Soil Physics-Informed Neural Networks to Estimate Bimodal Soil Hydraulic
Properties
Abstract
Pedotransfer functions (PTFs) are widely used to estimate soil hydraulic
properties (SHPs) from easily measurable characteristics. However, most
existing PTFs rely on unimodal hydraulic models, which fail to
accurately represent the bimodal SHPs caused by soil structure common in
field conditions. In this study, we developed new PTFs using two bimodal
soil hydraulic models and introduced soil physics-informed neural
networks (SPINN) to embed the models into PTF training. The results
showed that the new PTFs effectively captured bimodality in hydraulic
conductivity curves, achieving an RMSE of 0.578 in the test set,
compared to 0.709 for unimodal models. The PTFs also improved soil water
retention curve (SWRC) predictions but struggled with bimodal SWRCs for
some samples, likely due to the limited number of bimodal SWRCs in the
dataset. An independent dataset evaluation revealed that the RMSE for
hydraulic conductivity predicted by the new PTFs was approximately
one-third of that of classic PTFs. This underscores the significant role
of soil structure in SHPs, which classic PTFs fail to capture.
Additionally, PTFs developed using the SPINN method outperformed those
optimized fitted hydraulic parameters via machine learning, a common
approach in the literature. We also found that separate versus
simultaneous optimization of water retention and hydraulic conductivity
greatly affects PTF performance. Finally, we provided global 1
km-resolution maps of soil hydraulic parameters for the bimodal model.