Delineating a Watershed in Arcmap

welcome back to GIS analysis at the University of Alaska Fairbanks in this session we're going to delineate the watershed of the Yukon River in order to do that we're gonna use a suite of tools that are in the same tool set so if you search for watershed and go to that full set we're going to use the tools in this hydrology tool set to delineate our watershed first step is for every pixel what is the flow direction in that pixel use this tool flow Direction input is our raster that's a square raster in this case it's filled and then we'll output it will call this flow direction got TIFF the flow Direction geoprocessing tool outputs a raster that has these special values if we look at arcgis help it will describe what these categories represents the tool flow Direction creates a raster a flow direction from each cell to its steepest down slope neighbor and then this is what the raster values represent if you have a raster value of 1 it means the steepest gradient to the neighboring pixel will be to the right if you have a raster value of 16 it means the steepest gradient to any of these 8 neighboring pixels will be to the left our next step will be to use the flow accumulation tool which requires an input of a flow Direction raster our input flow Direction raster we just created and then our output will call it flow accumulation TIFF and then just ok and wait and wait and wait I computer it took 45 seconds to execute that geoprocessing tool so your computer it may take a few minutes now we have for every cell what is the number of cells that flows into each cell so if we zoom to layer let's zoom in on Western Alaska and let's change our symbology go to symbology and classify let's give it two classes and let's classify these two classes manually and let's give it a threshold of n thousand if it's less than 10,000 no color if it's a slow accumulation of 10,000 or above will color it some blue color now we have high flow accumulation raster's and what we want to do is find where the Yukon River enters the bering sea pixels with high flow accumulation we color coded blue so let's change the color code where we have sea level is zero for our elevation so if we go to that symbology let's color code that as no color and that will make it easier to find where the Yukon River Channel the last pixel so here's the Yukon flat so if we zoom in on the Yukon flats here's the Yukon River and if we follow the Yukon River it's going and it joins the Tanana River and then we're flowing down [Music] and here's where the Yukon River enters the Bering Sea so if we zoom in on that area what we want to do is find that pixel so if we do an identify on our flow accumulation that pixel has a flow accumulation value so what we want to do is isolate that pixel so we can use the con tool to isolate that pixel the pixel value was 8 3 7 5 8 4 so we're gonna say is the value greater than not the exact value but we'll use 8 3 7 5 8 0 and is the value ascend or equal to and we'll give it 9 0 so ctrl C to copy ctrl V to paste and if that's true we'll give it a value of 1 if it's false we'll give it a value of no data and we'll call that high flow accumulation and then just okay to execute that con test that condition there are two pixels that meet that condition and those two pixels are right at the outlet of the Yukon River so then we can use this to start the delineation of our watershed so we're gonna basically going to start at these two pixels and then go out until the flow Direction changes at a Ridgeline do that we'll use the watershed tool our flow Direction roster we created and our pour point roster is the output so the output is these two pixels which are our high flow accumulation that's going to create a raster of watershed and we'll output that to our home folder and we'll call this Conn River watershed if notice it does start at those two pixels and then it goes out until there's a change in flow direction then if we go to our properties let's do unique value and we'll give it some water color and then we'll give it a symbology under display of 50% transparent and we'll uncheck our flow accumulation and then zoom to layer here is the estimated area of the Yukon River watershed you notice along the Brooks Range it basically stops at the crest of the Brooks Range because this is the watershed draining to the North Slope of the Brooks Range and the same thing in the Alaska Range if we zoom to layer in the Alaska Range here it's flowing into the Copper River Basin not the Yukon River Basin our final step is we'll convert this watershed that's a TIF raster into a polygon use the raster to polygon tool to convert our watershed where one represents your in the Yukon watershed to a and we won't simplify we'll follow every grid cell and then we'll name this Yukon River and since we're going to into a folder it's gonna be a shapefile the advantage of that is we can symbolize our watershed polygon so zoom to layer will symbolize it as hollow and then give the outline color some blueish color a thickness of 3 here is a polygon representing the Yukon River watershed and then we can calculate what's the area that Yukon watershed in square kilometers so if we open up the attribute table we'll add a feel for area so a double precision and OB in square kilometers and then we'll calculate that watershed area in square kilometers and then we'll adjust that numeric value nothing to the right of the decimal add 1000 and show zero pad with zeros and Cho thousand separators our estimate of the Yukon River watershed is eight hundred and thirty-seven thousand five hundred and eighty six kilometer or square kilometers and let's go to properties and we'll give it an alias and then we'll give it one to the right of the decimal so there we have it

Channel: GIS Analysis NRM435 University of Alaska Fairbanks

Views: 335

Rating: 5 out of 5

Keywords: dem, arcmap elevation, watershed, basins, watershed delineation, watershed analysis

Id: Rq5KFnUcoZA

Length: 9min 15sec (555 seconds)

Published: Wed Apr 04 2018