Ecohydrology in semiarid urban ecosystems: Modeling the relationship between connected impervious area and ecosystem productivity

Abstract

In water-stressed, semiarid urban environments, connections between impervious surfaces and drainage networks may strongly impact the water use and ecosystem productivity of neighboring vegetated areas. We use an ecohydrologic model, the Regional Hydro-Ecological Simulation System (RHESSys), to quantify the sensitivity of vegetation water use and net primary productivity (NPP) to fine-scale impervious surface connectivity. We develop a set of very fine-scale (2 m 2 ) scenarios that vary both the percentage of impervious surface and fraction of this impervious surface with direct hydrologic connections to urban drainage systems for a small hillslope. When driven by Mediterranean climate forcing, model estimates suggest that total vegetation water use declines with increasing impervious area. However, when impervious area is hydrologically disconnected from the urban drainage network, declines in water and carbon fluxes with decreased vegetated area can be partially, or in some cases even completely, offset by increased transpiration and NPP in the remaining vegetation. Relative increases in water use and NPP of remaining vegetation are much greater for deeply rooted shrubs and trees and negligible for shallow rooted grasses. We extrapolate our findings to the catchment scale by developing a first-order approximation of fine-scale impervious connection impacts on aggregate watershed water and carbon flux estimates. Our approach offers a computationally and data-efficient method for estimating the impact of impervious area connectivity on these ecohydrologic fluxes. For our only partially urbanized Santa Barbara watershed, estimates of water use and NPP that account for fine-scale impervious connection differed by more than 10% from those that did not.

Watershed impacts of climate and land use changes depend on magnitude and land use context

“…In particular, county‐level estimates of LU change available in Wear () were further refined to a 30‐m spatial scale, consistent with the resolution of the NLCD. The ecohydrological model (RHESSys) used in the study is often used at a 30‐m or finer scales (Mittman, Band, Hwang, & Smith, ; Mohammed & Tarboton, ; Shields & Tague, ). Due to a combination of available data on land cover, digital elevation models and processing time (smaller spatial scales are more computationally intensive) resolutions of <30 m are generally used in smaller watersheds.…”

Modeling Runoff and Nitrogen Loads From a Watershed at Different Levels of Impervious Surface Coverage and Connectivity to Storm Water Control Measures

“…Since UI is inherently bounded by the watershed's TI, the ratio of the two (UI/TI) is another metric that characterizes the fraction of TI that is directly connected to the stream. This metric is analogous to drainage connection and effective impervious area fraction used in other studies (Shields & Tague, 2014;Walsh et al, 2005;Walsh & Kunapo, 2009).…”

“…This metric captures connectivity and extent of impervious surfaces. Modeling studies have shown that EI predicts runoff volumes and storm event peak discharges well (Dewals et al, ; Guo, ; Lee & Heaney, ; Palla & Gnecco, ; Schoener, ; Shields & Tague, ). However, there is currently a lack of empirical studies examining the watershed‐scale effects of connectivity to SCMs on water quality, and only a few modeling studies exist (Jefferson et al, ).…”

“…Urban streams experience increases in low flows when inputs from leaky water supply pipes and sewers migrate to the stream between storm events but also experience decreases because impervious surfaces reduce infiltration, the water table level, and therefore rates of groundwater flow entering the stream (Bhaskar et al, 2016). While the extent of urban impervious surfaces is an important control on watershed hydrology, the hydrologic connectivity of these surfaces to stream networks through constructed drainage channels and storm sewers also controls peak discharge and runoff volume (Alley & Veenhuis, 1983;Bell et al, 2016;Dewals et al, 2012;Shields & Tague, 2014;Shuster et al, 2005;Walsh et al, 2005;Walsh & Kunapo, 2009). These drainage networks cause runoff, rich in nutrients like nitrogen (N), to bypass biologically active zones in both the terrestrial and riparian environments, which limits the ability of urban ecosystems to remove or retain excess nutrients that enter the watershed by atmospheric deposition, imported food (subsequently released as wastewater), and applied fertilizer (Bernhardt et al, 2008;Duncan et al, 2013;Groffman et al, 2004;Hatt et al, 2004;Kaushal & Belt, 2012).…”

Projecting the CO2 and Climatic Change Effects on the Net Primary Productivity of the Urban Ecosystems in Phoenix, AZ in the 21st Century under Multiple RCP (Representative Concentration Pathway) Scenarios

“…Multiple lines of evidence indicated that urban ecosystem NPP is sensitive to environmental changes at various spatiotemporal scales. Previous studies have investigated inter-annual variability of NPP in response to climate change [10][11][12][13], the contributions of urbanization and climate change to NPP variations [14][15][16][17], and the effect of urban land transformation on NPP [18]. Most research has been focused on the effect of land use/land cover change and historical climate change; however, the dynamic of NPP in response to future climate/CO 2 change effects are still unclear.…”