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.
I'm a little lost in this...
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.
--
------------------------------
Soil and Water Laboratory
Biological and Environmental Engineering Department
Cornell University
Riley-Robb Hall
ITHACA, NY 14853 - USA
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@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.