[GRASSLIST:5487] Re: watershed and hidraulic gradient

[Soil & Water Lab.]

> I need to calculate hidraulic gradient for watersheds. Does anyone have a
> idea about how to do it? perhaps a cross with DEM to get the altimetric
> difference (how?), but maybe the length of the basin is the bigger problem.

The hydraulic gradient is the derivative of the hydrodynamic potential along 
space. The hydrodynamical potential \Phi is usually exprssed (in length 
units, viz energy per weight) as the sum
\Phi = h - z
where $h$ is the matric potential, and $z$ the vertical position (vertical 
axis oriented postively downwards).
$z$ will obviously depends on your DEM, but $h$ will depend on your soil 
characteristics. As a first estimation, you can leave out $h$ (viz, you 
asssume that capillary effects are negligble compared to gravity).

So, the hydraulic gradient would be approximated by$1$ in the vertical 
direction, and by the slope in the horizontal directions.

In GRASS, you get the slope with the "r.slope.aspect" command. Please check 
the doc for a complete description of the command. BTW, you should use the 
"percent" option, in order to get the slpe as \Delta X / \Delta z. You'll 
have to be cautious of your grid resolution": the coarser the grid, the more 
dubious the slopes value get.

Hope it helps
P.

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Soil and Water Laboratory
Biological and Environmental Engineering Department
Cornell University
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