GROUNDWATER-DRIVEN ALTERNATIVE STABLE STATES IN A LARGE NON-PERENNIAL RIVER

Non-perennial rivers and streams make up over half the global river network and are becoming more widespread, but our ability to predict and manage transitions from perennial to non-perennial flow is hindered by a lack of understanding of the hydrologic processes underlying these transitions. Here, we investigate drivers and feedbacks associated with shifts from perennial to non-perennial flow over the past 20 years in the Arkansas River near Larned, Kansas, USA. We find that the stream intermittency is characterized by periodic and alternating shifts between ‘wet’ (flowing) and ‘dry’ (no-flow) regimes that last multiple years. Multiple lines of evidence suggest these wet and dry regimes represent alternative stable states, including (i) significant jumps in the discharge time series that are not accompanied by jumps in flow drivers such as precipitation and nearby groundwater pumping; (ii) a multi-modal state distribution with 92% of months experiencing no-flow conditions for <10% of >90% of days, despite unimodal distributions of flow drivers; and (iii) a hysteretic relationship between climate and hydrologic state. Groundwater levels in the alluvial aquifer are the primary control over the system state, as wet regimes are accompanied by an upward hydraulic gradient (flow from the aquifer to the stream) while dry regimes are accompanied by a downward (stream to aquifer) hydraulic gradient. Recharge from the stream causes a rapid, high-magnitude increase in water levels response in the aquifer, while reductions in water levels due to pumping are lagged over multiple years. The water level response to climate is intermediate and primarily on seasonal to annual timescales. Our analysis suggests that cross-scale stabilizing feedbacks between stream-aquifer interactions, climate, and pumping have created alternative wet and dry stable states that are driven by groundwater levels in the alluvial aquifer at the site, and that regime shifts from perennial to non-perennial flow will be difficult to reverse.