Surface microtopography affects a series of complex and dynamic hydrologic and environmental processes that are associated with both surface and subsurface systems, such as overland flow generation, infiltration, soil erosion, and sediment transport. Due to the influence of surface depressions, overland flow essentially features a series of progressive puddle-to-puddle (P2P) filling, spilling, merging, and splitting processes; and hydrologic systems often exhibit threshold behaviors in hydrologic connectivity and the associated overland flow generation process. It is inherently difficult to realistically simulate the discontinuous overland flow on irregular topographic surfaces and quantify the spatio-temporal variations in dynamic behaviors of topography-dominated hydrologic systems. This dissertation research aims to develop a hydrologic model to simulate the discontinuous, dynamic P2P overland flow processes under the control of surface microtopography for various rainfall and soil conditions, and propose new approaches to quantify hydrologic connectivity. In the developed P2P overland flow model, the depressions of a topographic surface are explicitly incorporated into a well-delineated, cascaded P2P drainage system as individual objects to facilitate the simulation of their dynamic behaviors and interactions. Overland flow is simulated by using diffusion wave equations for a DEM-derived flow drainage network for each puddle-dominated area. In addition, a P2P hydrologic connectivity concept is proposed to characterize runoff generation processes and the related spatio-temporal dynamics. Two modified hydrologic connectivity indices, time-varying connectivity function and connectivity length of the connected areas and ponded areas, are proposed to quantitatively describe the intrinsic spatio-temporal variations in hydrologic connectivity associated with overland flow generation. In addition, the effects of DEM resolution, surface topography, rainfall distribution, and surface slope on hydrologic connectivity are also evaluated in this dissertation research. The developed model can be applied to examine the spatio-temporally varying P2P dynamics for hydrologic systems. This model provides a means to investigate the effects of the spatial organization/heterogeneity of surface microtopography, rainfall, and soil on overland flow generation and infiltration processes. In addition, the two proposed hydrologic connectivity indices are able to bridge the gap between the structural and functional hydrologic connectivity and effectively reveal the variability and the threshold behaviors of overland flow generation. / National Science Foundation under Grant No. EAR-0907588 / Department of Civil and Environmental Engineering, North Dakota State University / North Dakota Water Resources Research Institute

Spatial characterization of surface microtopography is important in understanding the overland flow generation and the spatial distribution of surface runoff. In this study, fractal parameters (i.e., fractal dimension D and crossover length l) and three hydrotopographic parameters, random roughness (RR) index, maximum depression storage (MDS), and the number of connected areas (NCA), have been applied to characterize the spatial complexity of microtopography. Clear and meaningful relationships have been established between these parameters. The RR was calculated as the standard deviation of the processed elevation, and the fractal parameters were calculated with the semivariogram method. The puddle delineation program was applied in this study to spatially delineate soil surface and to accurately determine MDS and NCA. It has been found that fractal parameters can better characterize surface microtopography. More importantly, fractal and anisotropic analyses can help to better understand the overland flow generation process.

The main objective of this study is to develop a modeling methodology that facilitates incorporating the plant pathway into environmental contamination models recognizing the fact that plants are dynamic entities that regulate their life cycle according to natural and anthropogenic environmental conditions. A modeling framework that incorporates the plant pathway into an integrated water flow and contaminant transport model in terrestrial systems is developed. The modeling framework is aimed to provide a tool to analyze the plant pathway of exposure to contaminants. The model developed using this framework describes the temporal and spatial variation of the contaminant concentration within the plant as it is interacting with the soil and the atmosphere. The first part of the study focuses on the integration of the dynamics of water and contaminant distribution and plant related processes within the vadose zone. A soil-plant system model is developed by coupling soil-water flow, contaminant transport, plant life-cycle, and plant pathway models. The outcome unifies single media continuous models with multimedia compartmental models in a flexible framework. The coupling of the models was established at multiple interfaces and at different levels of solution steps (i.e. model development phase vs. numerical solution phase). In the second part of the study, the soil-plant system model is extended to cover large spatial areas by describing the environmental system as a collection of soil-plant systems connected through overland flow and transport processes on the ground surface and through lateral interactions in the subsurface. An overland flow model is integrated with the previously coupled model of unsaturated zone soil-water flow and plant life-cycle by solving the flow model equations simultaneously within a single global matrix structure. An overland / subsurface interaction algorithm is developed to handle the ground surface conditions. The simultaneous solution, single-matrix approach is also adopted when integrating the overland transport model with the previously coupled models of vadose zone transport and plant pathway. The model developed is applied to various environmental contamination scenarios where the effect of the presence of plants on the contaminant migration within environmental systems is investigated.