sebastiano trevisani wrote:
I forgot to mention the fact that sometime when you detrend hydraulic
head data using topography it could happen that the remaining
residuals show a pure nugget effect (i.e. all the spatial continuity
is described by the trend).
Then, (do you agree?) if you use depth to water (i.e. calculate the
residuals subtracting the elevation) you could still have a trend.
Sometime, in mountain you have that the water depth decreases with
elevation: this is because the "water table" surface is not parallel
with topography but converges to topographic surface going downhill.
Yes, I think the answer to these questions would largely depend on the
idiosyncrasies of a given study area. If there's a lot of topographic
variation, e.g., mountainous with many catchments within the study area,
the concept of the water table being a "subdued replica of the
topography" probably is helpful, and the DEM might capture all the
spatial continuity of water table elevations. On the other hand, on an
alluvial plain with numerous pumping wells, depth to water and water
table elevation might be more related to proximity to pumping wells than
uniformly sloping topography, and the water table may locally be more
variable than the topography. Yet, if the pumps are turned off and the
water table recovers to regional trends, the water table and the
topographic slope may be closely related. Haitjema and Mitchell-Bruker,
Ground Water, 43(6), 2005, use dimensional analysis to argue that
mediation of the water table by topography depends on both topographic
features and hydrologic parameters (e.g., a highly permeable landscape
will have a water table less reflective of topography).
Regarding your second point, that the water table converges with
topography in valleys, one of the interesting things about the Desbarats
et al. paper was their use of the TOPMODEL topographic index as a
secondary variable. One would suppose that using the topographic index
(rather than elevation) would capture some of that convergence of the
water table and land surface between ridge tops and valley bottoms. As
it turned out, for their study, elevation provided a more robust
external drift model than topographic index. Nonetheless, exploiting
the simplified hydraulics contained in the topographic index is an
appealing approach.
Bob
--
Bob Harrington, Hydrologist
Inyo County Water Department
163 May St.
Bishop, CA 93514
(760) 872-1168
[EMAIL PROTECTED]
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