Site Overview
The redeveloped ASU Orange Mall is comprised of concrete and pavers
interspersed with stand-alone vegetation and planted bioretention
basins. The development introduced 525 m2 of bioretention basin and
ground cover, an approximate 12% increase in the overall amount of
permeable surfaces across the site. The site contains a total of 7
basins, which are divided into two primary (east and west) basin
systems.
The western half of the site contains 3 smaller basins – numbered
west basin 1 through 3 (WB1-WB3), which are 90, 40, and 15 m2 in
size, respectively (total = 145m2). The eastern half of the site
contains 4 larger basins – labeled east basins 1 through 4
(EB1-EB4), which are 165, 110, 20, and 85 m2 in size, respectively
(total = 380m2). Basins in each of the two subsystems are connected
to each other by a series of shallow runnels that channel water
between the individual basins with each system. An elevation
gradient directs flow from both the east and west basin systems to
converge at a final central collection box, where all stormwater
combines and flows south via a concrete pipe to an off-site
below-ground cistern. A variety of desert adapted native vegetation
species (n = 11) were planted across all of the Orange Mall basins.
Roughly 80% of the population is comprised of individuals from 5
species: Fan West Ash (Fraxinus Fan-West hybrid),
Tall Slipper Plant (Pedilanthus bracteatus), Desert Spoon
(Dasylirion wheeleri), Mexican Petunia (Ruellia brittoniani), and
Compact Jojoba (Simmonsdsia chinensis vista).
Surface inflows to the basins are varied and dispersed. EB1 and EB2
both receive significant point-source inputs at their eastern
terminus from curb cuts that drain surface flow from the remainder
of Orange Street, while WB1 receives significant inputs from Student
Pavilion roof drainage and asphalt roads to the north. However, the
basins also receive inflow from surface runoff flowing off all of
the concrete surfaces that surround them, making an accurate
estimate of total inflow volumes or rates difficult and therefore
not included here. Monitoring equipment were positioned to provide
insights into the general hydrological dynamics of the site. These
include characterizing water flow through the system and changes in
water quality as water flows through each of the two systems of
basins.
Water Quality
We used five ISCO® 6712 automated pump samplers to collect up to 9
discrete stormwater samples per sampler during storm events between
August 2018 and March 2019. We installed ISCO samplers at the inflow
and outflow of the western half of the site, and at the inflow,
mid-point, and outflow of the eastern half of the site (n = 5). ISCO
intake sampling lines were installed in the curb cuts or runnels
providing inflow or outflow to the basins so that only flow between
basins would be sampled, as opposed to standing water in the center
of a basin. ISCO® 720 bubbler modules were installed and used to
measure water stage and trigger sample collection based on water
stage; bubbler lines were installed parallel to the ISCO® 6712
sampling lines. The samplers were programmed to draw samples at a
water stage of 3.13cm or greater - the minimum depth required to
inundate the strainer at the end of the sample line. The samplers
were also programmed to sample at non-uniform fixed time intervals,
with sampling occurring more frequently during the beginning of
storms when water quality was expected to change most rapidly (i.e.,
first-flush effect; Lee et al. 2002). These fixed time sampling
intervals were set to 0, 5, 10, 15, 30, 45, 60, 90, and 120 minutes
following sampling program activation.
Stormwater samples were collected from the Orange Mall ISCOs within
12 hours of each event and transported to the ASU Wetland Ecosystem
Ecology Lab for processing. Subsamples for dissolved organic carbon
(DOC) and anion analysis were filtered through ashed Whatman GF/F®
47mm filters, and DOC samples were HCl acidified to pH = 2. Samples
were then transported to the ASU Goldwater Environmental Lab for
analysis. DOC samples were analyzed within 7 days on a Shimadzu TOC-
VC/TN analyzer (detection limit 0.04 mg DOC/L and 0.004 mg TN/L).
Unfiltered sub-samples were collected for total nitrogen (TN) and
total phosphorus (TP). Nitrate (NO3-), nitrite (NO2-) and ammonium
(NH4+) samples were centrifuged to remove particulates and along
with TN and TP analyzed on a Lachat Quick Chem 8000 Flow Injection
Analyzer (detection limit 0.85 µg NO3-N/L and 3.01 µg NH4-N/L). TN,
TP, NO3-, NO2- and NH4 samples were kept frozen after processing
until analysis. All methods for stormwater collection and analysis
were based on standard CAP LTER stormwater research to provide
cross-site comparability.
Water Quantity
To determine effluent discharge volumes from the basins, 90-degree
v-notch weirs were constructed and installed at the outflow
discharge points of the east and west basin systems (see figures
included with this data set for location details). ONSET HOBO U20L
water-level probes were installed 10 cm upstream from the weirs
inside the discharge pipe/channel to measure water stage. I used the
US Bureau of Reclamation (2001) Cone equation for v-notched weirs
was to calculate discharge rates at the weirs using these water
level measurements:
Q = 2.49h12.48
where Q represents the flow rate in m3/s, and h1 represents the
hydraulic head on the weir. Hydraulic head was derived by
subtracting the total distance from the bottom of the channel to the
bottom of the weir v-notch from the observed water stage. The
resulting value represented the hydrological head of water
overtopping the weir.
Soil Moisture
We measured continuous soil moisture content to characterize the
water retention capacity of soil amendments used in the basins. Soil
moisture data were collected in EB1 and EB2 using a series of ONSET
HOBO 10HS Soil Moisture Probes connected to an ONSET USB
Microstation datalogger, capturing continuous soil moisture data at
a 5 minute resolution. Soil moisture readings were averaged across
all sensors, as basin soil amendments were identical. ONSET HOBOWare
(version 3.7.15, ONSET, Bourne, MA) was used to download data from
all ONSET probes and loggers and transform raw data into the
appropriate units.
Transpiration
Orange Mall managers were also interested in understanding
comparative transpiration rates for the various species of
vegetation planted in the basins, given the important implications
this has for stormwater and microclimate management. Leaf-specific
transpiration rates were measured for the 5 dominant macrophyte
species planted throughout the system using a LICOR LI-6400 handheld
infrared gas analyzer (IRGA). Measurements were made on individual
leaves or leaflets of Pedilanthus bracteatus, Dasylirion wheeleri,
Ruellia brittoniani, and Simmonsdsia chinensis vista
plants. Gas flux data were collected continuously in 2-3 hour
sampling sessions on a random selection of individuals from each
species across all basins. For each individual, a spread of 3 leaves
representing the entire height and width of the canopy were chosen
and sampled using the IRGA. Several evening transpiration sampling
sessions confirmed night-time transpiration to be negligible for all
species (evening data not reported here).
