Materials and Methods
Site descriptions
The 2 DBs investigated during this current study, along with concurrent
studies investigating the effect of DBs on hydrology and nutrients
during the same storm events from 1 December 2017 to 30 November 2018,
were located on pastoral dairy farms in the north-western portion of the
Lake Rotorua catchment. Table 1 describes site characteristics reported
by the concurrent studies. The Oropi series soils at the the Hauraki
site, and Waiteti series soils at the Awahou site are both free
draining, with >72 mm/h permeability in slowest horizon
(Rijkse & Guinto, 2010).
Event types
Event types reported on in this, and concurrent studies, were
differentiated according to the mode(s) ponded water was discharged from
the DB. ‘Overflow Events’ occurred during larger runoff events when
inflow continued to be delivered to the pond after the pond height
exceeded the height of the upstand riser (Fig. 1 ). After 3 days
of ponding, any residual ponded water was evacuated when the outlet
valve was opened, creating ‘release discharge’. Therefore, ‘Overflow
Events’ had both overflow and release discharge components. In contrast,
‘Non-overflow Events’ were smaller storms that did not contribute enough
runoff to overtop the riser. Non-overflow Events included events when at
the end of the 3-day treatment period, either had a portion of ponded
runoff to discharge by opening the release valve, or all ponded runoff
leaked and infiltrated the soil so there was no water left to discharge.
Equipment
Isco® (California, USA) 6712 portable auto-samplers,
capable of filling 24 x 1 L bottles collected inflow and discharge
samples at each site when triggered by a telemetered
UNIDATA® Neon® 2013 F 3G External
Memory Metering Module data loggers linked to UNIDATA®6527 Starflow® QSD flowmeters. The auto-samplers were
triggered to collect 1 L samples when flows exceeded 7 L/s (Harmel,
King, Wolfe, & Torbert, 2002). Calibration and maintenance of the
monitoring equipment followed standard quality controls (NIWA, 2004).
Inflow auto-samplers collected a 1 L sample every 20 min for the first
10 samples, then one 1 L sample/h thereafter (Harmel, King, & Slade,
2003; Stanley, 1996). The mouth of a rain guarded 750-mL self-sealing
bottle using a ping-pong ball inside the bottle, was installed at ground
level near the pond outlet valve to capture a sample of the initial
flush of surface runoff generated before the inflow auto-sampler was
triggered. The ping-pong ball bottle sample was used as the
concentration of the initial runoff and used in calculating event inflow
yields
Discharge auto-samplers were programmed to collect a 1-L sample/h
(Harmel et al., 2003; Stanley, 1996). Sampled discharge flows were
generated if the pond height exceeded the upstand riser height during
pond filling (i.e. ‘overflow discharge’) (Fig. 1 ), and when the
valve at the base of the riser was opened to release the pond at the end
of the event treatment, typically on the third day of ponding (i.e.
‘release discharge’).
Throughout all ponding at both sites an intractable leak at the
connection point of the outlet valve pipe and the base of the upstand
riser generated a continual flow of ~2-4
m3/h. Attempts at sealing this leak during the study
period were unsuccessful. Under normal sampling conditions, the leak
flow was too low to trigger the auto-samplers, although leak samples
were collected during 4 events at the Hauraki site, and 1 event at the
Awahou site, in order to characterise the SS concentrations of the leak
discharge.
Water samples were collected from the field within 24 h of the end of
the ponding event and kept refrigerated at 4 °C prior to subsampling
(within ~24 h of collection). Two separate subsamples
(~30 mL) were taken from the field sample after
vigorously shaking the bottle, to analyse total and dissolved N and P.
The remaining field sample was kept refrigerated until being analysed
for SS concentrations used in this current study, following the standard
procedure from the American Public Health Association (2005).
Calculations
Mean flow proportional
concentrations
Event and annual mean flow-proportional (MFP) SS concentrations were
calculated by dividing the inflow and discharge loads by their
respective volume (Tanner & Sukias, 2011). The average difference
between the event MFP inflow and leak samples collected during 5 events
was +3%, with no consistent increase or decrease. Due to the negligible
difference between the MFP inflow and leak concentrations, the MFP
inflow concentration was applied to the entire leak volume for each
respective event in which the leak discharge was not sampled. The
applied leak concentration was used to calculate the event MFP discharge
concentrations and event discharge loads. All inflow and discharge MFP
concentrations will be referred to only as inflow and outflow
concentrations.
Loads and yields
calculation
Loads of SS in inflows and each discharge type were determined for all
ponding events. Inflow loads of SS were calculated by multiplying the
measured concentration of the runoff samples collected by the ping-pong
ball sample bottle and auto-samplers, and using interpolated
concentrations based on the linear rate of change between measured
concentrations, by the interval flow volume measured every 5 minutes.
Inflow loads were corrected on a pro rata basis (15% increase at the
Hauraki site and 9% increase at the Awahou site) to account for the
small catchment area between the inflow monitoring site and the DB
(Table 1) .
Discharge loads were calculated for overflow discharge (combining
upstand riser and spillway breaching), release discharge (which occurred
during Overflow events and Non-Overflow events), and leak discharge. The
load of each discharge type was calculated from flow measurements and
sample concentrations taken from the DB outlet pipe, except for
emergency spillway breaching. Emergency spillway loads were calculated
by applying the MFP concentration of the overflow discharge generated by
ponded water discharged by going over the upstand riser to the volume
breaching the spillway calculated by the concurrent study reporting on
hydrology. Yields refer to the load per unit of contributing catchment
area and expressed as mm for runoff volumes, and kg
ha-1 for SS loads.
Data analysis
Water sample data
analysis
Events were grouped by site, then analysed for overall annual results,
and results for each event type. Changes to concentrations were
calculated as the percent difference between inflow and discharge
concentrations. The percent difference between inflow and discharge
yields were reported as ‘yield treatment efficiencies’. Inflow yield
data for each site was also organised by austral seasons (i.e. summer
from December to February) to compare differences between the sites and
identify seasonal patterns for SS inflow yields.