Alternative stable states and hydrological regime shifts in a large intermittent river

Abstract

Non-perennial rivers and streams make up over half the global river network and are becoming more widespread. Transitions from perennial to non-perennial flow are a threshold-type change that can lead to alternative stable states in aquatic ecosystems, but it is unknown whether streamflow itself is stable in either wet (flowing) or dry (no-flow) conditions. Here, we investigated drivers and feedbacks associated with regime shifts between wet and dry conditions in an intermittent reach of the Arkansas River (USA) over the past 23 years. Multiple lines of evidence suggested that these regimes represent alternative stable states, including (i) significant jumps in discharge time series that were not accompanied by jumps in flow drivers such as precipitation and groundwater pumping; (ii) a multi-modal state distribution with 92% of months experiencing no-flow conditions for <10% or >90% of days, despite unimodal distributions of precipitation and pumping; and (iii) a hysteretic relationship between climate and flow state. Groundwater levels appear to be the primary control over the hydrological regime, as groundwater levels in the alluvial aquifer were higher than the stream stage during wet regimes and lower than the streambed during dry regimes. Groundwater level variation, in turn, was driven by processes occurring at both the regional scale (surface water inflows from upstream, groundwater pumping) and the reach scale (stream-aquifer exchange, diffuse recharge through the soil column). Historical regime shifts were associated with diverse pressures including network disconnection caused by upstream water use, increased flow stability potentially associated with reservoir operations, and anomalous wet and dry climate conditions. In sum, stabilizing feedbacks among upstream inflows, stream-aquifer interactions, climate, vegetation, and pumping appear to create alternative wet and dry stable states at this site. These stabilizing feedbacks suggest that widespread observed shifts from perennial to non-perennial flow will be difficult to reverse.

Groundwater connections and sustainability in social-ecological systems

“…Such research often aims to identify complex system attributes and behaviours (e.g., Figure 2b). For instance, methods to detect early-warning signals for regime shifts in complex systems (Scheffer et al 2009) are only just beginning to be applied to groundwater-connected systems (Zipper et al 2022).…”

Groundwatersheds of protected areas reveal globally overlooked risks and opportunities

“…Aquatic ecosystems can also be threatened by human activities transmitted through groundwater flow. Groundwater pumping, for instance, can reduce streamflow and drive streams from perennial to intermittent, ephemeral, or even disconnected [19][20][21][22][23] . In addition, since groundwater provides distinct chemical and temperature attributes and can transmit contaminants such as nutrients 24 , changes in groundwater levels and flow can introduce pollutants or otherwise alter water quality in protected areas 25 .…”

The Communication Distance of Non-Perennial Streams

“…Integration of subsurface connectivity into our methods would allow more holistic considerations of human impacts on the water cycle, as groundwater pumping can cause streams to transitions from perennial to non-perennial flow regimes (Zipper et al, 2022a). Numerous approaches exist to model groundwater pumping impacts on surface streamflow (Zipper et al, 2022b), and analytical models for distributing depletion within stream networks are codified in the R package streamDepletr (Zipper 2020).…”

“…problematic in non-perennial streams because certain stream locations (e.g., surface flow bottlenecks) may have inordinately large effects on stream networks (Godsey and Kirchner 2014;Zipper et al 2022a).…”