Monitoring and predicting the impacts of trees on urban stormwater volume reduction

Background

The use of green infrastructure in urban areas has increasingly been accepted as a means to reduce adverse impacts of stormwater on sewer systems that service Great Lakes communities and Great Lakes water quality. Over the last decade, much has been learned about how effectively individual green infrastructure practices can reduce stormwater volume; however, one element that remains poorly understood is the role of urban trees in stormwater detention. Research is needed to provide quantitative data on how trees affect the urban water cycle as trees have a unique influence on storage and losses. Trees may influence the water cycle by mechanisms such as throughfall, evapotranspiration and subsurface flows Furthermore, research in this area is needed due to the different spatial scales and complex urban mosaics in which bioretention and street trees are planted. While there is a body of evidence to suggest trees are an integral part of urban watershed management, few field studies have quantified the stormwater volume reduction capabilities of urban trees or accounted for trees in the context of the whole-water cycle. A synthesis of past research by Kuehler et al. (2016) and Berland et al. (2017) revealed that several knowledge gaps remain on the role of trees in stormwater management. Many of these gaps center around the need for a more holistic understanding of urban tree canopy, understory, and subsurface effects on urban water cycles including, but not limited to: variation attributed to tree species, age, seasonality (leaf-on, leaf-off), regional climate, interactions with the immediate urban-suburban ecosystem, and the role of trees in the context of other green infrastructure practices (e.g., trees planted in rain gardens). These knowledge gaps apply to trees throughout the urban ecosystem (in urban forest patches, private residential and commercial parcels, as well as the public right of ways), not only because of needs regarding urban water budgets, but also with respect to how these flows influence ecohydrological functions that relate to vegetation, public health, the removal of pollutants in the air and water cycle, among other ecosystem services.

Approach

The U.S. Geological Survey (USGS), U.S. Forest Service (USFS), Environmental Protection Agency - Office of Research and Development (USEPA-ORD), and the University of Wisconsin - Madison (UW) led a field scientific investigation that filled some of the data gaps identified by Kuehler et al. (2016) and Berland et al. (2017). The primary objective of the study was to quantify the effect of tree removal on the urban hydrologic cycle and to measure the impact that trees have on stormwater runoff detention volume. The study used a paired catchment statistical design and analyzed a continuum of storm event hydrographs from 2018 through 2020. Additional monitoring data (e.g., soil moisture and groundwater level) were also measured to characterize how water cycles with and without the presence of trees. 

In addition to characterizing hydrologic processes, monitoring data was used to verify, validate, and calibrate surface runoff and base flow simulations using the i-Tree suite of forest analysis models.  The i-Tree Hydro program computes surface runoff volume and baseflow groundwater contribution to surface water flow based on weather, soil conditions, impervious cover, tree characteristics, and other vegetation parameters.  The USFS structured the i-Tree Hydro model to evaluate conditions in the Test catchment before and after tree removal to help assess and improve tree planting designs to reduce runoff and improve water quality. Discharge data collected during the calibration period of this study was used to calibrate and validate i-Tree Hydro’s simulation of pre- and post-treatment conditions. Using actual data to confirm or directly modify established i-Tree computations will improve future applications of the model regarding reporting of tree- and runoff-related benefits and other ecological services provided by vegetation. Model validation will help scientists, resource managers, and communities better understand the role vegetation management can have in stormwater management, not only in the test area, but also in other Great Lakes watersheds and beyond.

Site Description

This study characterized the impact of street tree removal on stormwater runoff characteristics from two medium-density residential catchments in Fond du Lac, Wisconsin (figure 1). Catchments were comparable in size and consisted of single-family housing with approximately 0.25-acre parcels with trees interspersed along the right-of-way that flanks each street. These neighborhoods have separate septic-storm sewer systems. Stormwater runoff was conveyed via curb and gutter to stormwater inlets as the entry point to the centralized storm sewer collection system.

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Using the paired-catchment study design, one catchment was designated the ‘Control’ and the second catchment the ‘Test’ (figure 1, table 1). Trees in the area, both street trees and landscape trees, appear to be similar in age and were likely planted during the time of housing construction in the late 1980s.

The primary abundance of tree species lining the streets of each study catchment include ash, maple, and honey locust (table 2). Approximately one-half of the street trees in the Test catchment are Green Ash (Fraxinus Americana), a species of tree that is subject to disease through infestation by Emerald ash borer (Agrilus Planipennis). As such, the city of Fond du Lac removed all ash trees before infestation could occur. The removal of ash trees prior to infestation provided a unique scenario to quantify the influence that street trees can have on stormwater runoff volume. The project team established a baseline water budget by measuring the hydrologic response of each catchment before ash trees were removed. Monitoring during this calibration period spanned approximately 2 years, beginning in early spring 2018 before leaf-on and continuing through the fall of 2019. In March 2020, approximately 60 percent of ash trees lining the street in the Test catchment were removed. The e ensuing change in hydrologic response was attributed to the sudden absence of trees, mostly through the loss of interception capability due to the loss of leaves, trunks and stumps.

Table 2. Species abundance and canopy cover provided by the street trees in the Control and Test catchments.

Results

A total of 135 warm-season precipitation events, each with precipitation depths greater than 0.5 mm, were measured over the 15-month monitoring span, 93 and 42 during the calibration and treatment phase respectively. Approximately 60 percent of storms measured during the calibration period (2018 and 2019) had depths of 10 to 12 mm or less, yet these storms produced less than 15 percent of all measured runoff in the control and test catchments (figure 2). The percentage of cumulative volume in the control and test catchments follow a similar trajectory during the treatment period; however, accumulation of runoff volume in the test catchment is observed earlier than in the control, as indicated by greater separation between curves at appreciably lower rainfall depths. The departure between response curves remains consistent across all except the largest of storms (>85 mm). We attribute this difference to a potentially interactive mechanism of surface runoff production (infiltration-excess vs. saturation-excess) and degree of storage versus throughfall in the canopy. From figure 2, tree canopy appeared to be more retentive during the calibration period when street trees were in place. Removal of street trees reduced the interception and thus storage capacity, and this was observed across a wide range of precipitation depths (2.55 - >12.46 mm).

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To test the stormwater volume reduction efficiency of street tree canopy, storm event volumes from the control catchment were paired with those from the test catchment to establish and test a linear regression (figure 3). According to the paired-catchment approach, any change in the relation between the control and test catchments during the calibration period can be attributed directly to activities related to street tree removal. The magnitude of change reflects the cumulative effects of all changes including those related to loss of interception and storage by the canopy and branches and changing antecedent moisture conditions in the street right-of-way following removal of roots of removed street trees.

To better understand the reductive effect tree canopy exhibits across an array of precipitation depths, paired event volumes presented in figure 3 were discretized into smaller ranges. Because varying the precipitation thresholds for each range would affect statistical outcomes, precipitation ranges detailed were selected to provide granularity across the full range of depths while maintaining a similar number of events within each range during the treatment period. Results from statistical tests showed two of the five precipitation ranges had significant changes in storm event volume after street trees were removed (p<=0.10) (table3).

Table 3. Results from the ANCOVA test for paired event volumes in the control and test catchments during the calibration and treatment periods across a gradation of precipitation ranges. Statistical significance of the difference between slopes and intercepts are indicated by the corresponding probability values (p). A positive percent change indicates an average increase in event volume after removal of street trees compared to what would have been predicted with trees present using the pre-treatment regression equation. Values in bold indicate significance at the 90 percent confidence level (p <= 0.10). [n, number of events; --, not significant].

For each precipitation range identified as statistically significant in table 3, the increase in volume during the treatment phase is expressed as a percentage change between the average predicted and observed values. Using the average values gives some indication of the relative change for each precipitation range but does not provide enough information to determine the cumulative increase in runoff volume for individual events.  To better quantify the volumetric increase for all storms, observed and predicted runoff volumes during the treatment period were summed for each precipitation range. The difference between these two sums represents an estimate of the increase in runoff due to the removal of street trees (table 4).

Table 4. Estimated increase in stormwater runoff volume through removal of street tree canopy in the test catchment during the treatment period. Estimates are based on the difference between the sum of predicted and observed event volumes for each precipitation range. Only precipitation ranges that meet statistical significance (p <= 0.10) are presented.

The two ranges in table 4, when combined, accounted for an increase of 198 m³, which was equivalent to 4 percent of the total runoff volume measured in the test catchment during the treatment period. Storm events with precipitation depths in the <=2.45, 6.11 – 12.45, and >=25.16 mm ranges were not statistically different from the control and, therefore, not considered when assessing the overall increase in runoff volume because of tree loss.

A total of 31 street trees were removed at the onset of the treatment period, resulting in a loss of 2,990 m² of canopy over streets, driveways, sidewalks, and grassed areas. Each of these surfaces provide variable contributions of runoff to nearby storm drains during a rain event with impervious surfaces transferring surface runoff more quickly that pervious surfaces. An increase in runoff volume of 198 m³ indicates the normalized, aggregated volume reduction capacity of the removed canopy to be approximately 66 L/m² (6.6 cm equivalent water depth) over the 42 storms that occurred during the five months of May through September 2020.  Together these values represent the cumulative impact on stormwater generation from changes (interception, transpiration, and infiltration) that are associated with removing mature Fraxinus pennsylvanica street trees from the test catchment.

Implications for Stormwater Management

Although the runoff reduction volumetric benefits reported in this study reflect only green ash, a review of leaf area index shows the Fraxinus genus to be a good average representation of the diverse species of urban street trees commonly used in the Midwest (Ma et al., 2020). Understanding the value of urban tree canopy as a tool for stormwater management can help cities assess how removal or planting of street trees may influence the volume of stormwater runoff reaching receiving water bodies. Previous research has primarily focused on individual hydrologic components of trees with few studies examining trees holistically within the context of the urban water cycle. An extensive literature review by the Center for Watershed Protection (2016) identified 33 studies characterizing rainfall interception or transpiration of urban trees, most of which occurred in semi-arid climates. Similar studies covering a broad range of tree species commonly found in humid climates would be helpful to improve understanding of regional variability. Quantifying the combined effects of a tree’s ability to intercept, transpire, and infiltrate water into soils at the sewershed or watershed scale would be beneficial to limit variability and uncertainty inherent in studies of a single tree or at the plot scale.

References

Berland, A., Shiflett, S.A., Shuster, W.D., Garmestani, A.S., Goddard, H.C., Herrmann, D.L. and Hopton, M.E., 2017. The role of trees in urban stormwater management, Landscape and Urban Planning, 162, pp. 167 – 177.

Center for Watershed Protection, 2016, Review of the available literature and data on the runoff and pollutant removal capabilities of urban trees, available at: https://owl.cwp.org/mdocs-posts/review-of-the-available-literature-and-…, accessed on April 7, 2021.

Kuehler, E., Hathaway, J., and Tirpak, A., 2016. Quantifying the benefits of urban forest systems as a component of the green infrastructure stormwater treatment network, Ecohydrology 10(3), 10 p. 

Ma, B., Hauer, R.J., Wei, H., Koeser, A.K., Peterson, W., Simons, K., Timilsina, N., Werner, L.P., and Xu, C., 2020, An assessment of street tree diversity: Findings and implications in the United States, Urban Forestry & Urban Greening, 56, 13 p., http://dx.doi.org/10.1016/j.ufug.2020.126826