For analysis of plant water uptake, particularly in water scarce environments, knowledge of the soil water diffusivity function at low water content is needed. There is limited measurements of soil water diffusivity on intact soil.

We have developed a method for measuring soil water diffusivity. The method involves measuring the cumulative evaporation through time by frequently weighing an intact soil core that is sealed at the base and subjected to a turbulent air stream with a constant dewpoint between 0-1 deg C. The core sits permanently on a balance and it's weight measured after stopping the air flow briefly to allow the balance to stabilise.

The analysis is a recursive two step process: Firstly the cumulative evaporation through time is modelled by numerically solving the diffusion equation. Then the soil water diffusivity function is optimised to minimise the difference between the experiment and model.

Close agreement between the model and experiment is obtained with a 2^nd order polynomial diffusivity function of parabolic form. The diffusivity reaches a minimum of approximately 10 cm² d^-1. This is roughly an order of magnitude greater than other results, mostly on repacked soil, found in the literature.