Example: Create Watershed Areas

We can create Watersheds areas using the Watershed : areas transform operation.  Watershed areas show regions of common drainage.  


This topic should be read together with other watershed topics:











We begin with the Montara terrain elevation surface, seen below, as used in the Watersheds topic.



With the focus on the surface, in the Transform pane we choose the Montara image and we choose the Tile field.   We double-click on the Watershed template to launch it in the Transform pane.



In the Watershed template, we choose channel 0 as the Channel.   Single channel images (as often are used for terrain elevation data) will automatically have channel 0 loaded into the Channel box.


As an Output option we choose the areas operation.   We enter 500 for Minimum flow.   We specify Watershed areas 500 for the name of the New drawing to be created by the template.  As we enter the name for the drawing, the pane will automatically fill in an analogous name for the table.   We can change that if we like.


For a preview of what the template will do, press Preview.



Pressing the Preview button shows the previewed results of the operation using blue preview colors along with a blue preview caption bar at the top of the window with the name of the template used for the preview.  Previews are shown on top of all map layers.   To close the preview, right-click on the caption bar and choose Hide Preview.



Previews are opaque by default, hiding all layers below the previewed objects or raster.   If we would like to see layers below, we can right-click onto the preview caption bar and choose partial opacity, for 75%, 50%, or 25% opacity.   50% opacity is shown above.    A close look shows that the borders between the various watershed areas are the ridge lines of the terrain.



We can also right-click onto the preview caption bar and choose Left or Right to split the display so only the left half or the right half shows the preview, and then we can drag the vertical blue splitter line to the left or right to increase or decrease the part of the display that shows the preview.  We can combine split screen with opacity settings at the same time.


To apply the transform operation, press Transform.


A new drawing called Watershed areas 500  appears in the Project pane.   We drag and drop the new Watershed areas 500 drawing into the map as a layer:



The new layer appears in the map using default formatting.   Each polygonal area is a drainage basin that has 500 or more total flow units.


No watershed areas have been created in the drainage leading immediately to Pilarcitos Lake because the "lake" is actually a reservoir, walled off from flow further downstream by a dam.   In hydrology terminology it is a sink, that is, a catchment basin from which there is no onward flow.  It is not the entire closed drainage basin, but is just what is left when the rest of the closed drainage basin is subdivided into watershed areas having a flow of at least 500 units in each area.


We could have used the Watershed Prepare transform template to eliminate sinks in the terrain, for continuous areas in the region of Pilarcitos Lake as well.  See the discussion in the Watershed Areas, Sinks topic and in the and the Watershed Prepare template.



Opening the drawing's table, we can see the fields created by the template.   The total flow originating within each watershed area is given by the Value field.   The total flow through each watershed area including all flow from upstream watershed areas as well as flow from within the area, is given by the ValueSum field.



We can style the watershed areas using a thematic format that colors the areas in a range from white to dark blue, based on the ValueSum value for each area.  



The resulting coloration shows watershed areas that have more cumulative flow in darker blue.  



To visualize the relationship between the total flow in the watershed areas and the terrain, we add a version of the Montara layer that has been styled in grayscale, called Montara Gray.   



We can then use the Layers pane to change the opacity of the Watershed areas 500 layer to 40%, so the Montara Gray layer can show through to provide some hill shaded terrain context.   We use the gray layer so the coloration of the Montara layer will not change the colors seen from the Watershed areas 500 layer from white to blue.


Another way to see the relationship between watershed areas that are created and the underlying terrain is to use a transparent fill color for the watershed areas, using black for the borders.



We can do that with a click of the Area Fill button in the Style pane, to replace the thematic format with transparent color.  



We can now see through the areas to see how watershed areas are computed from the terrain.   The borders of the watershed areas are ridge lines, where on either side of a ridge line the water drains to either the one side or to the other.


We zoom into the region shown near the beginning of this topic.



The borders of watershed areas follow ridge lines which divide flow into different drainage regions.   For example, we can see a watershed area boundary should fall somewhere on the ridge line between the outward flowing arrows on the hill seen above.



Following the boundaries of the watershed areas drawn over the hill, we can see they follow the ridge lines between drainage areas.   Each drainage area has a total of at least 500 units of flow.   They are not further subdivided into smaller areas that cover a total of less than 500 units of flow.



We can create labels using the ValueSum field, which gives total flow for each watershed area object, and drop those labels into the map as seen above, to report the total flow for each area.  We have formatted the labels to use the same thematic format based on the ValueSum field, from white to blue, to emphasize increased amounts of flow with more blue color.  All of the areas have a total flow greater than 500.  



Manifold vs Arc - Fifty times Faster than Spatial Analyst -  The first video in a series of comparisons. We compare Manifold Release 9 to ESRI's ArcMap with Spatial Analyst. ArcMap instead of ArcGIS Pro is used to ensure maximum possible speed with no slowdowns from AGOL connections. Starting with a terrain 5300 x 5300 elevation raster we compare Manifold workflow and speed creating streams (watershed lines) with ESRI ArcMap and Spatial Analyst doing the same task. ArcMap requires four operations calculating intermediate steps, taking a total of three minutes and 30 seconds to compute streams.  Manifold does the same job in a single operation in under four seconds, over fifty times faster than Arc, and with the convenience of a single click.  ArcMap plus Spatial Analyst cost over $5000 per seat while Manifold costs under $500.  As data sizes scale up, Manifold gets even faster than Arc. Works in the free Viewer, too!


Manifold vs Arc - Watersheds Sixty Five times Faster than Arc -  Another video comparing Manifold speed to ESRI ArcMap with Spatial Analyst, this time computing upstream watersheds on a 5300 x 5300 terrain elevation raster for a few dozen locations.  ArcMap requires three geoprocessing tool operations calculating intermediate steps, taking a minute and a half.  Manifold does the same job in a single click in less than 1.4 seconds, over 65 times faster than ESRI. The larger and more complex the geoprocessing, the greater Manifold's speed advantage.   ArcMap plus Spatial Analyst cost over $5000 per seat while Manifold costs under $500. Works in the free Viewer, too!


Manifold vs Arc - Seven Seconds vs Four Minutes - Finding basins in a 5300 x 5300 terrain elevation raster, we compare Manifold workflow speed and ease of workflow to ESRI's ArcMap with Spatial Analyst.  ArcMap Standard plus Spatial Analyst costs a total of $5250 so it should work better than a Manifold package that sells for under $500, right?  No way! Manifold absolutely crushes the comparison, taking only a single click and seven seconds to do what takes the $5000+ package three geoprocessing operations and four minutes, not counting the time to setup and launch three operations. Works in the free Viewer, too!


Manifold vs Arc - 100x Faster on an Affordable Desktop - Watch Manifold do in 0.9 seconds what takes ArcMap plus Spatial Analyst over a minute and a half.  That's over 100 times faster!   Some  comments on previous comparisons have stated that Manifold was so super fast compared to ESRI because tests were run on a high-end, Threadripper machine that could run 48 threads. This video shows Manifold is faster even with fewer cores on an affordable desktop system.  We re-run Manifold trials on a  Ryzen 9 3900x computer, with three different tasks taking only 0.9 seconds, 5.4 seconds and 3 seconds.  AMD's 3900x CPU now retails for as low as $450, setting a new baseline for affordable GIS desktop computing.  Everything shown in the video works in the free Viewer, too!


See Also













Style: Thematic Formatting


Transform Pane


Transform Topics


Transform Reference


Transform - Tiles




Example: Create Watershed Lines


Shreve Order and Strahler Order


Watershed Areas, Sinks


Upstream Areas and Lines


Watershed Prepare: Filling Sinks


Downstream Lines


Flow Direction and Accumulation