The effect of watershed structure and climate on streamflow response, hydrologic memory, and runoff source areas

Abstract

Watershed-scale hydrology research has long focused on understanding how various feedbacks in the soil-vegetation-atmosphere continuum affect streamflow. With this dissertation I sought to contribute to our understanding of how watershed characteristics (e.g. topography and vegetation) and climate affect various aspects of watershed hydrology, such as streamflow response times, watershed memory, and runoff source areas. Specifically, I was interested in 1) how watershed structure and climate affect inter- and intra-watershed variability in hydrologic response times, 2) how past precipitation and watershed memory affect runoff response on time scales of months to years, and 3) how runoff source areas vary through time. I approached these challenges/questions through a combination of spatially and temporally intensive and extensive observations synthesized as a) application of a simple lumped model to distill complex watershed behavior into comparable metrics across nested watersheds, b) empirical analysis of long-term hydroclimatic data sets to investigate the effect of watershed memory on the hydrologic response of watersheds, and c) the development of a parsimonious but fully distributed hydrologic rainfall-runoff model to characterize the effect of topographically driven lateral water redistribution and water uptake by vegetation on landscape scale hydrologic connectivity. We demonstrated that 1) differences in response times between watersheds were caused by differences in watershed structure while differences in response times between years were a function of maximum snow accumulation; 2) we found strong influences of past precipitation on runoff from monthly to annual time scales; 3) runoff source areas were highly variable over the course of two water years and exhibited hysteretic spatial behavior over the course of the snow melt seasons. This dissertation contributed new hydrologic understanding of how watershed properties (topography, geology, vegetation etc.), climatic variability, and the interactions between them affect hydrologic response at the watershed scale.