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 tension, because this is a critical calibration variable for modelling soil 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 Norway spruce stand in the Fichtelgebirge (NE Bavaria). Standard tensiometers were installed at three soil depths (20 each) on the whole experimental plot, as well as 45 microtensiometers as a dense grid in a small soil pit. Microtensiometry at the centimetre scale showed that, depending on rain intensity and initial soil water tension, even a soil without discernible macrostructure may show preferential water infiltration. 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 relationship between increasing spatial heterogeneity of tensiometer readings and increasing soil water tension was found, which was particularly pronounced after longer dry periods. Also at low soil water tension, where spatial heterogeneity was low, the calculation of water fluxes from tensiometer values was critical, owing to the fact that small differences in measuring soil water tension resulted in big differences in calculated water fluxes. At high soil water tension in summer the spatial heterogeneity of tensiometer readings was extremely high. At our experimental site, since 30% of the total rain in summer falls in events having a precipitation rate greater than 5 mm h⁻¹, preferential water and solute flow was an important phenomenon. We conclude that the validation of calculated water fluxes using measured soil water tension at the stand scale is not an appropriate tool, because of measurement difficulties, considerable spatial heterogeneity, especially in dry periods, and the great variability of soil hydraulic properties. © 1998 John Wiley & Sons, Ltd.