SCHRIFTENREIHE Institut für Pflanzenernährung und Bodenkunde ...

Chapter 4 Temporal stability of soil moisture and its application in model result validation
support requires therefore appropriate sampling strategies and monitoring sites
(Kamgar et al., 1993). Due to the high cost and time-consuming of long-term soil
moisture monitoring, it is rare that the monitoring sites are uniformly distributed
in the entire studied area. On the contrary, the design of monitoring sites is
usually irregular (Gomez-Plaza et al., 2000; Martinez-Fernandez and Ceballos,
2005; Lin, 2006). Consequently, selected monitoring sites may not represent the
field mean moisture in terms of the temporal stability concept, especially in an
area without grid samplings but under heterogeneous conditions of soil, plant
and topography. The alternatives to direct measurement are estimations by
remote sensing data or use of hydraulic models (Albertson and Kiely, 2001;
Cosh et al., 2004; Martinez-Fernandez and Ceballos, 2005). Both methods,
however, require in situ measurements for the calibration and validation steps. In
addition, in the study of hydraulic model, normally the observation or modeled
points are selected without the prior analysis of representativeness of the
selected points. To account for this uncertainty, the temporal stability concept,
combined with a hydraulic model applied in the derived time stability point (TSP),
should be valuable. However, till now this kind of study is still lack (Hupet and
Vanclooster, 2004). Moreover, selected monitoring sites according to the
temporal stability of soil surface moisture may not represent time-stable
conditions for the deep soil (Lin, 2006). As Martinez-Fernandez and Ceballos
(2003) and Pachepsky et al. (2005) pointed out, relatively less is known about
the temporal stability of soil moisture as a function of depth. Thus, the use of
hydraulic models is a very promising alternative to obtain soil moisture not only
for the topsoil but also for the subsoil.
The temporal stability possibly helps to provide valid data for hydraulic
models by which applied in the responsible points. However, until now this has
not been used and tested to estimate the field water content and flux for a given
probability level (Seyfried, 1998). Here we analyze the temporal stability of soil
moisture using data from four plots under different grazing intensities during 3-yr
measurement periods. At the same time, we run a hydraulic model HYDRUS-1D,
combined with in situ field monitoring soil moisture data, to evaluate the validate
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