To what extents do landscape structure and climate control watershed hydrologic response? Understanding this relationship is necessary to develop a watershed classification system that can help to build low-dimensional and falsifiable hydrologic models and more reliably predict the response of ungauged basins. We propose to develop and test a unifying similarity theory of watershed hydrology based on dimensionless indices that capture both the internal structure of landscapes (geomorphology, pedology and vegetation patterns) and prevailing climate characteristics. The innovation of the research lies in the fact that we will (i) apply bottom-up as well as top-down dimensional analyses to observations from a large number of U.S. watersheds in various climate regions, and (ii) regionalize these similarity measures in an uncertainty framework to derive constraints on hydrologic behavior in ungauged basins. This unifying similarity theory of watershed hydrology will enhance our fundamental understanding of landscape functioning in the hydrological cycle and will promote the development of techniques that permit an integrated analysis of water budgets at the watershed scale, as well as of the effects of ecosystem disturbance and land use change. The main research hypothesis is that advancement in understanding watershed-scale hydrologic response is possible by focusing on the geomorphologic and ecologic controls on hydrological processes of landscapes. The ultimate proposal objective is to improve our fundamental understanding of flow processes at the watershed-scale. Constructing such a novel similarity theory of watershed hydrology, including an associated suite of models, will provide new insights into the hydrologic driving mechanisms at a wide range of space and time scales. Understanding the required level of complexity of hydrologic models directly links our proposal to current questions of the operational forecasting community in the United States. The proposal includes graduate training in advanced hydrologic modeling and synthesis in a collaborative research environment across three different departments at three different institutions in the United States.
