Hydrology Help

Question

Hello,

I am trying to find  a way to find the upslope contributing areas  to  various pour points within one watershed to determine the land use percentage contributions to correlate to  water quality data collected at these pour points. I created a DEM from contours and followed the procedure of filling the DEM, creating a flow direction raster, a flow accumulation raster, imported my pour point data set which are water quality monitoring sites along the streams. I also used the snap pour point tool to connect them to the flow accumulation raster. But when I run the watershed tool it only returns a couple tiny contributing areas and the rest of the watershed is blank. I've read that if the pour points do not lie on the high flow accumulation path these errors occur and the snap pour point tool should be utilised but it doesn't seem to respond regardless of the threshold size.  Are there any other suggestions that I can use to determine what I am trying to find? Any help would be greatly appreciated.

curtvprice · Answer

Another issue I've run across is the processing extents being reduced to something less than the extents I want. Under Geoprocessing>Environments>Processing Extent choose a layer that covers the area you are processing. The fac grid would work here.

More more thing: if you're combining vector (points) with rasters, be sure and set the Snap Raster environment to the flow direction grid so your pour point temp raster will line up with your flow direction cells. You want to set both Extent and Snap Raster.

MarkBoucher · Answer

I usually use Arc Hydro and not the Spatial Analysts Hydrology tools. If you apply a color ramp to your flow accumulation grid (fac) you should be able to see they high accumulation flow paths. You can use this to check that the pour points are snapping to the fac. Also, I believe in the Snap Distance in the Snap Pour Point tool is in map units (feet or meters) and not the number of grids the point is away from the high fac path. Since the tool doesn't present units in the input dialogue, it is not immediately obvious. The help box, if shown, does say this though.

Another issue I've run across is the processing extents being reduced to something less than the extents I want. Under Geoprocessing>Environments>Processing Extent choose a layer that covers the area you are processing. The fac grid would work here.