[HOT] What's water?

Pat Tressel ptressel at myuw.net
Mon May 4 09:03:54 UTC 2015

> I'm seeing some rivers mapped in areas that didn't have high-res imagery,
>> that (in the DG imagery) cross over dry land, or through areas were tree
>> tops are visible against a dark background that could be either water or
>> shadow -- it's a dark gray-purple.  What I'm interpreting as the actual
>> river channel is more turquoise and has (what look like) white rapids.  The
>> imagery has no clouds, and land looks fairly dry, so this imagery may,
>> perhaps, be dry season.
>> Note the previously mapped rivers in this area are very rough -- points
>> are far apart -- which implies they were mapped from low-res imagery.
>> Some questions:
>> Is there a large change in water volume in rivers during the dry season?
>> I'm wondering if water recedes to the deepest channel, and does show more
>> whitewater then.
> Yes, changes in water volumes between wet and dry season can be immense in
> Nepal. Some stream beds may also go dry in the dry season.
> I would guess that some mountain streams may behave as you suggested, but
> generally, my expectation would be for more whitewater in the wet season.

Disclaimer:  Hydrology is not my field, so this is an amateur
description...  Whether there's whitewater may depend on the shape of the
riverbed and the depth.  If the river is shallow, then whitewater can be
caused by flowing over boulders or other irregularites -- if it's deep,
those boulders might be covered, so the surface would be more smooth.  If
the river has a U-shaped channel, i.e. it's constrained in a narrow channel
even as there is more water flowing, then there might be whitewater near
the edges or around bends.  If the channel is narrow at the deepest part,
and curves out, so that it floods out onto a much wider area as volume
increases, then the flow may slow down.

> Do trees grow in standing water (deep enough to appear dark) in Nepal?
>> That's not unheard of -- it's true in the Everglades in Florida.  Or is an
>> area with treetops and dark between more likely dry but shadowed?
> Generally, most of the water in Nepal flows fast (we have lots of
> elevation changes), and there are very rarely trees growing in
> standing/moving water as in Florida.

Ok, thanks!

> I don't understand your second question.

Is that this question? "Or is an area with treetops and dark between more
likely dry but shadowed?"  I see rivers mapped through areas with trees and
dark grayish-purple between.  (This is in an area where there is no Bing
imagery, so the river may have been just roughly mapped from low-res
imagery.)  In other areas that are clearly dry, that dark grayish-purple
seems to be shadow under the trees.  The imagery has long shadows, so the
sun is at a low angle, so wouldn't shine straight down between the trees.

Let me find a good example, in case you'd like to take a look...

The imagery is:
Right here:
28.149462, 85.0427058
is a river showing the turquoise and white color, then the trees and dark
grayish-purple beside it.  It's possible that whole area is flooded, but
there are a lot of dry streambeds elsewhere, which seems wrong for
monsoon.  So I'm wondering if some of that dark area is just shadow.  It
could be that some is water and some land -- that the shadow leads to both
having the same lack of color.  In areas with more widely separated trees,
one can see that the sun is low and in the south-south-east.

-- Pat
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