  SWRC Fit and unsatfit are programs developed for determining parameters of
the water retention and the unsaturated hydraulic conductivity functions of
soils for analyzing water movement in the unsaturated soil. The SWRC Fit is a
web application for drawing SWRC (soil water retention curves) of various soil
hydraulic models from the measured data with just one click, available at
https://purl.org/net/swrc/. The SWRC Fit depends on the Python library unsatfit
that implements various types of soil hydraulic models to determine water
retention and hydraulic conductivity parameters by nonlinear least-square
optimization. The implemented models are classified as DF3 (Brooks and Corey,
van Genuchten, Kosugi) models, DF4 (Fredlund, Fayer, Peters, bimodal-CH)
models, and DF5 (bimodal) models by the degree of freedom of the water
retention functions. Most of the hydraulic conductivity functions are derived
from the water retention function and the general hydraulic conductivity
function. A modified model to suppress extreme changes in hydraulic
conductivity in the near-saturated range can be applied to any hydraulic model.
The algorithm that was proved to work with various types of soils in Seki et
al. (2023) is provided as the sample Python codes. This paper describes how to
use SWRC Fit and unsatfit.
