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How to use SWRC Fit web interface

The Web interface of the SWRC Fit uses SWRC Fit to calculate soil water retention curve. This is the screenshot of the user interface.

input screen

The input data, i.e., the soil water retention curve, should be prepared like this and copied and pasted in the textbox. If you are using Microsoft Excel, you can copy the data from spreadsheet.

0 0.2628
20 0.237
30 0.223
40 0.211
50 0.2035
70 0.1855
100 0.169
200 0.151
430 0.1399
640 0.131
1050 0.1159

The first column is the suction head and the second column is the volumetric water content, where space is used as a delimiter. Any unit can be used as the input data, and the calculated data depends on the unit used as the input data.

Input data can also be selected from the sample soil water retention data in the UNSODA database (Nemes et al., 2001). Please note that the sample data from the UNSODA database is suction (cm) and volumetric water content (dimensionless or cm3 / cm3).

In other textboxes, the description of the soil sample, soil texture, and name can be written. The description written here appears in the results screen. The model can be selected from the 6 models shown, where unimodal models are selected by default. In the Figure option, "Show only one model" is selected, which means that out of all the fitted models, only the best model with the minimum AIC (Akaike's Information Criterion) is shown in the figure. Remove the check if you want to show all the fitted curves.

Many other options are available by clicking "Show more options." In calculation options, you can select if you want to fit the parameters θr and θs, respectively, or make them constant with specified values. Correction function in the Fredlund and Xing model (FX model) can also be specified. In the figure options, you can manipulate the apperance of the figure to be drawn.

The calculation starts by pressing the "Calculate" button. Please wait for several seconds until the result screen appears.

In the result screen, the result of the nonlinear fit is shown like this.

result screen

The models, equations, parameters, R2, and AIC values are shown in tabular form, and the fitting curves with measured data points are also shown in a graph. The accuracy of the fit with different models can be compared in R2 and AIC values, and also by the fitting curves. For comparing models with different numbers of parameters, AIC is a better indicator than R2. The description of the soil sample and the original data is also displayed in the results screen so that the users can print out and store all the necessary information.

The image files of the graphs are stored in the server until it will be overwritten by other figures at the next run. If you want to erase your own data from the server, please run the fitting of sample data after you finish your calculation.

Offline version of SWRC Fit can be installed on PC with various environment including Windows, Mac and Linux. It can be used from other program so that automatic calculation of datasets is available. For such programming, the source code of this web interface may be of some help. Therefore, for those who might be interested, you can read source code of the web interface.

Citation of this work

Please cite this paper when you publish your work using SWRC Fit.

BibTeX and EndNote files are also available.


  1. Nemes, A., M.G. Shaap, F.J. Leij, and J.H.M. Wosten: Description of the unsaturated soil hydraulic database UNSODA version 2.0. J. Hydrol. (Amsterdam) 251:151--162, 2001.

Author: Katsutoshi Seki