Spatio-temporal patterns and covariance structures of soil water status intwo northeast German field sites

Spatio-temporal patterns of soil moisture status highly affect the heteroge neity of soil water and solute transport and leaching of chemicals to the g roundwater. In order to quantify and describe spatial variability of ecolog ically highly relevant spatial and temporal processes Linked to soil moistu re at the land surface, the spatio-temporal covariance structure and the re asons for its change in time need to be identified. Therefore, soil water p ressure head was monitored in two surface horizons between April and Novemb er 1995 at two field sites with a shallow ground water table, a sandy loam and a heavy clay soil in north-east Germany. For the 10- and the 30-cm dept h of the sandy loam soil and for the 30-cm depth of the heavy clay soil the variance of soil water pressure head h (on log basis) was large under wet conditions. With decreasing soil water pressure head the variance of log(10 )(-h) decreased to a critical value, for which a spatial correlation struct ure disappeared. With further drying, the variance of log(10)(-h) increased again, and a spatial range of correlation existed. During drying, temporal ly stable variation patterns developed at both field sites. The change of v ariance of log,,( -h) which is probably associated with changing degree of heterogeneity of flow conditions validates the findings of Roth (1995) obta ined from stochastic flow model calculations. In general, the temporal corr elation length was better defined than that of spatial correlation. At both depths, correlation lengths of the sandy loam were larger than those of th e heavy clay soil. Random spatial variation of log(10)(-h) occurred under c onditions when the hydraulic gradient was close to zero. With our experimen tal design we could identify (i) temporal persistence of spatial patterns a nd correlation ranges of soil water status that can be used for representat ivity studies, (ii) the within-site variation of land surface moisture stat us, and (iii) a basis for description of spatial processes of effective soi l properties that are Linked to soil water status at the land surface. (C) 1999 Elsevier Science B.V. All rights reserved.