Introduction
Suspended sediments (SS) are organic and inorganic particles transported
in suspension by water (Bilotta et al., 2009). Rainfall and surface
runoff cause erosion, leading sedimentation in surface waters and
causing aquatic ecosystem degradation by disrupting aquatic habitats and
food webs (Howard-Williams, Davies-Colley, Rutherford, & Wilcock,
2010), and by delivering sediment-bound nutrients that contribute to
eutrophication (Dare, 2018). Land use developments clearing native
forests have accelerated the already naturally high rates of erosion
across New Zealand and caused significant sedimentation in lakes and
streams (Ministry for the Envrionment, 2019).
Pastoral agriculture in New Zealand is strongly associated with
eutrophication and degraded freshwater ecosystems (Verburg, Hamill,
Unwin, & Abell, 2010). Treading by grazing animals physically disrupts
the soil, and increases the likelihood of surface runoff and erosion by
decreasing infiltration rates and porosity, and impairing plant growth
(Bilotta, Brazier, & Haygarth, 2007; R. W. McDowell et al., 2003; Ward,
Talbot, Denne, & Abrahamson, 1985). Year-round grazing and high
stocking rates used to graze crops are common practices in New Zealand
and contribute to increased erosion (Monaghan et al., 2007). Addressing
erosion is a challenge for pastoral farmers in New Zealand, particularly
those on sloping landscapes, due to variable precipitation patterns with
very wet winters, and dry summers interspersed with large storms (R.
McDowell, Wilcock, & Hamilton, 2013). Erosion is likely to be
intensified by climate change causing more dramatic hydrologic
conditions (Ministry for the Envrironment, 2019; Ockenden et al., 2016).
Since the 1960’s water quality in Lake Rotorua, in the Bay of Plenty
Region on the North Island of New Zealand, has declined due to nutrient
inputs from residential, commercial, industrial and agricultural
developments (Environment Bay of Plenty, 2009). An estimated 43% of the
annual total phosphorus (P) delivered to the lake comes from pastoral
dairy and drystock farms which cover ~48% of the 42,000
ha Lake Rotorua surface catchment (Bay of Plenty Regional Council,
2012). Between 71-79% of the anthropogenic P delivered to the lake is
sediment bound (Hamill, 2018), and a portion of that may become
biologically available under anoxic conditions which occur in Lake
Rotorua and contributes to lake eutrophication (Abell & Hamilton,
2013). Lake sediments release an estimated 36 t P y-1,
accounting for ~48% of the total annual P loading (Bay
of Plenty Regional Council, 2012). Intensified anoxic conditions during
periods of warmer temperatures have cause significantly greater
contributions to annual P loads by lake sediments (Burns, McIntosh, &
Scholes, 2005).
The 2012 Lake Rotorua Management Plan has set a target to reduce annual
TP loads delivered from the catchment by 10 t P y-1 by
2022 from the estimated 39 t P y-1, in order to
restore lake water quality (Bay of Plenty Regional Council, 2012).
Achieving Lake Rotorua water quality targets by addressing P loading
from pastoral agriculture will require combining multiple appropriate
nutrient mitigation strategies and potentially developing new
technologies (R. W. McDowell, 2010).
Mitigation strategies that increase stormflow residence time have been
found to decrease surface runoff flows, leading to increased sediment
deposition by decreasing the kinetic energy of flowing water (Dosskey,
2001; McKergow, Tanner, Monaghan, & Anderson, 2007; Stanley, 1996).
However, the type of mitigation strategy affects the duration sediments
are attenuated. Studies have found that sediment retention times are
more brief (weeks to months) where sediments accumulate in more
concentrated areas, such as narrow grass filter strips and constructed
treatment wetlands, compared to strategies where sediments are blanketed
over a wide area, which may have retention times of up to hundreds of
years (McKergow et al., 2007).
Previous research has found that ponding surface runoff can decrease
discharge concentrations and loads of sediments and particulate bound P
by decreasing the kinetic energy of flowing water (Brown, Bondurant, &
Brockway, 1981; Harper, Herr, Baker, & Livingston, 1999; Levine et al.,
2019; R. W. McDowell et al., 2006; Stanley, 1996). Detainment bunds
(DBs), are earthen storm water retention structures constructed on
pastures across the flow path of targeted low-order ephemeral streams
capable of impeding stormflow and temporarily ponding up to 10,000
m3 of surface runoff. The structures were implemented
in the Lake Rotorua catchment in 2010 as a mitigation strategy to target
P losses from pastures (Clarke, 2013). A concurrent paper by Levine et
al., (In review) investigating the strategy’s effect on stormwater
volumes leaving the same DB catchments during the same storm events as
this current study presents further detail on DB design and function.
Preliminary studies of DBs in the Lake Rotorua catchment have found that
P enriched sediments were deposited in DB ponding areas (Clarke, 2013),
and that a DB effectively decreased runoff volumes, and sediment and P
loads, discharged during 3 non-consecutive ponding events (Levine et
al., 2019). A concurrent study investigating the same ponding events at
the same DB sites as this present study reported 31 and 43% and noted
that deposited sediments could be developing a less permeable surface
soil layer and/or clogging soil pore spaces and causing infiltration
rates to decline in the ponding areas (Levine et al., In review).
Although erosion is recognised for its potential impact to aquatic
ecosystems, there is room to progress our understanding of the transport
and fate of sediments from intensively managed pastures (Haygarth et
al., 2006). To determine if DBs could provide a viable strategy for
pastoral farmers to improve Lake Rotorua water quality, it is important
to quantify their ability to decrease SS loads. The main objective of
this study was to measure the effect of the DB strategy on SS
concentrations and yields delivered to 2 DBs with 55 ha and 20 ha
catchments, located on pastures in the Lake Rotorua catchment, and
identify the factors influencing the results. Previous studies on DBs,
and related mitigation strategies, suggest ponding surface runoff
facilitates sedimentation, although there is currently no definitive
research quantifying the impact of the DBs on annual sediment loads
transported from pastures in the Lake Rotorua catchment. We hypothesised
that ponding surface runoff will facilitate sedimentation and result in
lower discharge concentrations, which, combined with soil infiltration
lowering runoff discharge volumes reported in a concurrent study by
Levine et al. (In Review), will decrease annual SS loads discharged from
the treatment catchments.