Introduction
Gully erosion is globally recognised as a significant source of soil
loss and can directly influence the water quality of downstream aquatic
ecosystems by the presence of fine suspended sediment (<63 µm)
and suspended sand (63-2000 µm) (Vercruysse et al., 2017; Walling, 2006;
Walling et al., 2016). The impact to water quality from gully erosion
can vary depending on the particle size distribution and sediment yield.
For example, small gullies that form in recently tilled agricultural
land can affect local waterways (Capra et al., 2002), whereas larger
landscape-scale gullies can impact the water quality of connected
waterways for hundreds of kilometres downstream (e.g.,
>40% of the sediment polluting the Great Barrier Reef
Marine Park, in the coastal waters of Northern Australia, is generated
by gully erosion that occurs hundreds of kilometres upstream) (Brooks et
al., 2013; Olley et al., 2013; Wilkinson et al., 2015).
Gullies are often formed by water flowing over land at a sufficient
velocity to incise the soil and form a deep exposed gap, typically in
areas with poor soil cohesion (Casalí et al., 2009). Over time the area
of gullies expands due to active erosion caused by repeated flow events
associated with intense rainfall (Poesen et al., 2003). Gully erosion is
a natural process, however, the rate of erosion can drastically increase
as a consequence of anthropogenic land use change (e.g., installation of
roads and other infrastructure or through agricultural activities, such
as land clearing and livestock grazing) (Nyssen et al., 2002; Wilkinson
et al., 2018).
To date, suspended sediment monitoring in gullies has been conducted
using methods designed for rivers and streams (e.g., automated discrete
samplers (autosamplers), rising stage samplers (RS samplers), and
turbidity loggers) (Baker et al., 2016; Bartley et al., 2017; Caitcheon
et al., 2012; Nistor et al., 2005). However, gully systems present a
unique set of hydrological and operational challenges for which these
various sampling and measurement methods have not been thoroughly
evaluated. The method considered to be the most accurate for measuring
suspended sediment, flow-proportional manual sampling (Horowitz et al.,
2008; Perks, 2014), is often infeasible or unsafe to conduct in gully
systems due to the unpredictability of rainfall events, the remoteness
of many gully landscapes, and the instability of gully channels and
banks.
Currently, the operational challenges associated with measuring water
quality in gullies are overcome using autonomous sampling or surrogate
measurement techniques (autosamplers, RS samplers, and turbidity
loggers) and remote sensing methods (time lapse cameras or light
detection and ranging (LiDAR) techniques) (Casalí et al., 2009; Castillo
et al., 2012). These methods are established monitoring techniques with
well understood capabilities and limitations (Table 1). Many of these
methods, however, are too expensive to implement over the large spatial
network of actively eroding gullies within a catchment (e.g.,
autosamplers and turbidity loggers), and others provide incomplete
information when deployed in isolation (e.g., RS samplers and time-lapse
cameras). To address this gap, our study includes the recently developed
Pumped Active Suspended Sediment (PASS) sampler; an automated,
time-integrated, and in situ sampling device, as a low-cost
approach that could be used to monitor gully erosion over large spatial
scales (Doriean et al., 2019; Nunny 1985; Phillips et al., 2000). As
this work is primarily focussed on approaches that measure water quality
associated with suspended sediment (i.e., suspended sediment
concentration and particle size distribution) remote sensing techniques
(i.e., LiDAR) that are used to estimate soil loss using landscape scale
volumetric analysis will not be included in this evaluation.
Here we aim to systematically evaluate and compare the capabilities and
limitations of a variety of suspended sediment monitoring methods (i.e.,
autosampler, RS sampler, turbidity logger, and PASS sampler) when
applied to gully systems. The methods are compared under controlled
laboratory conditions and in the field to assess the relative ability of
each method to provide accurate measurements of suspended sediment
concentration and particle size distribution.