The objectives of the research are to: (1) develop improved, multi-scale methods to quantify the puddle-filling and puddle-merging (P2P) overland flow process under control of microtopography, with focus on characterizing discontinuity, variability, and hierarchy of overland flow; (2) develop a P2P overland flow modeling system that integrates the new methods and modeling techniques in a user-friendly Windows interface; and (3) improve hydrology education at all levels by providing an interactive, education-enhanced P2P teaching-learning software.

Preliminary field data and previous studies highlight the important role of microtopography in overland flow generation and evolution, and emphasize the need for bridging the macro- and micro-scale hydrologic studies. Major challenges herein are: (1) effects of soil surface roughness on the P2P overland flow process are scale dependent; and (2) simply incorporating micro-scale effects of soil roughness in a larger-scale modeling framework can be computationally prohibitive. In this project, multi-scale methods, which involve dynamic puddle delineation and "point" and "area" modeling at two scale levels, will be developed to cope with these challenges and improve computational efficiency. Specifically, a quasi-three-dimensional model will be developed to simulate vertical infiltration into layered soils and rainfall excess at "points," and mass exchange and hydraulic connections between "points" over "areas" (puddle filling-connecting-merging process). The "point" modeling will be implemented on the cell/grid scale, while the "area" modeling will explicitly account for the effects of microrelief of cells on small-scale overland flow processes, such as flow types (microchannel flow or sheet flow) determined by an inundated factor. The P2P overland flow mechanism will also be examined by a series of laboratory and field experiments that reflect variability in soils, their spatial combination, roughness, and rainfall characteristics. Microrelief of the soil surfaces will be measured by using the laser scanner. The fractal model will be used to quantify soil surface roughness and scale effects of roughness will also be analyzed.

A Windows-based P2P overland flow modeling system will be developed to enhance applications of the new methods. An interactive P2P teaching-learning software will be further developed for improving hydrology education at all levels. The educational software, with enhanced visualization capabilities, will integrate the new modeling techniques, computer-guided self-learning center, and a set of education-oriented tools and databases in a user-friendly Windows interface. The teaching-learning software will be used as the core for graduate and undergraduate hydrology courses, as well as other outreach programs.

Intellectual Merit and Broader Impacts: The research on multi-scale P2P overland flow methods should be the first investigation of characterizing microtopography-controlled overland flow from a multi-scale perspective. The new methods will improve the understanding of intrinsic natures of overland flow: discontinuity, variability, and hierarchy. The models will be valuable tools for analyzing overland flow generation and evolution, and quantifying dynamic changes in the contributing areas of storm runoff, which is critical to nonpoint source pollution. Thus, this study will also have substantial impacts on environmental and ecological research and broad interdisciplinary application potentials. The user-friendly Windows P2P modeling system will particularly improve the accessibility to the new methods by the entire hydrology community. As an integral part of the NSF-funded LTER program, this unique hydrologic study will also provide valuable support for other LTER projects.

The state-of-the-art, interactive software will be the first comprehensive teaching-learning tool specially designed for hydrology education at all levels. The software, with enhanced visualization capabilities, the computer-guided self-learning center, and hydrologic tools and databases, will not only effectively promote students' learning and interest in hydrologic sciences, but also benefit instructors by creating an active and innovative teaching-learning environment in their classes. Most importantly, the work will give the underrepresented students an excellent opportunity to conduct the cutting-edge hydrologic research and gain skills and experiences that are invaluable to their studies and future career.

Project Report

Natural land surfaces are rarely smooth. They are characterized by numerous depressions, mounds, ridges, and channels across scales. This research is intended to address how overland flow is generated under the influence of surface microtopography and to further understand where the water goes. To do so, we first proposed a new concept of "puddle-to-puddle" (P2P) overland flow to characterize the dynamic P2P filling, spilling, merging, and splitting overland flow processes. We then developed physically-based mathematical models to identify puddles on topographic surfaces and their hierarchical relationships, track the flow paths, and simulate the microtopography-controlled P2P dynamics. At a smaller scale, the P2P models show how much water infiltrates into soils and how wetting front moves down through soil profiles, how much runoff water is generated locally, how a single puddle is filled, when and where it spills its water, when and how two puddles merge and form a larger higher-level puddle, and when and how a higher-level puddle splits into its embedded lower-level puddles under a dry condition. At a larger scale, the P2P models reveal how a water-ponded area expands, how different water-ponded areas are connected, and eventually, how a cascaded drainage system is developed. The P2P overland flow methodologies and modeling techniques developed in this project are designed to tell the whole story of puddles and their dynamic life. To facilitate the P2P modeling and education, a comprehensive Windows-based P2P modeling system was developed. It incorporated a group of models for simulating the P2P dynamics under various hydrologic conditions and a set of tools for 2D/3D visualization and hydrotopographic data processing and analyses. The P2P education system in the software package was specially designed for all educational levels, from K-12 to undergraduate and graduate levels. In addition to the modeling work, a wide range of laboratory and field overland flow experiments were conducted in this project. Our combined experimental and modeling studies emphasized the important role of surface microtopography in overland flow generation, and demonstrated that surface microtopography dominated the P2P filling-spilling-merging-splitting dynamics and significantly influenced the spatio-temporal distributions and occurrence timing of surface ponding, hydrologic connectivity, surface runoff, infiltration, and unsaturated flow. This research also has broader impacts and application potentials in agricultural engineering, environmental engineering, geography, ecology, and other related areas. This NSF grant provided excellent training opportunities for both undergraduate and graduate students who were involved in the project and a number of K-12 students who participated in the P2P outreach activities.

National Science Foundation (NSF)
Division of Earth Sciences (EAR)
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Thomas Torgersen
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North Dakota State University Fargo
United States
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