2.5. Soil moisture data
A TRIME-IPH TDR system (IMKO, Germany) was employed to sample soil
moisture in each plot in the 0 depth range. In order to capture detailed
temporal patterns of soil water use of jujube trees in the main root
soil profiles, soil water content was also recorded using EC-5 sensors
(Decagon Inc., USA) every 10 minutes in one plot for each of the four
treatments. To install soil moisture sensors, a 320-cm-deep pit was dug
and the soil moisture sensors were inserted horizontally at depths of
10, 20, 40, 60, 100, 160, 220, and under each of the four plots. Soil
water data for the first month after sensor installation were excluded.
The soil moisture outputs (θE ) obtained via
the EC-5 sensors were calibrated at the study site in order to improve
accuracy. According to Li et al. (2018), the calibration equation was:
(1)
where denotes volumetric soil water content derived by multiplying
gravimetric water content by dry soil bulk density.
In this study, daily soil water observations were used to analyze
temporal patterns of water use by jujube trees. Daily soil water data
clearly has less noise than the 10-minute and hourly data; however,
using daily data can obscure the effect of gravity drainage following
rainstorms when quantifying soil water use, because the relatively high
infiltration capacity of loess soil means that gravity drainage is
expected to last less than one day (Zhu et al., 1983). Daily soil
moisture data were derived by arithmetically averaging soil moisture
data at the 10-min resolution over a whole day.
Here, the 0-100 cm soil layer was defined as “shallow”, because this
layer clearly exhibits higher temporal variations in soil moisture (Gao
et al., 2014) and higher fine root density (Li et al., 2017) than the
layers below 100 cm. Furthermore, this definition of a shallow layer can
also be found in other regions (e.g., Broedel et al., 2017; Yang et al.,
2017).