Simulation of water flow and calculation of the related physical quality
indices as influenced by soil water retention curve fitting methods
Abstract
Accurate fitting of soil water retention curve (SWRC) parameters is
crucial in the modeling of soil water flow and the assessment of soil
quality. The un-weighted least squares regression (ULS) is the most
common approach applied for fitting the SWRC functions to the observed
data-points in order to optimize their parameters. However, the variance
of SWRC data varies in different water contents; therefore, unlike the
wet-end of the SWRC, the ULS method may not be sufficiently effective in
estimating its dry-end. This study examined the differences between
parameter approximations achieved by the ULS and the weighted
least-squares (WLS) in the SWRC. Then, an analysis of both approaches in
the simulation of water redistribution and the related soil physical
quality indicators (SPQIs) was done. Accordingly, the measured SWRC data
in six replications were fitted to the SWRC equations to optimize their
parameters, through either WLS or ULS. The results showed that despite
the increase of error in the SWRC estimation by the WLS method
(RMSE=0.027 and 0.043 cm3 cm-3 in the ULS and WLS, respectively), WLS
increased the accuracy of the estimations at the lower water contents
(dry-end), when compared to the ULS. The WLS regression resulted in
different values of SPQIs (e.g., S-index = 0.033 and 0.042 or RFC
(relative field capacity) = 0.57 and 0.62 in the ULS and WLS methods,
respectively). Furthermore, the simulated soil water movement in either
wet or dry water conditions was different for the SWRC parameter
estimated by WLS and ULS regressions.