SCHRIFTENREIHE Institut für Pflanzenernährung und Bodenkunde ...

tool for sampling strategies and for verifications of hydraulic process.
Keywords: Time stability; Soil moisture; Hydraulic model; Sample strategy
1. Introduction
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Temporal stability of soil moisture is essential to understand the variability of
structures and functions over an area, as well as the underlying hydrological
processes (Grayson et al., 2002; Lin et al., 2006). It is characterized by the
moisture status of soils which in some areas may be consistently wetter or drier
than in other areas, or compared with the average moisture content across the
whole area (Vachaud et al., 1985). Therefore, the temporal stability suggests
that the field mean moisture could be determined from few point measurements
of time-stable sites. This aids to reduce the sampling number and frequency,
thus increase sampling efficiency without a significant loss in accuracy.
Using the temporal stability concept, the sampling strategy is to investigate
if some locations can represent the field mean moisture and if this character can
conserve during the considered period. This has been proofed by many reports
(Kachanoski and de Jong, 1988; Lin et al., 2006; Parent et al., 2006), which
showed that certain sampling locations represented the mean of the whole study
area reasonably well. Moreover, Vachaud et al. (1985) found that some locations
could represent the mean field water content at any time of the year. Therefore, if
the time stability concept can be successfully applied, the selected points may
be assumed to accurately represent the mean soil moisture beyond the
measured period. However, the selection of the time-stable sampling locations is
based on a posteriori information derived from previous data analysis, thus it is
not really practical. Hence, an apriori approach to estimate time stable locations
is of more interest from a practical view. Time-stable locations have been found
to be related to the averaged soil and plant properties, e.g. soil particle size and
vegetation coverage (Vachaud et al., 1985; Hupet and Vanclooster, 2004; Starr,
2005). Thus it should be possible to choose some parameters as a covariance of
soil moisture for an apriori procedure to estimate time-stable locations.
The high spatial variability of soil moisture and the small measurement