Use of secondary information in space–time statistics for biomonitoring studies of saline deposition

Atmospheric salinity is often neglected in studies of atmospheric pollution, despite its proven hazardous effects in man‐made structures, vegetable organisms (crops and natives) and soil/water resources. Biomonitoring with lichens, which has been extensively used in assessing metal element deposition, was applied to the measurement of the deposition of saline elements, in a sampling programme established on the south‐west coast of Portugal, for a period of 52 months. The concentration of chloride and sodium, used as tracers of saline deposition, was measured in thalli of Ramalina spp., and was shown to be dependent on the distance from the coast, the intensity of rain observed before the sampling date and the dry period before sampling. These three factors were used in the definition of a space–time multiplicative regression model that reproduces the space–time trend of the data, and was used as secondary variable in kriging with external drift. Results of kriging estimation on a generalized space–time domain, showed better results when using secondary information as external drift, in comparison with ordinary kriging results. Copyright © 2001 John Wiley & Sons, Ltd.