Using DEM in QGIS to Find Watershed Area for Each Point in River Network

grassNetworkqgiswatershed

I followed @Micha's instruction and successfully used the modules in GRASS.

I use r.watershed to get the basin raster and the drainage line：
enter image description here

and I use r.water.outlet to get the watershed of one outlet：
enter image description here

I got the whole basin area of the green outlet point, but all I want is the little part of the original basin area, like this：part B is what I want
enter image description here

This schematic photo shows the idea of what I want：
enter image description here

Can I achieve this goal using r.water.outlet or other modules in GRASS？ Or all I can do is using the intersect tool in Geoprocessing tools in QGIS？

I want to find the watershed area, but using QGIS.

I have following files to work with：

stream network shapefile(polyline)
point shapefile clipped from the polyline shapefile
a whole island(the stream is inside the island) DEM(I got 2 types:.lan & .tiff)

I want to find the total watershed area located upstream of every point in the network, here's the schematic diagram：

enter image description here

the green, red, and yellow line enclose the watershed of each point(here I only take 3 points as the example), and I want to calculate their area.

I already got the DEM file, and I must cut the region that contains the stream off, should I polygonize it first？

Can I directly use this form(.lan or .tiff) to work with GRASS？

Best Answer

You will definitely need an elevation raster (DEM) to do what you want. With the DEM you can use two GRASS modules to get individual drainage areas for points along the river network as follows: First calculate a flow direction grid from the DEM with r.watershed:

r.watershed elev=DEM thresh=<your threshold> drain=flow_dir_grid

See the r.watershed manual for more details Next loop thru the X-Y values of the points along the river network to calculate each watershed with the r.water.outlet module.

r.water.outlet drain=flow_dir_grid basin=basin1 easting=X1 northing=Y1

Details in the r.water.outlet manu page

For doing the loop, you might be able to get some ideas in this post

Related Solutions

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

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.

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

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

Related Question