SPATIAL HETEROGENEITY AND TEMPORAL DYNAMICS OF SOIL-WATER TENSION IN A MATURE NORWAY SPRUCE STAND

In ecosystem research great effort is made in measuring soil water ten sion, because this is a critical calibration variable for modelling so il water fluxes. In this paper the spatial heterogeneity and temporal dynamics of soil tensions and their consequences for the determination of water fluxes are investigated. Studies were carried out at a Norwa y spruce stand in the Fichtelgebirge (NE Bavaria). Standard tensiomete rs were installed at three soil depths (20 each) on the whole experime ntal plot, as well as 45 microtensiometers as a dense grid in a small soil pit. Microtensiometry at the centimetre scale showed that, depend ing on rain intensity and initial soil water tension, even a soil with out discernible macrostructure may show preferential water infiltratio n. At the stand scale the variability of soil hydraulic properties and tree root distribution causes substantial heterogeneity of soil water tension, as observed by standard tensiometers. A functional relations hip between increasing spatial heterogeneity of tensiometer readings a nd increasing soil water tension was found, which was particularly pro nounced after longer dry periods. Also at low soil water tension, wher e spatial heterogeneity was low, the calculation of water fluxes from tensiometer values was critical, owing to the fact that small differen ces in measuring soil water tension resulted in big differences in cal culated water fluxes. At high soil water tension in summer the spatial heterogeneity of tensiometer readings was extremely high. At our expe rimental site, since 30% of the total rain in summer falls in events h aving a precipitation rate greater than 5 mm h(-1), preferential water and solute flow was an important phenomenon. We conclude that the val idation of calculated water fluxes using measured soil water tension a t the stand scale is not an appropriate tool, because of measurement d ifficulties, considerable spatial heterogeneity, especially in dry per iods, and the great variability of soil hydraulic properties. (C) 1998 John Wiley & Sons, Ltd.