INTRODUCTION
Intermittent streams are closely tied to local climate and geology and
reflect the hydrologic cycle within their watershed (Baker et al. 2004).
Intermittent streams are naturally dynamic systems that often follow
periodic cycles of drying and wetting (Arthington et al. 2014).
Intermittent streams may fully dry, but often retain isolated pools
during low flow periods (Busch et al. 2020). Intermittent streams are
common and critical components of aquatic ecosystems that occur across
most biomes and are estimated to account for up to 60% of the length of
all defined stream channels (Larned et al. 2010; Datry et al. 2014;
Costigan et al. 2016; Messager et al. 2021). Due to climate change,
intermittent streams will likely become more widely distributed and many
will likely dry for longer periods of time as precipitation patterns
shift (Larned et al. 2010; Jaeger et al. 2014; Costigan et al. 2016;
Zipper et al. 2021; Datry et al. 2022).
Despite drying for parts of the year, intermittent streams support
diverse and complex community assemblages (Meyer et al. 2007, Datry et
al. 2016), are valuable components of aquatic ecosystems (Colvin et al.
2019), play a role in the persistence of fishes by providing seasonal
habitats (Vander Vorste et al. 2020), transport nutrients and material
(von Schiller et al. 2017) and support terrestrial biodiversity
(Sánchez-Montoya et al. 2022). The availability of habitats and
resources provided by intermittent streams is variable within and
between water years, influencing community dynamics (Fausch and
Bramblett 1991) and driving patterns of community structure (Olden and
Kennard 2010). Moreover, intermittent streams provide valuable habitat
for native species and contain variable community compositions compared
to perennial streams (Rogosh and Olden 2019). For example, wet season
habitats can provide complementary habitats for different life stages of
aquatic organisms (Labbe and Fausch 2000) and increase connectivity and
therefore colonization rates (Franssen et al. 2006). In contrasts,
during summer drying, fish communities shift as intermittent systems
reduce to isolated pools, whether by reducing as conditions harshen, or
by as available habitat shrinks (Capone and Kushlan 1991; Pires et al.
1999; Taylor and Warren 2001; Hopper et al. 2020). The longer a no-flow
period persists, the more potential exists for long-lasting impacts on
the aquatic community, as no-flow periods represent a significant
disturbance (Poff et al. 1997).
While both high- and low-flow conditions are integral aspects of the
flow regime (Poff et al. 1997), low-flow conditions such as no-flow
duration, dry-down period and no-flow timing drive intermittent stream
ecology (Poff et al. 1997; Olden and Poff 2003; Zipper et al. 2021).
Ecologically relevant low-flow conditions can also be described by
simple classifications such as whether or not a stream fully dries or
dries to isolated pools, and what proportion of time the stream spends
in each state (Gallart et al. 2017). The temporal and spatial components
of low flow hydrology are closely linked and require understanding of
both physical and anthropogenic factors (Smakhtin 2001). Climatic
patterns, ground water, water table interactions, geology, and watershed
shape, structure and size are all key components influencing low flow
periods (Smakhtin 2001, Snelder and Biggs 2002, Hammond et al. 2020). At
relatively fine spatial scales, geology and land use characteristics may
be stronger drivers of flow patterns than climate (Snelder and Biggs
2002, Ssegane et al. 2012). Geology can impact stream flow through
groundwater storage, recharge characteristics and flow capabilities
(Mwakalila 2003). Anthropogenic alterations may also influence stream
intermittency patterns, with changes in land cover influencing timing
and magnitude of flow patterns, especially in regard to flow variability
and low flow periods (Ficklin et al. 2018; Datry et al. 2022).
A wide range of metrics have been developed and used to characterize the
flow regime and compare variations in hydrologic patterns across space
and time. Many of these metrics are simple calculations that correspond
to ecologically important aspects of flow, are often highly correlated
(Smakhtin 2001), and serve to represent the complexity of a riverine
system (D’Ambrosio et al. 2017). Olden and Poff (2003) examined 171
hydrologic indices to evaluate informativeness, redundancy and
ecological importance and concluded intermittent flow regimes are best
characterized by unique indices in comparison to more stable systems.
The time scale at which flow patterns exist is highly variable, from
hours to years, potentially requiring long-term observation to fully
quantify variability (Poff et al. 1997; Richter et al. 1997; Leasure et
al. 2016). Many streams have established, long-term monitoring in the
form of gauging stations, such as those maintained by the United States
Geological Survey (USGS), providing accurate discharge data (Falcone
2011). However, stream gauges are often not suitable for highly dynamic
stream systems, and alternate data collection techniques have met
varying levels of success (Blasch et al. 2004). Additionally, most
intermittent streams are not gauged (Eng et al. 2015), limiting readily
available long-term flow data.
Evaluating flow conditions in the absence of stream gauge data can be
analytically and conceptually difficult, but is important given the
increased interest in intermittent streams (Sivapalan 2003; Datry et al.
2016; Zipper et al. 2021) and in understanding how hydrology and
hydrologic indices predict ecological patterns and processes (Clausen
and Biggs 2000; Meyer et al. 2007; Booth and Konrad 2017). One approach
to quantify dry periods of intermittent streams is to use water
temperature data, which can be subject to interpretation (Blasch et al.
2004; Sowder and Steel 2012). Another technique is to model streamflow
using nearby gauged streams, a data intensive methodology that may miss
inherent variation between stream segments (Black 1972; Mwakalila 2003;
Zimmer et al. 2020). Spatial scale is an integral aspect of flow
modeling, as conditions between nearby streams can be highly variable,
often as a result of local precipitation (Yuan 2013). The Soil and Water
Assessment Tool (SWAT) is a widely used model for evaluating streamflow
characteristics and potential changes in flow regime, predicting
streamflow patterns and soil or nutrient transport from various
processes including climate, hydrology, vegetation, management and
precipitation (Cibin et al 2014; Gassman et al. 2014; Jajarmizadeh et
al. 2015). In ungauged systems, SWAT models may not represent flow
patterns accurately (Qi et al. 2020) and can be difficult to calibrate
and validate due to a lack of empirical data (Sivalapalan et al. 2003;
Beven and Smith 2015). Regardless, there is a need for low-cost
techniques to monitor ungauged intermittent streams (Blasch et al. 2004;
Chapin et al. 2014) to better understand the link between stream drying
and ecological responses.
Our objective was to describe patterns of drying, rewetting, and flow
variability across a gradient of intermittency at ungauged streams. We
used two low-cost techniques to identify drying extent (e.g.,completely dry, isolated pools, or retain connectivity) and quantify
metrics describing the flow regime of intermittent streams (timing,
duration, magnitude, rate of change, and frequency of high and low flow
events). In addition to describing ecologically relevant flow metrics
that can be derived from low-cost field methods, we also identified
landscape and geophysical drivers of stream intermittency and hydrologic
variability. We hypothesized watershed size and slope would be primary
drivers of intermittency, with smaller and steeper watersheds exhibiting
earlier and more frequent periods of no-flow. We aim for the field
methods, hydrologic metrics, and observed variability in stream drying
described in this paper to help initiate larger efforts to monitor
hydrologic variability of intermittent streams.