Stream flow generation and hillslope hydrology
Ultimately, the goal of watershed hydrology is to understand specifically where water introduced as precipitation spends time before leaving a watershed as runoff. Hillslope hydrology represents an attempt to introduce basic categories of storage and stream flow generation that are the primary controls of watershed behavior. These categories are determined by the lateral movement of water toward the stream created by partitioning at surface detention, soil water, and groundwater movement. This module introduces the concepts of hillslope hydrology and the mechanisms of stream flow generation that provide a more detailed perspective on watershed function (4:35 min).
Contents of this module
Mechanisms of overland flow and the variable source area concept
Partitioning of stream flow generation from surface detention is often called infiltration excess overland flow, due to the primary cause driven by precipitation rates in excess of soil infiltration capacity. This mechanism of stream flow generation is so inextricably linked to Robert E. Horton that it is often referenced as Hortonian overland flow (5:49 min).
The intersection of the water table with the land surface determines the potential for saturation excess overland flow (6:53 min).
The dynamics of the expansion and contraction of the watershed area contributing to saturation excess overland flow is the foundation of the variable source area concept (12:39 min).
TopModel and the topographic wetness index
The topographical wetness index is based on fundamental principles of where water should be expected to accumulate within a watershed. While plenty of complexities confound the influence of topographic convergence and local slope, there are few generalizable models that are more reliable in predicting the spatial distribution of "wetness" within a watershed (9:16 min).
Let's review an example of the implications of the topographic wetness index based on interpretation of a topographic map (5:03 min).
While the fundamental logic of the applicability of the topographic wetness index is undeniable, an understanding of the assumptions that make it more or less directly applicable in hydrologic models is critical (3:46 min).
The topographic wetness index provides a useful approach to predicting the areas of the watershed more likely to become part of the variable source area during precipitation events. Let's review the link between topographic position and the potential for saturation excess that underlies the TopModel approach to watershed modeling (3:40 min).
Interflow or subsurface storm flow
Subsurface storm flow (or interflow) is the lateral movement of water in the unsaturated zone during precipitation events. The mechanisms of interflow are less intuitive than other pathways of stream flow generation (5:38 min).
The role of macroporosity
Macroporosity is a modern enigma of watershed hydrology. Relatively simple conceptual models that can capture the influence of macroporosity on both water resource quantity and quality at watershed scales remains a hot topic in hydrologic science (4:03 min).
I have included photographs of hydrologists responsible for seminal works in my slides in an attempt to humanize the science. Hydrology has emerged from the ideas of ordinary people, and it is important to recognize that anybody motivated enough to push through mediocrity can make a contribution. But this review of the fundamentals is based on decades-old ideas that are accompanied by stale notions about societal roles, and it is important to recognize that the faces of a bunch of old white dudes are not representative of the modern faces of hydrology. Diversity in perspective has been critical to recent advances and is critical to future advances of the science (2:21 min).
Source water separation in stream flow generation
Having categorized stream flow generation into three basic categories, the next step is to define the basic causal factors that might be used to predict the relative importance of those three mechanisms in understanding watershed response to precipitation (5:57 min).
The holy grail of watershed hydrology is thus tied to hydrograph separation at the level of detail related to the principles of stream flow generation derived from hillslope hydrology (5:14 min).
Summary and supporting materials
Study guide
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Study guides are designed to summarize the vocabulary, concepts, and mathematics learned in this module.
study_guide_stream_flow_generation.pdfReadings from Dingman (3rd ed)
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A list of associated readings from Physical Hydrology by S. Lawrence Dingman (3rd edition)
dingman_3ed_stream_flow_gen.pdfSlides used in videos
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Useful materials for further study or skill development
Classic papers
A seminal paper from Keith Beven and others on using the topographic wetness index in watershed modeling
Beven et al. 1984 in Journal of Hydrology
A seminal paper from Kevin Bishop and others on the transmissivity feedback concept
Bishop et al. 2004 in Hydrological Processes
A seminal paper from Jeff McDonnell describing the role of macroporosity in the rapid response of old water to precipitation