A model is presented for the simultaneous, dynamic simulation of soil water movement and plant root growth. Root apices are translocated in individual growth events as a function of current local soil conditions. A threedimensional finite-element grid over the considered soil domain serves to define the spatial distribution of soil physical properties and as framework for the transient water flow model. Examples illustrate how field-observed morphology of root systems can be approximated by including even a coarsely discretized description of the soil environment. Intended as a tool for testing of hypotheses on soil-plant interaction, simulations can be performed for different levels of model complexity, depending on how much information is available. At the simplest level, root growth is simulated without soil water uptake, whereas the most comprehensive level includes growth of the shoot and dynamic assimilate allocation to root and shoot. \ c J j J ! J J J P Fig. 4. A: Crypsis aculeata, loamy sand near a salt pond (Kutschera and Lichtenegger, 1982; used with permission) B: Simulated root system.

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Previously, soil water retention and hydraulic conductivity curves have been scaled independently with separate scale factors calculated for each curve. Here we provide a method to scale both hydraulic functions simultaneously to yield a single set of scaling factors. An algorithm was developed to scale the hydraulic data, either as a function of degree of saturation S, or normalized water content ©. Optimization using Powell's method in combination with a Newton-Raphson procedure resulted in one set of parameters representing the scaled mean hydraulic functions and one set of scale factors describing the variability of both the retention and conductivity relationships. We present and compare results obtained from (1) scaling water retention data only; (2) scaling hydraulic conductivity data only;(3) simultaneous scaling of soil water pressure head h and the natural logarithm of hydraulic conductivity K; and (4) simultaneous scaling of the logarithms of both h and K. Sets of scale factors for two data sets were analyzed for distribution type and autocorrelation.

X‐ray computed tomography (CT) offers distinct advantages to study fundamental physical processes of water movement and contaminant transport in porous media. Tomography provides nondestructive and noninvasive cross‐sectional or three‐dimensional representations of porous media and has the potential to measure phase distribution and species concentration at the pore scale. Sources of error are discussed for the application of industrial microfocus CT to quantitative studies of flow and transport. Specifically, effective resolution and measurement uncertainties due to photon randomness are considered for a miscible displacement experiment. A calibration method for the measurement of solute concentration is proposed that accounts for the effect of beam hardening, which is characteristic for polychromatic industrial X‐ray sources. The results of an X‐ray microfocus CT experiment are presented, emphasizing the need to correct for beam hardening and describing the inherent spatial variability of solute breakthrough through a glass‐bead porous medium with an effective spatial resolution of approximately 85 μm.

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