Temporal Common Trends of Topsoil Water Dynamics in a Humid Subtropical Forest Watershed

Research oriented toward understanding the hydrologic functioning of the relict “laurisilva” evergreen forests is scarce. This study focused on the analysis of temporal changes in soil water status under such humid subtropical stands and explored to what extent hydrologic fluxes may explain topsoil water dynamics. Hydrologic fluxes (potential evapotranspiration, canopy fog water dripping, and rainwater below the canopy) were computed for a 2-yr period using in situ micrometeorological measurements in the Garajonay National Park cloud forest (Canary Islands). Time domain reflectometry (TDR) data were used to characterize soil water status at 0.15- and 0.30-m depths in plots located at 1145, 1185, 1230, and 1270 m above sea level. The resulting eight daily TDR data sets were studied with dynamic factor analysis. The variability in the soil water status time series was simplified and successfully described (coefficient of efficiency = 0.717) with a single temporal trend dynamic factor model (DFM), representing unexplained variability common to all plots and monitoring depths. Comparison of DFMs with and without explanatory variables (i.e., hydrologic fluxes) indicates that unexplained variability in the observed data was partially reduced by the information provided by the hydrologic fluxes. The rainfall contribution to the soil surface, and to a lesser extent forest potential evapotranspiration, were necessary variables for describing temporal changes in topsoil water status; however, dripping fog water was found to be a negligible contributor. Dynamic factor analysis proved to be useful for studying the variability in multivariate hydrologic time series without the need of a priori detailed information about the underlying mechanisms governing soil water dynamics.

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