SCHRIFTENREIHE Institut für Pflanzenernährung und Bodenkunde ...

Chapter 4 Temporal stability of soil moisture and its application in model result validation
and further select a modeled location (model certainty) with a prior time-stable
analysis. Simply, we will therefore focus on the following steps: at first, we detect
the best TSP in each plot, and then run the HYDRUS-1D model with this point to
get the water dynamics throughout the whole monitoring time. Finally, we
compare the model results applied in TSP with the measured value both from
TSP and field fixed monitoring site to address the validity of this method. The
van Genuchten hydraulic parameter of TSP is derived from data of soil texture
and bulk density by the neural network prediction tool ROSETTA (Schaap et al.,
2001). The calculated parameter is further optimized by in situ measurements of
hydraulic conductivity using a Mini-disk Infiltrometer in 0.5 cm suction (Zhang,
1997).
To link the spatial patterns of soil moisture and provide a validating data for
a hydraulic model, in situ filed monitoring soil moisture was measured using
horizontally inserted Theta-probes in three depths at 5, 20 and 40 cm. At each
plot three replicate profiles were installed and connected to one solar powered
automatic data-logger (type DL2e, Delta-T Devices, Cambridge, UK), and the
depth-averaged soil moisture at 30-min intervals was used. During the
experimental period, the in situ weather station recorded the precipitation and
other variables necessary to estimate the reference evapotranspiration from
FAO Penman-Monteith equation (Allen et al., 1998). Plant parameters, such as
vegetation coverage, leaf area index, and plant height, were taken from
vegetation measurements. Root samples were taken with soil root auger and soil
cores were separated into five depths of 0-10, 10-20, 20-50, 50-70 and 70-100
cm (see details in Zhao et al., 2008b).
3. Results and discussion
Temporal stability of soil moisture
To investigate whether temporally stable locations exist within the field, we
perform a visual inspection of the graphs presenting the ranked mean relative
differences (MRD) with their errors (Figs. 2-5). In general, the MRD shows a
strong temporal stability for most points at each plot during the period studied.
The MRD ranges from –15.9 to +11.9% at UG 79, from –10.8 to +14.7% at UG
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