2.2.2 Identification of rainfall process and soil wetting
event
An automatic weather station was set up at the bare land uphill slope
site. Rainfall was recorded every minute by a tipping bucket rain gauge
(0.2 mm/tip; Jiangsu Naiwch Corporation Inc., China). The definition of
a new rainfall process was when the 1-h rainfall amount was larger than
0.4 mm (surpassing the threshold of instrument noise) and recorded after
the period without any rainfall record exceeding 48 h. We defined the
end of the process when the rainfall amount was less than 0.2 mm for 48
h (i.e., the level of instrument noise). Within the rainfall process, we
defined the timing of a rainfall record with an interval of more than 24
h from the previous rainfall record as the initiation of an individual
rainfall event.
The concept of a soil wetting event was adopted, in which a significant
increase in SM as the result of rainfall infiltration into the soil can
be observed (Lozano-Parra et al., 2015; McMillan and Srinivasan, 2015).
For all the probes, we defined the start time of the soil wetting event
if the increase in accumulated SM was >0.2 vol.% greater
than the initial SM (Hardie et al., 2013; Lin and Zhou, 2008). In our
study, we found that SM was subject to soil temperature fluctuation that
might cause a decrease at a rate of 0.1 vol.% per h during the daytime.
An SM increment of 0.2 vol.% was selected to exceed the device noise
(0.1 vol.% for the EC-5 SM probes) (Rosenbaum et al., 2010).
Furthermore, we used the time without an increase in SM at 6 h as the
end time of the soil wetting event process.