Limitations of the Model Structure and Soil Hydraulic Property
Observations in Pedotransfer Function Development
Abstract
The commonly applied pedotransfer functions (PTFs), which predict soil
hydraulic properties (SHPs) from easily measured soil properties such as
texture information, often account only for capillary forces. Recent
advances in soil hydraulic modeling suggest that, to improve the
prediction of SHPs under dry conditions, the impact of adsorption forces
has to be taken into account. However, the lack of observations in
particularly dry conditions, due to the difficult and time-consuming
measurement, hinders the development of PTFs that predict SHPs from
saturation to oven dryness. In this paper, we first present a simple
method for predicting complete SHPs with limited measurements that cover
only a relatively high matric potential range. With this method, we
extended a public dataset to cover dry conditions, and then applied it
to develop PTFs that can predict SHPs from saturation to oven dryness.
This was achieved by applying the complete soil hydraulic model proposed
by Wang et al. (2021), which accounts for both capillary and adsorptions
forces and overcomes the unrealistic decrease near saturation for
fine-textured soils. The impact of vapor diffusion was also considered.
We further applied this method in extending an existing capillary-based
PTF to dry conditions. The results showed that: 1) the proposed method
performs very well in describing SHPs over the entire moisture range; 2)
the PTFs developed with the extended observations and the complete model
show a superior prediction performance, especially for the hydraulic
conductivity; and 3) the extended capillary-based PTF improves the
performance in describing SHPs under dry conditions.