Results for trend and breakpoint analyses from: Losing flow in free-flowing Mediterranean-climate streams

Stream drying is happening globally, with significant ecological and social consequences. Most examples of stream drying come from systems influenced by dam operations or those with highly exploited aquifers. Stream drying is also thought to be happening due to climate change, but examples are surprisingly limited. We explored flow trends from the five Mediterranean-climate regions with a focus on unregulated streams with long-term gauge records. We found consistent evidence of decreasing discharge trends, increasing zero-flow days, and steeper downward discharge trends in smaller basins. Beyond directional trends, many systems recently shifted flow state, including some streams that shifted from perennial to intermittent flow states. Our analyses provide evidence of stream drying consistent with climate change, but also highlight knowledge gaps and challenges in empirically and statistically documenting flow regime shifts. We discuss the myriad consequences of losing flow and propose strategies for improving detection and adapting to flow change.

Methods
To document flow change, we compiled gauge records from five Mediterranean-climate regions of the world, including California (U.S.), Chile, South Africa, Spain, and Western Australia. For each gauge, we downloaded daily discharge records from public sources. Next, we limited our analysis to gauges located in Mediterranean-climates zones by retaining the subset of gauges located in Köppen-Geiger climate classes Csa, Csb, Csc (i.e., areas with a dry summer) using maps from Beck et al. 2018. Second, we identified gauges located in minimally disturbed basins. In the US and Australia, we used “reference” gauges identified by the USGS and Bureau of Meteorology, respectively. In South Africa, Chile, and Spain - where reference gauges have not been designated by agencies – we instead used aerial image analysis of upstream watershed conditions to identify basins with no evidence of significant reservoirs or large water infrastructure projects. We note that our determination of “reference-quality” gauges in Spain [excluding Catalonia] is consistent with Messager et al. 2021. Third, we identified gauges with daily data from 1980-2019 (i.e., most recent 40 years in common across the five regions) and no more than one year of missing data.

Overall, we identified 158 gauges that met our criteria for inclusion (i.e., Mediterranean-climate, reference-quality, 40 years of data from 1980-2019, and no more than one year of missing data, WebPanel 1, WebFigure1). To reduce noise in zero-flow conditions, we defined “zero flows” as flows < 0.1 cfs. Finally, for our analysis of zero-flow trends, we used a liberal definition of “intermittent” and included the subset of streams with ≥ to 1 day/year of zero-flow on average, i.e., ≥ 40 days across the 40 year study, following Messager et al. 2021.

Using the population of gauges that met our criteria for inclusion, we conducted trend analyses on daily discharge (for each gauge in our population) and on the annual number of zero-flow days (for the subset of intermittent gauges) across the time series by means of non-parametric Mann-Kendall tests. 

We next explored evidence of flow regime shifts. Specifically, we conducted a breakpoint analysis on the zero-flow days per year using the ‘strucchange’ package in R. We constrained the analysis to test for evidence of a maximum of one breakpoint (indicating a state shift).