Modeling linkages between effective impervious surface and urban vegetation productivity in semi-arid environments

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1 Modeling linkages between effective impervious surface and urban vegetation productivity in semi-arid environments Catherine Shields PhD candidate, Bren School of Environmental Science and Management University of California at Santa Barbara

2 Urban areas, vegetation, and semi-arid environments Majority of world s population live in urban areas Urban vegetation provides a range of services Locally: shade/cooling, aesthetic values, erosion control Downstream: nutrient uptake/retention Semi-arid areas: water is limited Water stress can limit plant growth Providing water can be difficult/expensive or unsustainable Ideal: maximize productivity with minimal extra water photo: inhabitat.com

3 Landscape patchiness in the natural environment Landscape heterogeneity plays a key role in ecohydrology and productivity of semi-arid regions Runoff from bare patches can be routed to vegetated patches, boosting productivity Connectivity of bare and vegetated patches can be more important than total vegetated area (Bautista et al 2007) photo: Hugh Stimson (hughstimson.org)

4 Patchiness in the urban environment Urban areas also have patches of bare (impervious) and vegetated surface However! Urban patches are designed The connectivity between vegetation and nonvegetated patches is engineered by humans Water inputs are augmented by irrigation Mission Creek catchment near Rocky Nook Park, Santa Barbara, CA

5 Urban vegetation productivity What matters? Vegetation type Irrigation Placement of impervious surface (upslope, downslope) Connectivity?

6 Effective vs. total impervious area Total impervious area: any surface through which water cannot infiltrate Effective impervious area: direct hydrologic connection to the stream TIA only EIA + TIA

7 Connectivity in Urban Vegetated Patches In an urban context in semi-arid water limited areas: What is the influence of effective impervious area (EIA) compared to total impervious area (TIA) on vegetation water use? Does lower EIA compensate for increases in TIA? Dfd Dg Productivity Dfhd fd 0 Fgh fgfg Low EIA High EIA Xytu x Percent Impervious cover

8 Study site: Mission Creek Watershed Located in Santa Barbara, CA Has been steadily urbanizing over the past century Urbanization has been shown to affect streamflow (Beighley et al) Semi-arid climate (~500 mm/yr rainfall) Water for plants is scarce!

9 RHESSys model Spatially distributed ecohydrologic model Accounts for where as well as how much is input and how much is used Catchment is divided into patches based on elevation Model is primarily focused on water, but can also be used to model carbon and nitrogen

10 RHESSys model Water moves both horizontally and vertically through the watershed

11 RHESSys Modeling in Mission Creek Model has been calibrated to Mission Creek Model performance compares well to other studies for this type of climate NSE: Log(NSE): Percent error: %

12 Methods Generate a range of EIA and TIA scenarios for a test hillslope 5-65% TIA 5-75% of TIA as EIA e.g., 50% TIA, 50% EIA= 25% disconnected impervious 25% connected impervious 50% vegetated Irrigated and non-irrigated lawn grass scenarios Does the model show an EIA effect?

13 Annual photosynthesis Irrigation No irrigation Increasing impervious surface decreases photosynthesis Irrigated scenarios show more variability between EIA levels, lowest PSN for high TIA/high EIA scenarios Declines in photosynthesis largely due to decrease in vegetated area, lower EIA appears to slightly offset impact of increased TIA Total area average Vegetated area average

14 Seasonal trends With irrigation, not all scenarios show sharp summer PSN decline Sustained summer PSN in lower (5,25%) EIA scenarios Also seen with lower TIA values in high EIA scenarios No irrigation Irrigation Low TIA Typically low EIA No separation evident without irrigation High TIA Typically high EIA Is lower EIA boosting irrigation effectiveness?

15 Low EIA and irrigation impact Why do higher EIA scenarios not see a consistent irrigation PSN boost? Higher EIA = faster routing of water out of the hillslope Less water stored during rainy season Water stores may be below a critical threshold Current irrigation practices may not be sufficient to boost PSN if water stores are too low going into summer months However, other factors may be at play No obvious EIA effect at lower levels of TIA Specific spatial location of impervious surface seems most likely variable unaccounted for With less TIA, more opportunity for spatial variability in placement Opportunity for variability declines as TIA increases, EIA effect more noticeable Is there a further TIA threshold at which TIA is so great that EIA effect ceases? Here, no scenarios with >65% TIA, >65% TIA with only 5% EIA difficult to achieve

16 Conclusions While TIA is a key control on total system productivity, influence is largely due to total decline in vegetated area Decreasing EIA as a fraction of TIA may increase effectiveness of management practices such as irrigation at higher levels of TIA. Multiple factors likely contribute to productivity of urban vegetation

17 Questions?