HR: 08:00h
AN: H51H-01    [Abstracts]
TI: The effects of spatial structure and connectivity on hydrologic response: non-linear response timescales as an emergent property in subsurface hillslope runoff.
AU: * Harman, C J
EM: charman2@uiuc.edu
AF: Department of Geography University of Illinois at Urbana Champaign, Room 220 Davenport Hall 607 South Mathews Avenue, Urbana, IL 61801, United States
AU: Sivapalan, M
EM: sivapala@uiuc.edu
AF: Department of Geography University of Illinois at Urbana Champaign, Room 220 Davenport Hall 607 South Mathews Avenue, Urbana, IL 61801, United States
AU: Sivapalan, M
EM: sivapala@uiuc.edu
AF: Department of Civil and Environmental Engineering University of Illinois at Urbana Champaign, 205 North Mathews Ave, Urbana, IL 61801, United States
AB: Landscapes exhibit spatial structure at all scales. It is increasingly apparent in hydrology that this structure leads to the emergence of processes at larger scales that cannot be modeled using spatially averaged versions of smaller scale process models. The concept of ‘connectivity' has emerged as a potentially useful framework for developing a new understanding of landscape processes. In this work, the effect of spatial structure on lateral flow through a perched hillslope aquifer is explored using a numerical model, and the connectivity of flow pathways is invoked to understand the results. The effect of spatial structure is examined by comparing a two-dimensional distributed model in which the conductivity and bedrock topography are allowed to vary, and a (non)linear reservoir model. The reservoir model has a single parameter, a response timescale which is assumed to vary with the storage. This timescale qualifies as a simple closure relationship for the behavior of the whole hillslope. A dimensionless framework is used to classify regimes of hydrologic response to storms, simplifying the analysis. Three modes of behavior are identified. The first two modes are observed in homogenous hillslopes: an ‘advective' mode in which the response timescale increases with the storage in the hillslope and a ‘diffusive' mode in which it is invariant. The third mode, in which the response timescale decreases with storage, only occurs in hillslopes with spatially variable conductivity or topography. Under this mode, a hillslope aquifer will initially drain rapidly after a storm, but then release water very slowly as the storage decreases. In a hillslope with sufficiently variable bedrock topography, the response timescale becomes infinitely large for non-zero storage, creating a threshold-like ‘spill and fill' effect. Stochastic rainfall is applied to the hillslopes to determine how the three modes of hydrologic response are manifested in the filtering of a rainfall signal into runoff. In hillslopes where spatially variable connectivity creates both relatively fast and slow response timescales, peak runoff can respond to variability at both short timescales (within-storm intensity fluctuations) and the long timescales (the history of recharge events). This ‘preferential' third mode is an example of an emergent process. It occurs at a larger scale (the hillslope) as a result of spatial structure and interactions at a smaller scale. It can be explained physically by the connectivity of fast and slow pathways within the hillslope. Water is rapidly released through the fast pathways that are well connected to the hillslope base. Water that must pass through slow pathways before reaching the outlet remains in the hillslope for longer periods of time. The preferential mode is most pronounced where the connectivity of the fast pathways is highest, and connectivity of the slow pathways to the outlet is lowest.
DE: 1829 Groundwater hydrology
DE: 1839 Hydrologic scaling
DE: 1847 Modeling
DE: 1869 Stochastic hydrology
DE: 4435 Emergent phenomena
SC: Hydrology [H]
MN: 2007 Joint Assembly