accumulation grid from a depression filled/breached DEM, which is the standard practice for this type of operation, then your Type III will have an outflow and it will have a greater flow accumulation value than the total of the input (by the area of the polygon assuming a single outflow). If you haven't hydrologically corrected the DEM, then the outflow will equal zero (i.e. there is no discharge from the polygon) and that is your answer...your inflow versus outflow is simply whatever goes in divided by zero. – WhiteboxDev Sep 16 '14 at 17:56
• Ultimately the problem here is that you are using flow-accumulation as a simple surrogate for discharge but in your case you would need to know more, e.g. the amount of evapotranspiration, the retention times within the features. You'd need to hook this up to a hydrological model that can handle wetlands of varying types. – WhiteboxDev Sep 16 '14 at 18:00
• I DO need to know more information. I'm intending to use this drainage layer as just one piece of a larger model that includes landcover type, soil type, etc. In regards to wetlands, many of my sites are wetlands, but they do not show up on the National Wetlands data layer because they're too small. That's where I was hoping to develop a drainage model/calculation that could differentiate my wet sites. – KKL234 Sep 16 '14 at 18:04