Interpretation of water retention field measurements in relation to hysteresis phenomena

Knowledge of the soil water retention function is fundamental to quantifying the flow of water and dissolved contaminants in the vadose zone. This function is usually determined by fitting a particular model (see, for example, van Genuchten (1980) or Brooks-Corey (1964)) to observed retention points. Independent of the model chosen, interpretation and identification of the water retention parameters are subjective and prone to error, particularly as it is common that the hysteresis history in measured data points is unknown. Experimental data sets from three different field soils are used to clearly demonstrate how the lack of hysteresis knowledge can lead to an inconsistent and incorrect interpretation of the retention data, and therefore to the incorrect estimation of soil hydraulic parameters. By using a hysteresis model to interpret this same data set, it is easily shown that consistent and reliable estimates of soil retention parameters can be obtained. This is true for any physically based hysteresis model. The difficulty in reading water retention measurements may be evident when both drying and wetting data are measured. However, in practice, users are rarely aware of this problem since generally only one set of drying data is measured, making comparison impossible. Such erratic interpretation of water retention field data in the literature will be probably far more common than expected.