[GRASSLIST:5491] Re: watershed and hidraulic gradient

[javier garcia - CEBAS]

Just a remark.
Hydraulic gradient will depend on several hydrologic factors:
f(dem, rain, faults, soil characteristics, wells). Depending on your aim, to 
trust a dem as the only source of information can be hairy.

-- 
A. Javier Garcia
Water and Soil Conservation Department
CEBAS-CSIC
Campus Universitario de Espinardo
Apartado 4195
30080 Murcia (Spain)
Phone: +34 968 39 63 90
Fax: +34 968 39 62 13
email: rn001 at cebas.csic.es
-------------------------------------------------

El Jue 06 Feb 2003 17:34, Soil & Water Lab. escribió:
> > 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.