Nonstationarity of low flows and their timing in the eastern United States

Abstract

Abstract. The analysis of the spatial and temporal patterns of low flows as well as their generation mechanisms over large geographic regions can provide valuable insights and understanding for climate change impacts, regional frequency analysis, risk assessment of extreme events, and decisionmaking regarding allowable withdrawals. The goal of this paper is to examine nonstationarity in low flow generation across the eastern US and explore the potential anthropogenic influences or climate drivers. We use nonparametric tests to identify abrupt and gradual changes in time series of low flows and their timing for 508 USGS streamflow gauging sites in the eastern US with more than 50 years of daily data, to systematically distinguish the effects of human intervention from those of climate variability. A time series decomposition algorithm was applied to 1-day, 7-day, 30-day, and 90-day annual low flow time series that combines the Box-Ljung test for detection of autocorrelation, the Pettitt test for abrupt step changes and the Mann-Kendall test for monotonic trends. Examination of the USGS notes for each site showed that many of the sites with step changes and around half of the sites with an increasing trend have been documented as having some kind of regulation. Sites with decreasing or no trend are less likely to have documented influences on flows. Overall, a general pattern of increasing low flows in the northeast and decreasing low flows in the southeast is evident over a common time period , even when discarding sites with significant autocorrelation, documented regulation or other human impacts. The north-south pattern of trends is consistent with changes in antecedent precipitation. The main exception is along the mid-Atlantic coastal aquifer system from eastern Virginia northwards, where low flows have decreased despite increasing precipitation, and suggests that declining groundwater levels due to pumping may have contributed to decreased low flows. For most sites, the majority of low flows occur in one season in the late summer to fall, as driven by the lower precipitation and higher evaporative demand in this season, but this is complicated in many regions because of the presence of a secondary low flow season in the winter for sites in the extreme northeast and in the spring for sites in Florida. Trends in low flow timing are generally undetectable, although abrupt step changes appear to be associated with regulation.

Drought in a human-modified world: reframing drought definitions, understanding, and analysis approaches

“…For the recent multi-year drought in California this has led to discussion about the role of groundwater abstraction (AghaKouchak et al, 2015a). Additionally, observed trends in measured low flows and drought are influenced by human activities (Sadri et al, 2016), probably even when only unregulated catchments are selected (as noted by Hisdal et al, 2001;Stahl et al, 2010). This undermines our understanding of the effects of climate change on low flows and droughts and increases the uncertainty in projections for the future (Forzieri et al, 2014).…”

“…Another large-scale data analysis method that has great potential for use in drought research is comparative analysis (Wagener et al, 2007), which aims to find patterns by analysing a large set of catchments with a wide range of characteristics, both in terms of natural and human processes (e.g. Price et al, 2011;Eng et al, 2013;Sadri et al, 2016). This method is especially valuable if it is combined with qualitative data to explain the patterns found.…”

Does nonstationarity in rainfall require nonstationary intensity–duration–frequency curves?

“…The multiple tests allow a more rigorous and comprehensive assessment of the overall trend in the time series since certain tests are complementary to each other (Sadri et al, 2016;Yilmaz et al, 2014Yilmaz et al, , 2017. Figure 2 shows the schematics of the overall analysis.…”

Multiple causes of nonstationarity in the Weihe annual low-flow series

“…Therefore, the investigation of the magnitude and frequency of low flows is of primary importance for engineering design and water resources management (Smakhtin, 2001). In recent years, low flows, as an important part of river flow regime, have been attracting an increasing attention of hydrologists and ecologists in the context of the significant impacts of climate change and human activities (HAs; Bradford and Heinonen, 2008;Du et al, 2015;Kam and Sheffield, 2015;Kormos et al, 2016;Liu et al, 2015;Sadri et al, 2016). In general, under the im-B.…”

“…The significant factors such as precipitation, temperature, evapotranspiration (EP), streamflow recession, large-scale teleconnections and human forces may play important roles in influencing low-flow generation (Botter et al, 2013;Giuntoli et al, 2013;Gottschalk et al, 2013;Kormos et al, 2016;Sadri et al, 2016). Gottschalk et al (2013) presented a derived low-flow probability distribution function with climate and catchment characteristics parameters (i.e., the mean length of dry spells λ −1 and recession constant of streamflow K ) as its distribution parameters.…”