Journal of the Japan Society of Erosion Control Engineering, Vol.59,No.3,2006
Methods for determining soil hydraulic parameters related to vertical rainwater
infiltration
Weili LIANG Ken'ichirou KOSUGI Yuki HAYASHI Takahisa MIZUYAMA
Abstract
Hydrological models, which simulate rainfall infiltration and groundwater generation
at forested hillslopes, have been proposed for predicting shallow]landslides.
However, model performances were not adequately evaluated in comparison with observed
rainwater infiltration processes. These models require soil hydraulic parameters,
which describe water retention characteristics and hydraulic conductivities, as
input data. While these parameters were usually determined in a laboratory by
using soil samples, it was suggested that in]situ infiltration processes are not
adequately described by using the laboratory]determined hydraulic parameters.
In this study we examined three methods to determine hydraulic parameters; the
traditional laboratory methods for measuring retention characteristics and saturated
hydraulic conductivity with estimating unsaturated hydraulic conductivity by Mualem's
model (Method 1), the combination of the laboratory and field methods (Method
2), and the field method which uses vertical profiles of water content and matric
pressure observed in]situ during a storm event (Method 3). The accuracy of each
method was evaluated by comparing observed and predicted water contents and matric
pressures for 10 storm events. Results showed that Method 1 tended to overestimate
water storage capacity of soil and under estimate hydraulic conductivity, resulting
in a delayed wetting front propagation. On the average, the error between simulated
and observed water content changes for Method 1 was 5.84 times greater than that
for Method 3. Method 2 produced better prediction results than Method 1, but performed
worse than Method 3. In conclusion, Method 3 was the suitable method to determine
hydraulic parameters for describing rainwater infiltration processes at forested
hillslopes.
Key words:Rainwater infiltration, Soil water retention, Hydraulic conductivity,
Richards equation, Numerical simulation
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