Watershed Areas, Sinks

A watershed that has no outlet, for example, Crater Lake in Oregon, the Dead Sea in the Middle East, or the Caspian Sea between Europe and Asia, is called a sink.   A sink is a closed drainage basin, or, more technically, an endorheic drainage basin.  


The Watershed : sink areas transform operation creates areas that show sinks, regions that are closed drainage basins.  The areas created do not depend on minimum flow, since each area will cover all territory where rainfall drains into the same sink.


This topic should be read together with other watershed topics:











Creating watershed sink areas (closed drainage basins):


  1. Open an image that represents terrain elevation.

  2. In the Transform pane, choose the image and the Tile field in that image.

  3. Double-click the Watershed template to launch it.

  4. In the Watershed template options, choose the Channel desired.  Single channel images will automatically have channel 0 loaded.

  5. Choose the sink areas operation for the Output.

  6. Leave the Keep areas flowing outside box checked.

  7. The Result is always a new drawing and table.   Specify the names desired.

  8. Press Transform.



We begin with the Montara terrain elevation surface, as shown below.  



With the focus on the map window, 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 sink areas operation.   We check the Keep areas flowing outside box.


The Keep areas flowing outside box chooses how to handle the edges of the terrain elevation image.  By default, the box is checked, which means the edge of the image is considered a closed barrier through which water cannot drain and so areas which flow outside the image are retained as sinks.  That covers the entire image with closed basins.  To see just those drainage basins that are closed which do not drain over the edge, we can uncheck the box, in which case sink areas that flow outside the image will be discarded.


We specify Watershed sink areas 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 layers below.   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 allows part of the layer below to show through the preview.  To make the preview layer fully opaque, choose 100% opacity.


To apply the transform operation, press Transform.


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



The drawing appears using default formatting.  We can Style the drawing for a more informative display.



We use Style with a thematic format to color the different areas in different colors, using a white border line for the areas to set them apart.   We use the Layers pane to set 50% opacity for the drawing layer, so the Montara terrain underneath shows through.

Discard areas flowing outside

To see just those drainage basins that are closed which do not touch the edge, we can run the  Watershed : sink areas operation with the Keep areas flowing outside box unchecked.  



We do that to create a drawing we call Watershed sink areas non-edge.



The remaining, non-edge, sinks are shown above, with the view zoomed out slightly and opacity increased to 70% for the sinks layer. The Montara region has many such closed basins due to the use of dams.



We can take a closer look at the downstream edge of one of the sink areas, seen above, where a dam closes off flow.  



Double-clicking the sinks drawing layer tab to turn the layer off, we can clearly see the dam.

Watershed : areas compared to Watershed : sink areas

It might be confusing to consider the difference between watershed areas, that is, drainage basins, and watershed sinks, that is, closed drainage basins.  


All water falling on a drainage basin will drain to some common point.    In the case of watershed areas (not sinks), that common point will be located on the edge of the area, where the water will flow on to the next watershed area.  Watershed streams show the location on the edge of the watershed area where the drainage from that basin collects and drains to the next watershed area.  That continues until a region is reached that is either the edge of the raster terrain elevation image, or is a closed drainage basin from which there is no outflow.



The image above shows areas created with the Watershed : areas operation using a Minimum flow of 500.  The areas have been thematically colored in shades of gray by their Shreve order.   At the zoom level used for the illustration, two clear regions are visible as not being filled in with watershed areas.   



We have highlighted one of the clear regions by outlining it in yellow color.  It is easy to think that the filled-in, gray areas are watershed areas that are drainage basins, and that the highlighted, clear area is the closed drainage basin, but that is not precisely correct.   We can see that by adding the sinks layer computed earlier.



In the illustration above we have added the sinks layer with partial opacity, and we have also added a layer with a white border around each sink area, to more clearly delineate each sink area.  We can see that the clear region is only the last, most downstream part of the larger, green, closed drainage area.    


We can add watershed lines to see the drainage flow patterns more clearly.



The closed drainage basin, what the operation calls a sink, is the entire region from which water flows to a dead end.  In the case of the green sink area, any drop of water that falls with the white boundary of that green region will flow downstream, collecting together in flows with other drops of water, until it flows into Pilarcitos Lake in the clear region, from which there is no downstream exit because of the dam that forms the lake.  Any drops of water which fall onto regions outside of the green sink region will flow into one of the other drainage basins, but will not be trapped within the green, closed drainage basin.


The watershed areas seen in the gray colored areas in the Watershed areas 500 layer are local catchment basins where each basin accumulates a minimum flow of at least 500 units, with any water falling within that catchment basin collecting together as it drains down to the next basin.  The clear region is just that part which is left over, from which there is no exit, and within which no watershed area can be constructed that has at least a flow contribution of 500 units.



Consider a zoomed-in view of the clear region, showing Pilarcitos Lake and the hills immediately surrounding it.  Clearly, rain that falls on those hills will flow downhill into the lake.  But no region of common flow, such as one of the small ravines leading into the lake, accumulates enough flow to reach the minimum of 500 units required to build a watershed area,  based on the 500 unit Minimum flow we specified when we ran the operation to create the Watershed areas 500 layer.   Had we used a smaller Minimum flow value, we would have built smaller areas that would cover at least part of those hillsides.



In the illustration above, we have created a watersheds areas layer, areas 100,  using Minimum flow of 100, and dropped it into the map under the 500 layer.  We can see how many smaller watershed areas have been created, each of which would contribute a minimum flow of at least 100 units.    


Why are not all of the hillsides surrounding Pilarcitos Lake covered by smaller areas, such as the larger, knobby hills on the South side of the lake?   When rain falls on those rounded hills water flows in all directions downhill and is not concentrated by a ravine or valley.  The result is that the rounded hill would be covered by very many watershed areas, each with a minimum flow well under 100.  


The essential message is that the clear regions left over after creating watershed areas for a given minimum flow indeed do indicate the presence of a sink, but the clear region is not the entire closed drainage basin.  It is simply what is left over, at the tail end of the drainage, after the drainage basin has been subdivided into watershed areas that each have the specified minimum flow, and each of which locally is not a closed drainage basin.


Sink is slang - The word sink is used casually and imprecisely in the name of the Watershed : sink areas operation.  The operation creates areas that are closed drainage basins, where all rainfall that falls anywhere within the area drains to a common point somewhere within the interior of the area.   Strictly speaking, that common point is the sink.   As a practical matter, sinks are usually not single points but are themselves areas, such as lakes, within the closed drainage basin into which all water drains.    However, sink is such a short, convenient word that it has become popular as a synonym for closed drainage basin.  ESRI calls sinks basins, and other packages may call sinks pits.


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 Areas


Example: Create Watershed Lines


Shreve Order and Strahler Order


Upstream Areas and Lines


Watershed Prepare: Filling Sinks


Downstream Lines


Flow Direction and Accumulation