Influence of drought identification methods on analyzing and assessing responses of water quality to droughts

Hydrological drought, a global extreme event impacting water resources and ecosystems, is characterized by absolute (fixed drought threshold, FDT) or relative (variable drought threshold, VDT) low streamflow metrics. These metrics influence water quality analyses during droughts, a previously overlooked aspect. This study compares FDT and VDT to assess their effects on water quality responses in Harp Lake catchment, Ontario, using long-term (1978–2018) streamflow and water quality data (e.g., dissolved organic carbon, DOC; total nitrogen, TN; total phosphorus,TP). Our findings demonstrate that methodological disparities in hydrological drought identification substantially influence water quality responses to droughts, with divergent thresholds yielding distinct dynamics. Specifically, (i) drought events identified by FDT exhibit higher concentrations of DOC, TP, and TN than those by VDT (e.g., the DOC concentration recognized by FDT is 23.11% higher than that of VDT). However, their fluxes are restricted by reduced streamflow (e.g., the DOC flux recognized by FDT is 80.15% lower). (ii) Both methods show drought duration and severity positively correlate with nutrient concentrations, but FDT exhibits faster response rates. (iii) Discrepancies arise because FDT identifies longer, cross-seasonal droughts (25.35% longer average duration), while VDT detects short-term wet-dry fluctuations, fragmenting events. The study reveals that apart from the perturbations caused by drought itself on water quality, the drought identification method used can also make remarkable difference in the assessment of these perturbations, which calls more research or attention.