DISCUSSION
Our low-cost field methods allowed us to calculate five flow metrics corresponding with potentially ecologically relevant aspects of the flow regime (Poff et al. 1997; Clausen and Biggs 2000). Demonstrating the capability of using low-cost and robust techniques in flashy systems is an important step towards monitoring and understanding the flow regime of intermittent streams. The use of redundant methods of stream flow measurement allowed us to collect variations in data inherent to each method, such as when sensors sat in small, isolated pools of water at substrate level, had residual water on the electrodes, or from high humidity levels. Additionally, the initial dates of first noflow period varied slightly between methods, likely for the same reasons. The sensor data produced significant advantages: data processing time was faster and less prone to error because no manual evaluation of stage height was required. The sensors also provided stream temperature data, which can be of high biotic importance, especially in intermittent streams experiencing extreme temperatures during drying. The methods presented in this study can be used to quantify variation in intermittent flow at other ungauged sites.
Current modeling strategies may not accurately or fully depict the flow regimes of small, highly variable watersheds. Predictions by the Oklahoma HAWQS model were within the range of what we observed in the field for some metrics but underestimated the number of no-flow days. The number of no-flow days is a simple, but ecologically and conceptually important metric describing stream drying (Poff et al. 1997; Olden and Poff 2003; Zipper et al. 2021). Prior calibrations of hydrologic models have reported varying success in calculations of hydrologic metrics, often overestimating low flow conditions (Kiesel et al 2017; Pool et al. 2017), a direct contrast to our observations, wherein the HAWQs models underestimated low flow conditions. Other observations in the region report levels and durations of stream drying consistent with our observations (Homan et al. 2005). Generally, our streams exhibited higher numbers of no-flow days than predicted in a localized framework developed by Leasure et al. (2016), implying that environmental conditions may be harsher than anticipated in Ouachita streams.
Watershed area, a measure of stream size, is often considered to be one of the defining variables in flow patterns. Larger streams typically have less extreme variations in flow, with lower relative extremes of both high and low flows (Chiang et al. 2002). Slope is also frequently regarded as an important driver of flow, given that slope plays a role in determining the speed at which water moves through a system, with steeper slopes more likely to shed water faster (Harr 1977; Paznekas and Hayashi 2016). We observed some of our strongest correlations between watershed area and slope with our flow metrics, however these relationships were surprisingly variable. Because the gradient of stream sizes included in our study was relatively narrow, other variables may be exerting strong influences on streamflow patterns, such as watershed and ecological processes (Snelder and Biggs 2002). The impact of the relatively fine spatial scale of this study may have also influenced the lack of significant links between land use or geological features and flow patterns. Landscape factors typically used to predict hydrology may not have varied enough within our study area to predict flow dynamics, however variability and high flow conditions may be better explained at the landscape level than drying trends. Our observed variability in hydrologic metrics and the relatively weak observed relationships among landscape factors and metrics highlight the challenge of predicting ecologically meaning measures of stream drying in small streams. However, we acknowledge the exploratory nature and small sample size in this study limited our ability to statistically detect strong relationships among landscape variables and hydrologic metrics. There is a need for more field observations of stream drying to understand drivers of variation in stream drying dynamics at intermediate spatial scales relevant to ecological process and land management.
In addition to watershed characteristics, climate maintains a well-established link with flow regime (Poff et al. 1997; Snelder and Biggs 2002; D’Ambrosia et al. 2017). Within runoff driven systems, such as the Glover River basin, local variation in precipitation likely serves as a driver of variation in stream drying, a pattern seen in other intermittent systems (Costigan et al. 2015). Southeastern Oklahoma is characterized by high intensity and short duration precipitation events, with two primary rainy seasons (Oklahoma Climatological Survey 2023). During the sampling period, we observed variation in local precipitation between adjacent drainages. Variation in rainfall and hydrograph responses can shift within a storm, based upon storm direction, speed, movement patterns and watershed shape (Roberts and Klingeman 1970; Jensen 1984; Singh 1997). Given the relatively small area of our study extent, the scale of precipitation variability is somewhat surprising and difficult to account for. Measuring site-specific precipitation would be beneficial to further inform and understand the drivers of small intermittent streams.
Climate change projections for the region predict an increase in short duration, high intensity precipitation events, potentially less overall annual precipitation (Karl et al. 2009; Marion et al. 2013; USGCRP 2018), and increased drought potential (Strzepek et al. 2010). These projections will likely result in more frequent no-flow periods, interspersed with large high-flow events. Beyond the obvious risk of more streams completely drying in response to longer no-flow periods, longer drying events also stress aquatic organisms where isolated pools remain. In isolated pools, increased temperatures and decreased dissolved oxygen levels can pass lethal thresholds for aquatic organisms, leading to mortality for less tolerant species (Capone and Kushlan 1991; Hopper et al. 2020). More intense precipitation events may increase the intensity of disturbance, potentially impacting reproductive success of resident organisms (Humphries et al. 1999) or influencing community structure (Bernardo et al. 2003).
Long-term field observations of stream drying are needed to understand drivers of variation in stream intermittency. This study documents the ability to measure ecologically relevant hydrologic metrics, however long-term data would serve to smooth the inherent year to year variability in precipitation, both annually and between sites, and more accurately represent between-site variability in flow patterns (Poff 1997). Without long term flow data, hydrologic dynamics can be difficult to predict or model, limiting the applicability of environmental flow standards (Zhang et al. 2016). Our comparison of HAWQS model predictions to observed data support previous observations that current long-term stream gauges in the US can provide potentially misleading or flawed information about no-flow periods, highlighting the need for increased data collection of streamflow data in smaller, intermittent streams (Zimmer et al. 2020).
Both stream intermittency sensors and trail cameras can serve to provide valuable information about stream flow regimes. We successfully evaluated the five major aspects of the flow regime for a gradient of intermittent streams. These techniques readily allow for classification of flow regimes into three ecologically relevant categories, but given the small sample size, both temporally and spatially, identification of watershed level impacts on flow regime remains less clear. Within the Glover River basin, our ten streams exhibited flashy intermittent flow patterns, with variable signatures among systems. Although we found some evidence of watershed characteristics correlating with flow patterns, many of the other drivers of local flow regime remained elusive, potentially due to highly variable precipitation patterns. Evaluation of the selected hydrologic indices serves to underscore the highly variable nature of intermittent streams in Southeastern Oklahoma, and the difficulty in linking watershed scale factors with local flow regime.