The transport of reactive solutes in the subsurface is influenced by a variety of physical and chemical processes. The processes are characteristically heterogeneous and often operate simultaneously at different temporal and spatial scales. In modeling reactive solute transport different models take different approaches, dependent on the scale of the system and the objective of the study. Two major approaches have been used to incorporate heterogeneous rate-limited mass transfer into mathematical models for solute transport. One focuses on processes operative at the microscopic scale and associated grain-scale heterogeneity, while the other stresses the macroscopic variability of the medium and the field-scale behavior of solute transport. In this work, I first examine the conceptual framework and model formulation of these two approaches in an attempt to evaluate potential commonality, then present a two dimensional numerical model that integrates the first approach with traditional stochastic modeling for reactive transport. In this model multiple processes are explicitly accounted for, including spatially variable flow, spatially variable sorption, locally heterogeneous diffusive mass transfer, locally heterogeneous rate-limited sorption, and locally heterogeneous first-order degradation. Finally, the model is used to (1) examine the individual and concurrent effects of multiple heterogeneous processes on reactive transport, and (2) evaluate the impact of microscopic-scale mass transfer heterogeneity on field-scale transport in systems for which hydraulic conductivity is spatially variable. The comparison of the two approaches shows that despite differences in conceptualization and formulation, both microscopic and macroscopic based models produce comparable behavior for smaller-scale systems. However, greater deviations are observed at larger scales. This suggests that caution should be taken when using mathematical modeling for elucidating the specific processes that may be influencing reactive-solute transport for a given system. Results from 2-D simulations of the new model reveal that inclusion of locally heterogeneous mass transfer does not appear to significantly influence the mean transport behavior for systems with field-scale heterogeneity. However, it does appear to influence low-concentration tailing. For simulations of reactive transport over extended distances, models with locally heterogeneous mass transfer may "preserve" the non-equilibrium effects associated with rate-limited mass transfer better than the models incorporating locally uniform mass transfer when both pore-scale and field-scale heterogeneity coexist.

Field-measured data collected from infiltration experiments conducted at the Maricopa Agricultural Center, Arizona, and one-dimensional numerical models were used in an iterative approach to develop conceptual models of unsaturated flow and transport. Conceptual models were developed using field data, estimates of hydraulic and transport properties, and assumptions regarding the types of processes occurring in the field plot. Using the field data and numerical models in an iterative approach revealed that adjustments to the parameterization of the conceptual models were necessary to allow for acceptable predictions of flow and transport. In addition, the numerical models delineated where the conceptual models needed adjustment and indicated where further analysis of the field data was necessary. Specifically, in regards to the unsaturated water flow, it was found that: (1) without increasing the laboratory determined value of Ks tenfold, it was impossible to make acceptable predictions for the infiltration experiments; (2) the laboratory-derived hydraulic properties provided a superior first approximation to texturally based hydraulic properties taken from an internal database; (3) incorporation of hysteresis into the soil hydraulic functions resulted in predictions of the water contents through time and soil water content profiles that were marginally better than not incorporating hysteresis; and (4) based on the mean square errors of the predictions and a post-audit based on the adjusted hydraulic properties, the use of a homogeneous soil profile was supported. In regard to the unsaturated solute transport, it was found that: (1) inversion techniques to estimate the unsaturated transport properties at each location (local approach) yielded the best results with respect to the fit of the data compared to the techniques which fit data from all depths simultaneously (global approach); (2) predictions using unsaturated transport properties averaged from the global approach yielded the best results compared to the average local approach properties; (3) incorporating hysteresis had little effect on the predicted bromide breakthrough curves and concentration profiles, yielding only marginally better results than predictions from models with no hysteresis; (4) it was impossible to accurately predict the breakthrough of bromide at depths deeper than 200 cm without using a retardation factor less then one.

General Circulation Models are important tools in the study of the earth's climate system. The terrestrial surface forms the lower boundary to such models over continents and a well-defined lower boundary is crucial for reliable climate simulations because the Earth interacts with the atmosphere via this boundary. The primary motivation for this research is to improve the parameterization of these interactions in General Circulation Models using field data and calibration techniques. For this purpose, a recent version of Biosphere-Atmosphere Transfer Scheme was selected, studied, and then calibrated for five different vegetation types using multi-criteria calibration techniques. The associated parameter sets were then tested in a ten-year climate integration with Version 3 of the Community Climate Model. The present study explored the methodology needed to use the growing number of relevant field data sets effectively and efficiently better to parameterize the land surface in a GCM. It showed that such field data can, indeed, be used in this way, not only to improve simulations but also to understand models' capabilities and deficiencies. Calibrating the land surface parameterization significantly improved simulations relative to the original default parameterization but several physically based land surface models studied, once calibrated, were found to give equally good simulations of the land surface processes. The primary results are that it is possible to obtain a single preferred parameter sets for different vegetation types using multi-criteria calibration, and that using calibrated parameter sets in climate models can improve the representation of surface exchanges and the modeled climate given by a GCM.

Measuring the relative apparent dielectric permittivity of the subsurface is an easy and inexpensive way to indirectly obtain the volumetric water content. Many of the instruments that measure the dielectric, specifically borehole ground penetrating radar, rely on the travel time of an electromagnetic wave through a moist soil. Through inversion of the travel time, the water content can be calculated provided the path over which the wave travels is known exactly. In traditional interpretations of water content, the travel path of the electromagnetic wave is assumed to be direct from the transmitting antenna to the receiving antenna, irregardless of the propagation velocity structure. A new analysis is presented for the interpretation of first arrival travel time measurements from borehole ground penetrating radar during zero-offset profiling that considers critically refracted ray paths. By considering critical refraction at interfaces between contrasting propagation velocities, the travel path becomes dependent upon the velocity structure. Several infiltration experiments were performed to test whether critical refraction occurs in the subsurface. The infiltrating water will change the velocity structure of the subsurface in a predictable manner The interpretations of travel time were then compared to predictions made with an unsaturated flow model and supporting instrumentation. It was found that when critical refraction was not considered, the volumetric water content was underestimated by up to 30%. Correcting for critical refractions, therefore, becomes an important step in properly characterizing the subsurface. The new analysis presented herein may improve our ability to use direct measurements in water resource management practices to assess water availability in semi arid regions.

This study consists of a theoretical analysis of the energy balance equation for a partially covered body of water, and experimental analyses of the energy balances of partially covered insulated evaporation tanks. The theoretical analysis indicates that surface reflectance for solar radiation and infrared emittance are the most important cover properties. White colored materials were found to satisfy the requirement that both these parameters be as large as possible. Experiments were conducted using covers of foamed wax, lightweight concrete, white butyl rubber, and styrofoam. A variety of shapes and sizes were tested. Cover radiative properties were again noted to be most important, and thin covers proved to be slightly more efficient than thick insulated covers of the same size. Evaporation reduction was found to be proportional to the percent of surface area covered, the constant of proportionality depending upon the color and type of material used. For the white, impermeable materials tested, the constant of proportionality was near unity. It was also noted that reduction in evaporation and reduction in net radiation, as compared to an open tank, were highly correlated. Evaluation of two Dalton-type expressions, the Bowen ratio method and the combination method, for predicting evaporation from an open water surface, showed the combination method to be better under conditions of this experiment. Based on this finding, a modified combination method was derived. This modified equation proved valid for predicting evaporation from a partially covered body of water. The use of insulated evaporation tanks also provided an easy and accurate method of predicting net radiation over other surfaces, and long-wave atmospheric radiation.