THE ROLE OF HYDROLOGIC CONNECTIONS BETWEEN IMPERVIOUS SURFACES AND VEGETATED PATCHES IN SEMI-ARID URBAN ENVIRONMENTS (Invited Presentation)

Urban environments are characterized by fine-scale heterogeneity, with patches of impervious surface and vegetation. In semi-arid regions where precipitation patterns are episodic, this fine scale structure may be a significant control on both storm-event runoff and seasonal water yields. From an ecological perspective, the productivity of urban vegetated patches is often limited by available water or is supported by irrigation inputs. Hydrologic connections between fine-scale impervious-surfaces and urban vegetation may influence not only runoff but also useable water for urban vegetation. Semi-arid natural ecosystems are also often characterized by fine scale vegetated - bare soil patch structures and hydrologic connections between these patches has been shown to be a significant control on shrub and forest productivity. In this paper we use an ecohydrologic model, RHESSys, to quantify the sensitivity of vegetation water use, net primary productivity and runoff to fine scale impervious surface connections in a semi arid urban settings. We use simulations on a theoretical hillslope where we can systematically vary impervious surface-vegetation fractions as well as connectivity between these patches to examine sensitivity of runoff, vegetation water use and productivity for a range of semi-arid climate forcing. We combine this analysis with watershed scale models for sites in Santa Barbara and San Diego California to develop a first-order approximation of the potential impact of accounting for fine scale impervious connections on aggregate watershed ecohydrologic flux estimates. Results indicate that while large increases in impervious surfaces will reduce transpiration and photosynthesis, due to the sheer loss of vegetated area, the effect of decreased vegetated area can be partially or in some cases even completely offset by increased transpiration and productivity in the remaining vegetation that occurs when water is routed from impervious to vegetated areas. These results highlight the importance of accounting for fine-scale heterogeneity and hydrologic connections between patches. We conclude by presenting a modelling framework that can be used to account for these interactions.