SCHRIFTENREIHE Institut für Pflanzenernährung und Bodenkunde ...

Chapter 3 Factors controlling spatio-temporal variability of soil moisture using multivariate geostatistics
Many studies envisioned soil moisture variability should be strongly
controlled by variations in hydraulic conductivity (K), especially under wet
conditions (Hawley et al., 1983; Chanzy and Bruckler, 1993; Hébrard et al.,
2006). Unexpectedly, we did not detect a correlation between K and SWC (Table
4). K depends strongly on pore size and continuity, wetting properties and SWC.
Considerable temporal variability in all of these properties, combined with
different impacts on K depending on antecedent environmental conditions, may
have confounded any clear trend being observed. The only significant
relationship between K and SWC was found for UG 79 when dry. This trend
could be caused by the influence of SOC on wettability when the soil was dry, as
indicated by the strong relationship between K and WDPT (Zhao et al., 2007).
Animal trampling in HG may have caused the negative correlation between
shear strength and SWC. The recovery of soil physical structure, reinforcement
by plant roots and cohesion by SOC in UG 79 may have reversed this trend as a
positive correlation was found.
Plant heterogeneity may also control soil moisture patterns as rooting depth,
surface cover, and species composition affect evapotranspiration processes.
SWC was significantly correlated with vegetation coverage (VG) and
aboveground biomass (AGB) in the continuously (CG) and heavily grazed plots
(HG), irrespective of moisture condition (Table 4). However, in the ungrazed
plots, correlations were only observed under a wet water status in UG 79, and
under medium and wet water statuses in UG 99. Reynolds (1970), Hawley et al.
(1983) and Francis et al. (1986) found that vegetation coverage was a major
factor influencing soil moisture variability. Hawley et al. (1983) further noted that
differences were often greater in wet conditions than in dry conditions, but our
study did not support this finding. In the HG plot, the negative correlation
between AGB and SWC showed the influence of plant transpiration. However,
VC was positively correlated to SWC in the HG plot, illustrating the deleterious
impact of grazing intensity on both plant coverage and soil physical structure.
With minor exceptions, neither surface factors, e.g. relative elevation, slope and
aspect (local control) nor subsurface factors, e.g. curvature, upslope contributing
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