Geoscience Reference
In-Depth Information
2.5 m
(a)
0.25
0.2
2.0 m
0.15
0.1
0.05
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0
0
0.5
1
1.5
2
2.5
Water Content (m
3
m
-3
)
x-Direction (m)
2.5 m
(b)
0.25
0.2
2.0 m
0.15
0.1
0.05
0
0.00
0.05 0.10 0.15 0.20 0.25 0.30 0.35
0.40
0
0.5
1
1.5
2
2.5
x-Direction (m)
Water Content (m
3
m
-3
)
fIGURe 25.10
Simulated soil water content distributions of the 2.5 × 2 m two-dimensional flux field (left)
and within two depths: 35 cm (thin line) and 125 cm (bold line) 13 h after onset of irrigation. (a) Shows
water content assuming a layered structure and (b) resulted from a random distribution of the soil hydraulic
properties.
25.4 ConClUSIonS
In sandy soils as they have been investigated in this study, GPR is a valuable technique for map-
ping soil water content at an intermediate scale in between point (TDR) and area measurements
(remote sensing). The observed strong correlation between the change of the reflection time of
the electromagnetic wave and the change in soil water content as determined from gravimetrical
measurements (r² = 0.82) confirmed the accuracy of the GPR technique. The popular Topp equa-
tion proved suitable for the registration of the seasonal water status development in the top soil.
However, the analysis of the velocity of the electromagnetic wave alone does not provide depth-
dependent information about the soil water regime. A more detailed inspection of the radargram,
especially of the reflection amplitudes, is necessary when information about a change of soil struc-
ture or soil water content with depth is needed. In addition, the statistical analysis of the reflection
amplitudes or other parameters obtainable from the GPR measurements might be a way to reveal
the heterogeneity of the soil water content and the related water flux field.
First results presented in this study indicated that the assumption of a heterogeneous distribu-
tion of soil hydraulic properties coincides better with the geophysical data than a generated homo-
geneous case (homogeneous soil properties within a single soil layer). It can be concluded that even
in homogeneous sand profiles, local variabilities dominate water flow which confirms previous
findings from other studies in which, for instance, the dye tracer technique has been used to identify
flow patterns (Flury et al., 1994). The variances of the soil hydraulic and geophysical parameters
differed, although in general the heterogeneous case performed better than the homogeneous case.
The next step essential for evaluating the potential of the method would be using the analyzed vari-
ance of GPR amplitudes for each depth interval directly in the numerical models for simulating water
flow. Comparisons of water discharge rates derived from numerical models based on soil hydraulic
parameter distributions determined from the GPR analysis with real measured data will show if a bet-
ter agreement of model and experiment can be reached. We expect a better description of the water
flux variation by using the information about soil heterogeneity derived from GPR measurements.
