D. Mallants et al., PREDICTED DRAINAGE FOR A SANDY LOAM SOIL - SENSITIVITY TO HYDRAULIC PROPERTY DESCRIPTION, Journal of hydrology, 206(1-2), 1998, pp. 136-148
Prediction of unsaturated how phenomena at field-scale requires a set
of hydraulic functions that capture local-scale variability which is p
resent in all natural soils. Several sets of hydraulic functions measu
red on different core sizes collected across the field were used to pr
edict drainage from a saturated soil profile. Simulations were carried
out with a one-dimensional numerical model, based on the Richards equ
ation. Four methods were considered: (1) Monte-Carlo simulation using
500 unimodal retention and conductivity functions representing theta(p
si) and K-S data measured on 0.05 m diameter 0.051 m long core samples
; (2) multiple simulations with a set of 60 multimodal retention and c
onductivity functions which better represented the measured theta(psi)
data of method 1; (3) one single simulation with a set of hydraulic f
unctions obtained from a gravity-drainage experiment on 15 l m long 0.
3 m diameter soil columns collected from the same field; (4) one singl
e simulation with a set of hydraulic functions obtained by scaling the
retention and conductivity data from method 3. A perfect match of the
final mean outflow was obtained when scaled retention data was used i
n combination with scaled K-S values (method 4). All other cases under
estimated the total outflow to a varying degree: 30% for method 1, 29%
for method 2, and 21% for method 3. The results further revealed that
none of the four methods was able to completely describe the mean obs
erved drainage from the start until the equilibrium condition. This wa
s further demonstrated by the disparities between the mean observed so
il water content profile and the simulated values using hydraulic func
tions from method 4. Especially after the first day, differences were
large, presumably because macropore flow could not be described using
the Richards flow equation. Despite the introduction of multimodal ret
ention and conductivity functions which better described the retention
behaviour of small soil cores (method 2) in comparison with unimodal
retention functions (method 1), mean predicted outflow for both method
s was nearly identical. (C) 1998 Elsevier Science B.V. All rights rese
rved.