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).