Interannual climate variability and cover texture in the surface Rn-222 flux from a shallow Ra-226 source

Engineered soil covers limit Rn-222 release by gas diffusion that depends on soil water content, whereas compliance with a flux limit is verified by single-day measurements. Rn-222 transport through clay loam, loam and sandy loam covers over a shallow Ra-226 source was simulated with HYDRUS-1D under five observed years of monthly climate from coastal northern Taiwan and, separately, under their average. (1) Cover material set the mean release and its variability in opposite directions: the clay loam cover released less than one fiftieth of the sandy loam cover under the averaged climate and about one seventh under the observed record, but varied by orders of magnitude within a year, whereas the sandy loam cover was almost invariant. (2) Almost any survey date reproduced the multiyear mean of the sandy loam cover within a factor of two; on the clay loam cover, even a full-year window left about a third of surveys outside that range. (3) The averaged climate, which contains no month of net evaporation, understated the clay loam mean by a factor of about eight and removed the overlap between its daily range and that of the sandy loam cover. (4) Under the averaged climate, thickening the cover from 10 to 30 cm reduced the flux more than tenfold for clay loam but by about a fifth for sandy loam. The cover with the lowest mean release was therefore the hardest to verify.