SOIL-WATER DEPLETION BY OAK TREES AND THE INFLUENCE OF ROOT WATER-UPTAKE ON THE MOISTURE-CONTENT SPATIAL STATISTICS

The space-time statistical structure of soil water uptake by oak trees was investigated in a 3.1-m-diameter closed top chamber using a three -dimensional measurement grid of soil moisture and pressure, and measu rements of tree transpiration. Using the time domain reflectometery (T DR) measured moisture content, resistance block measured soil water pr essure, and a compact constant head permeameter measured saturated hyd raulic conductivity, the soil hydraulic properties for the chamber wer e first estimated. Then, dimensionless statistical measures that utili ze the soil water pressure were proposed and used to assess the relati ve importance of lateral to vertical flow. On the basis of the measure d statistical properties of the soil-water pressure, it was found that the vertical flow is at least an order of magnitude larger than the l ateral flow, and thus a one-dimensional flow approximation to continui ty was utilized. Using continuity and a first-order Taylor series expa nsion of the Buckingham-Darcy vertical flux about the spatial mean moi sture content state, an approximate relation for the time variation of the spatial mean moisture content was derived and tested with the TDR measurements, Despite a large spatial coefficient of variation in the TDR measured moisture content (which was also shown to be comparable to reported values from larger-scale field experiments), good agreemen t between mean moisture content predictions and measurements were foun d for two separate drying cycles. The approximate Taylor series flux e xpansion was utilized for deriving an analogous relation for the time variation of the spatial moisture content variance. The resultant vari ance budget was used to assess the role of root water uptake on the sp atial variability of moisture content. It was found that the root upta ke component, which resulted from a covariance between the root water uptake and moisture content spatial perturbations, is comparable to th e contribution from soil hydraulic properties and soil water redistrib ution, One of the main findings in this study is that root water uptak e is central to the moisture content spatial variance dissipation espe cially for dry soil moisture conditions. These results were further in vestigated using Monte-Carlo simulations.