[GIS] How to calculate for every basin/sub-basin, drainage area at every elevation contour along stream

arc-hydroarcgis-10.0watershed

With DEM as a source, I created all necessary files like flow direction, accumulation, slope, streams, watersheds, pour point etc… In my watershed (basin), I have many sub-watersheds (sub-basin) with streams and outlets at various elevations.
I need to do analysis on the following two basis.

I need to calculate for every basin/sub-basin, the drainage area at every elevation contour along the stream. Please ref figure for better understanding my view.
I the figure there are six elevation contours (2 black top-bottom, 4 red in between) in the basin/sub-basin. In the figure, outlet is at the bottom elevation. Now I want to change outlet to all above elevations along the stream. And for each elevation point outlet along the stream, I need to calculate drainage area and volume.

I am trying various options with ArcGIS, Global Mapper, AutoCAD, Surfer, MicroStation and others…

Please give me some idea in doing this watershed analysis with less effort with any of the above said software/tools/extensions/toolbox with better accuracy.

Best Answer

For Q1, if you aren't recalculating the watershed at each contour line, you can use the Split Polygons tool in ArcMap to chop up your watersheds. You need an ArcEditor or ArcInfo license to use it. It will chop the watersheds by the contour lines and retain attributes.

So, a possible process could be to have an acreage field from the original watersheds, split the watersheds, add another acreage field and calculate acreages on the new polygons, then use the Field Calculator on a third field to determine percent of total acreage in each subwatershed.

This method assumes you have converted watersheds to vector data and that the contour lines are also vector data.

Related Solutions

[GIS] Automatic Watershed Delineation in ArcGIS Desktop and Arc Hydro

The areas you are looking to create are called "subwatersheds" or subsheds. You can use the "Batch Subwatershed Delineation" tool in ArcHydro under the "Watershed Processing" menu. This will calculate subsheds from a set of points you define. The other input is a flow direction grid. This is a raster where the value of each pixel represents the direction water would flow from this pixel (N, NE, E, SE, S, SW, W, NW).

The data you have in your screenshots look like the 1:100,000 resolution National Hydrography Dataset (NHD). If this is the case, the NHDPlus dataset has a flow direction grid already calculated that you can download. Otherwise, you will need to use the "Flow Direction" tool in ArcHydro under the "Terrain Preprocessing" menu to manually calculate flow direction from an input digital elevation model (DEM).

Note that the spatial resolution of your subwatersheds is limited to the spatial resolution of the input DEM.

[GIS] How to find the watershed for each outlet point and sum values within it

Assuming that each withdrawal point is related to the closest stream segment, we can do the following:

Use the split line at point tool to split the stream line into segments based on outlet points (depending on your data you may already have this). Make sure that each point has an ID, which should relate to stream order (see one method below for an example in Python).
Use the generate near table tool to generate a table that will link your withdrawal points to the nearest stream segment. Check the only get closest checkbox so you only get one result for each point.
Join the withdrawal amout to the near table to associate with the segment IDs.
Use the summary statistics tool to sum the withdrawal amount grouped by segment ID.
Join the output withdrawal sums to your stream segment, select those segments upstream from the point of interest, and sum the total withdrawals (again, see below for one way to do this).

And example of IDing the segments using Python.

import arcpy

#Written assuming arcGIS 10
#Note this will require modification to run...

start_pts = {}
end_pts = {}

for segment in arcpy.SearchCursor("segmented_line"):
    start_pts[segment.SEGMENT_ID] = segment.SHAPE.getObject(0) #SHAPE returns an arcpy.Array
                                                               #object containing points for
                                                               #geom type line
    end_pts[segment.SEGMENT_ID] = segment.SHAPE.getObject(segment.SHAPE.count)

try:
    #cleanup as you can only have one cursor open at once, otherwise wackiness
    del segment
except:
    pass

rows = arcpy.InsertCursor("start_pts")
for start in start_pts:
    row = rows.newRow()
    row.SHAPE = start_pts[start]
    row.SEGMENT_ID = start
    rows.insertRow(row)

try:
    #more cleanup
    del row
    del rows
except:
    pass

arcpy.SpatialJoin_analysis(start_pts, outlet_pts, "start_outlet_pts",
                           "JOIN_ONE_TO_ONE", match_option="WITHIN_A_DISTANCE",
                           search_radius="some metres")

arcpy.JoinField_management("segmented_line", "SEGMENT_ID", "start_outlet_pts", "SEGMENT_ID", "OUTLET_ID")

################################################################################
##Repeat for end_pts
################################################################################

Once I get a bit of time later I'll come back and update this answer with a bit of script to build a network using networkx to help sum the values...

Hope this helps!

Best Answer

Related Solutions

[GIS] Automatic Watershed Delineation in ArcGIS Desktop and Arc Hydro

[GIS] How to find the watershed for each outlet point and sum values within it

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