Dynamics of DOC concentration and flux in different propagation stages of hydrological drought: Patterns and drivers

Abstract

Droughts influence dissolved organic carbon (DOC) export. DOC concentration and flux dynamics during different propagation stages of hydrological drought (HD) are complex and poorly understood. Long-term (from 1978 to 2018) weekly (biweekly) DOC concentration and daily streamflow at four headwater streams of Harp Lake catchment, Ontario, Canada were used to examine the effects of HD on the dynamics. Identified HD were divided into four propagation stages: pre-drought (P1), intensification (P2), recovery (P3) and post-drought (P4). The potential mechanisms underlying the patterns in DOC were also investigated. First, HD reduced DOC concentration by an average of 19.24% relative to non-HD periods. DOC concentrations and fluxes of streams were different during different propagation stages. Second, DOC flux patterns among streams were consistent with “fast decline-slow rise-fast rise” trends over the P2, P3, and P4, relative to each previous stage. However, DOC concentration pattern differed among four streams, showing a “fast decline-slow decline-fast rise” trend across four stages in two streams, but a “fast decline-slow rise-fast rise” pattern at other two. Initially, DOC flux during HD was primarily dominated by streamflow, but DOC concentration eventually became an important co-factor. Third, the rapid decrease (or increases) in streamflow during P2 (P4) relative to P1 (P3) led to a fast decline (or rise) in DOC concentration. Differences in DOC concentration responses among streams were probably due to differences in local hydrologic features (e.g., wetlands) and unnatural activities. pH and SO4 can also influence DOC concentration dynamics during HD, but just under the specific hydrothermal combinations. This study provides an updated perspective to understand the dynamic patterns of DOC concentration and flux and their drivers during droughts, and it could be used to improve the modeling and management capability of water quality.