A. Gottlein et B. Manderscheid, SPATIAL HETEROGENEITY AND TEMPORAL DYNAMICS OF SOIL-WATER TENSION IN A MATURE NORWAY SPRUCE STAND, Hydrological processes, 12(3), 1998, pp. 417-428
In ecosystem research great effort is made in measuring soil water ten
sion, because this is a critical calibration variable for modelling so
il water fluxes. In this paper the spatial heterogeneity and temporal
dynamics of soil tensions and their consequences for the determination
of water fluxes are investigated. Studies were carried out at a Norwa
y spruce stand in the Fichtelgebirge (NE Bavaria). Standard tensiomete
rs were installed at three soil depths (20 each) on the whole experime
ntal plot, as well as 45 microtensiometers as a dense grid in a small
soil pit. Microtensiometry at the centimetre scale showed that, depend
ing on rain intensity and initial soil water tension, even a soil with
out discernible macrostructure may show preferential water infiltratio
n. At the stand scale the variability of soil hydraulic properties and
tree root distribution causes substantial heterogeneity of soil water
tension, as observed by standard tensiometers. A functional relations
hip between increasing spatial heterogeneity of tensiometer readings a
nd increasing soil water tension was found, which was particularly pro
nounced after longer dry periods. Also at low soil water tension, wher
e spatial heterogeneity was low, the calculation of water fluxes from
tensiometer values was critical, owing to the fact that small differen
ces in measuring soil water tension resulted in big differences in cal
culated water fluxes. At high soil water tension in summer the spatial
heterogeneity of tensiometer readings was extremely high. At our expe
rimental site, since 30% of the total rain in summer falls in events h
aving a precipitation rate greater than 5 mm h(-1), preferential water
and solute flow was an important phenomenon. We conclude that the val
idation of calculated water fluxes using measured soil water tension a
t the stand scale is not an appropriate tool, because of measurement d
ifficulties, considerable spatial heterogeneity, especially in dry per
iods, and the great variability of soil hydraulic properties. (C) 1998
John Wiley & Sons, Ltd.