Approaches to modelling the behaviour of tritiated water in crops

Tritium in the form of tritiated water (HTO), is released routinely by nuclear installations as well as during accidents. Tritium behaviour in vegetation is very different from that of most other radionuclides and triated water may not always follow the flux of water in the plant canopy-atmosphere continuum. The controlling step for both uptake and loss of HTO appears to be flux through stomata and thus modelling of canopy processes is of importance in this context, and will be of general benefit for the development of SVAT models. This aim of this work is validation of the UK MAFF STAR-H3 model. A major difference between STAR-H3 and a major German model (UFOTRI) is that the mechanism of plant-atmosphere exchange is treated using an empirical rate constant in the former, while in the latter is described according to the model of Belot, with temperature, stomatal resistance and boundary layer resistance determining the uptake and loss of HTO by vegetation. HTO deposition from air to surfaces has been modelled either by use of a deposition velocity (v_g) or by use of an exchange velocity (STAR-H3). The advantage of the v_g approach is that values for the deposition velocity and the re-emission rate can be chosen independently unlike the exchange velocity method. Our validation study of STAR-H3 indicates that the loss of HTO from crop foliage is best described by a double exponential decay, rather than the default single exponential.