The raw data collected at the OnCenter and used in this study by the research team, along with a graphic illustrating days excluded per the criteria in Section 2
, can be accessed on the website (http://greenroof.syr.edu
), with a password that can be provided upon request by contacting the authors.
3.1. Weather during the Study Period
Weather during the study period was generally consistent with historic temperatures (Figure 3
). Total precipitation in the region was higher than the historic regional precipitation for 9 of the 21 months in the study period. A total of 1062 mm of rainfall was measured at the site during this period, excluding snow events. Measured total precipitation averaged between 1950 and 2010 at the Syracuse Hancock International Airport, located 8.3 km north of the OnCenter, is compared with the rainfall measurements taken at the OnCenter between November 2014 and May 2016 (Figure 4
). Daily total rainfall during these periods are comparable, with reasonable differences attributed to the spatial variability of rainfall.
Following removal of the non-qualifying events, 165 events remain. Rainfall duration and depth for each event are given in Figure 5
overlain by updated recurrence intervals for the region based on historic precipitation records through 2008 [28
]. Of these events, three exceed the 1-year recurrence interval—one of which nears 5-year recurrence. Despite the high frequency of events, the 84 very small (<2 mm) events comprise only 5% of the measured rainfall during the study period. The 11 large events (>20 mm), however, account for 38% of the total rainfall measured. It is important to note that the green roof was designed to hold a 25.4 mm rain event.
3.2. Green Roof Performance
Cumulative retention on the green roof is 56% over the study period, a total retained depth of 599 mm. Given the 5550 m2
roof area, a total of 3350 m3
of rainfall is retained during the study period. Full capture occurred for 106 events, or 64% of the events. Overall mean event retention is 85%. Nawaz et al. found that cumulative retention for 19 studies varied widely, between 15% and 83%, with average and median retentions of 57% and 59% respectively [9
]. One experimental study in New York City found a 55% cumulative retention during their study period for a roof with a 25.4 mm growth medium depth [9
]. Studies with growth media of similar depth to the study (70–100 mm) reported overall mean event retention between 52% and 74% [24
], though comparison to other studies has limited value as many factors influence performance of individual roofs. Note that three of the above-mentioned studies are also cited by Nawaz et al. [24
]. In contrast, a study in Norway reported an annual retention of 11%–30% across multiple green roofs including both rain and snow, with a higher range of 22%–46% reported between May and October for rain only [34
]. On ten plot-scale green roofs, Liu et al. reported overall mean event retention between 23% and 33.2% [35
]. Significant variation exists across experimental studies reported in the literature, and recent studies have begun to consider the influence of multiple factors on the reported performance [36
Detention metrics, summarized in Table 1
, are calculated for the 39 events where 5 min data are available and runoff occurred. Peak attenuation ranges from 0.11 to 5.2 mm/5 min, with an average of 1.3 mm/5 min. Runoff delay is calculated from peak to peak and centroid to centroid.
The peak delay, calculated for the 5-minute timestep, ranges from −0.92 to 30 h, with a mean and median of 3.3 and 0.75 h, respectively. Despite the wide range, most events have a short delay, with 71% falling between 0 and 2 h. Only one event had a negative delay. The peak in runoff occurring prior to the peak in rainfall is a result of the natural variability of rainfall during an event and calls into question the appropriateness of the peak delay metric. In contrast, the centroid delay for this event is 1.5 h, demonstrating that the center of the rainfall event still preceded the center of the runoff event. One long event with a 30-hour delay had 41.9 mm of rainfall over 61 h, with most of the rainfall occurring in the first few h of the event. The green roof growth medium had the capacity to retain almost all of the first few hours of rainfall. However, the event continued at a slower rate until the capacity of the roof was exceeded and some runoff occurred, with only 63% of the total event being retained. Both of these peak delays are a result of the temporal variability of rainfall within an event and the time scale over which the roof responds. Peak delay, among other detention metrics used in green roof research, is borrowed from the field of hydrology where response times on a watershed scale are considerably longer. Other researchers have found similar behavior and also question whether the peak delay provides an accurate interevent comparison [37
The delay between the onset of rainfall and runoff is, in part, a product of the antecedent soil moisture which will be discussed in Section 3.3
. As rainfall continues, the retention capacity of the substrate is exceeded, and runoff begins. The wide range of detention metrics results from the temporal patterns of precipitation within an event and the initial conditions of the substrate which also influence retention. Detention metrics cannot be separated from initial losses. After moisture within the substrate exceeds field capacity, runoff response is quick and predictable. Each peak in precipitation is followed by a corresponding peak in runoff, not unlike a small watershed whose runoff is strongly governed by hill-slope mechanics [40
]. The time response of the green roof results in multiple peaks within a single event definition. Previous observations on a plot-scale suggest runoff will follow rainfall almost immediately after the retention capacity of the roof is exceeded [38
]. Observations on this full-scale roof suggest a delay of 15 min after the retention capacity of the substrate is met, though some of this may be attributed to the time required to travel the distance between the roof drain and the in-line flowmeter and to the equipment measurement interval. Further, this roof design incorporated drain conduits across the roof, as shown in Figure 1
. As the designers intended these drain conduits aid in the removal of excess water from the growth medium, decreasing the detention time during large events.
3.2.1. Performance by Event Size
The largest events are responsible for a smaller portion of overall retention, while most of the rain falling as small and very small events is retained, as shown in Figure 6
. Only one event less than 2 mm experienced any runoff. This event generated 0.03 mm of runoff from 0.65 mm of rainfall occurring on an already very wet substrate (aVWC = 0.20). The event occurred just outside the 6 h event definition allowing it to be separated from the previous event. Only 24% of the measured retention occurred within the 11 large events, detailed in Table 2
. Further discussion of the effect of aVWC on evapotranspiration is included in Section 3.3
3.2.2. Performance by Season
Event average retention is lowest during the winter and not substantially different in the remaining seasons, as shown in Table 3
. Cumulative seasonal retention is highest in the fall and spring and lowest in the winter. Lower retention during the winter is explained by lower rates of evapotranspiration during interevent recovery periods. Climatic conditions during the summer months, however, promote the highest rates of evapotranspiration, which should result in high overall retention in the absence of other influencing factors. Yet summer cumulative retention is only 35%, lower than both the fall and spring. Rainfall patterns vary with the season. Summer has both less rain events and higher average event intensities than the spring or fall. During the study period, rainfall for the months of May, June, and July is higher than the median historical data. The average event peak intensity during the summer exceeds 8 mm·h−1
while the next highest, during the spring, does not exceed 5 mm·h−1
. It appears that the higher intensity, lower frequency rainfall patterns experienced during the summer contribute to the season’s lower performance.
Probabilities of exceedance for runoff depth separated by season are given in Figure 7
. Winter events are more likely to result in greater runoff depth, consistent with the behavior reported in the literature [24
], and the low average retention for this season. The results of a one-way ANCOVA found that the difference in runoff behavior is statistically significant at the 0.05 level only for winter and summer. Weather conditions between the spring and fall are not statistically different, but an increase in the number of events in future data collection periods along with consideration of other weather factors may provide more insight into these trends.
Retention performance has an inverse relationship with aVWC. The substrate has a finite capacity to hold water, after which all incoming precipitation enters the drainage structure. The difference between the soil moisture level in the substrate and its maximum capacity is the available water retention capacity. During interevent periods, the retention capacity of the substrate is restored via ET.
Daily ET rates for dry days averaged across a transect of the roof, as shown in Figure 8
, range from 0 to 2.5 mm·day−1
. On this same roof, daily ET measurements for both wet and dry days during warm months between 2015 and 2017 are found to range from 0 to 5.4 mm·day−1
, with a daily average of 0.76 [42
]. Plot-scale studies planted with Sedum mexicanum
and Disphyma australe
in New Zealand found ET rates from 1.9 to 2.2 mm·day−1
under unstressed water conditions [33
]. Measurements made on two green roofs in New York City in 2009 and 2013 found an average daily ET of 0.24 and 0.72 mm·day−1
in December and 4.80 and 4.94 mm·day−1
in July [43
]. The ET measurements here are within these ranges measured on other roofs. Differences between measurements made in Syracuse and New York City may result from differences in weather as well as in roof construction, i.e., substrate retention properties. A consistent drying curve for ET is visible for multiple periods throughout the data set as a series of points decreasing in a nearly vertical line. A longer dry period results in a nearly vertical line with slight curvature near the bottom of the graph, due to decreasing ET as the soil dries, e.g., between 25 April and 9 May 2015, as available soil moisture has a direct relationship with rates of ET. As weather cools through fall 2015 and the available energy for ET decreases, there are fewer points with large ET. The current method of estimating ET cannot be used with snowstorms, but there are some rains in December and February which enable estimates of ET that are less than 1 mm day−1
. Rising temperatures in March, April, and May result in increasing maximum values of ET. Between late May and early July infrequent and intense storms result in high ET rates immediately following an event and long consistent decay in the period June–July 2016.
ET rates are limited by the availability of energy and water. Daily maximum insolation and initial daily soil moisture are considered relative to daily ET rates as proxies for available energy and water on the roof, as shown in Figure 9
and Figure 10
, respectively. As insolation increases, the maximum ET rates (corresponding to sunny days) show an approximately increasing pattern. Days with low maximum insolation, less than 250 W·m−2
, are cold weather days in the period late November–December 2015, where lower ambient temperatures also contribute to the low ET rates. While cloudy summer days are included in the study, all days between 27 October 2015 and 6 February 2016 have a maximum solar insolation of less than 500 W·m−2
and insolation values measured for only 9–11 h. In contrast, many late spring and summer days report insolation measured for 14–16 h per day and reach maximum values above 750 W·m−2
Under conditions with significant available energy for ET, limited available water results in low rates of ET. During water-limited periods, the range of ET rates is small. In contrast, during periods of abundant available water, ET rates have a larger range as the amount of energy available for ET varies. Water-limited conditions, although infrequent, occur primarily in the summer. Under rare drought conditions in the summer of 2016, a minimum soil moisture of 0.009 m3
was recorded on 8 July 2016, after 13 mm fell in 39 days during a period of high incoming solar radiation. This value is significantly lower than the wilting points reported in the literature [33
]. With the annual cycling of incoming solar radiation, energy available for ET varies, influencing the available water retention capacity of the substrate and therefore seasonal retention performance. Field capacity on the roof as estimated from the observed soil moisture after runoff has ceased is approximately 0.22 m3
, but varies spatially across the roof due to localized differences in substrate structure and flow pathways.