Determining Soil Hydraulic Properties Using Gamma-Ray Attenuation in an Evaporation Experiment.

A popular laboratory method for determining retention and conductivity properties of soil samples is the evaporation method as proposed by Wind (1968), Schindler (1980) and Wendroth et al. (1993), among others. The method is interesting given its relative simplicity and small data demand and is normally performed with tensiometers as measuring devices. Common tensiometers measure within a pressure head range between 0 and ‑9 m, imposing limitations on the applicability of the method.

We performed evaporation experiments substituting the tensiometer measurements by gamma-ray attenuation readings. Undisturbed soil samples (7 cm diameter and 7 cm height) were taken from several soils. Samples were saturated by capillarity, closed at the bottom and then submitted to natural evaporation from their upper surface. Evaporation rate was determined by weighing. Measurements of attenuation of a collimated gamma ray, produced by a ¹³⁷Cs source were performed at several times and depths below the sample surface, during some weeks after the onset of evaporation, until samples reached a very low water content. Samples were finally oven dried and one last reading was performed to determine the attenuated reading of the dry sample. Water contents corresponding to attenuated readings were calculated using Beer´s law.

For the same soils, water retention curves were determined by the pressure chamber method, and results were used to fit the Van Genuchten retention parameters.

Observations from the evaporation experiments were used as objective function to determine Van Genuchten-Mualem parameters for water retention and conductivity using the inverse modeling option available in the Hydrus-1D software. This was done: (1) using the predetermined retention parameters and predict only the hydraulic conductivity parameters by inverse modeling and (2) determining all parameters by inverse modeling.

Results will be presented and discussed during the conference session.