Climatic and Hydrological Controls on Stream DOC Concentrations in Headwater Catchments as Revealed by Long-term, High Frequency Data

Abstract:

The seasonal variations of DOM concentrations and fluxes in streams can help identifying the major sources and processes at play. Recent data obtained on the environment research observatory (ERO) of Kervidy-Naizin, a 5 km² agricultural catchment located in France, revealed the role of water table fluctuations in wetlands and slopes on the variations in DOM composition and fluxes. Based on a detailed analysis of the variations of the seasonal patterns of stream DOM from 2001 to 2013, this study aims (i) to identify the main hydrological and climatic variables controlling DOM production and export (ii) to discuss the underlying mechanisms of these controls.

The time series of 12 years of daily DOM concentrations was divided into successive seasons based on hydrological functioning: i) continuous days of low water table levels in the riparian wetland soils (dry season), ii) progressive rewetting of the uppermost soil layers of the riparian wetland soils (season A), iii) hillslope water table rising and holding high (season B), iv) drawdown of the hillslope water-table, with less and less connection between topsoils and the stream (season C). Base flow and storm flow samples were also distinguished. Climate and hydrological variables and indices were included in partial least square regressions (PLSR) to explain variations of DOC concentrations and fluxes at annual and seasonal scales.

Mean annual concentrations (Cy) are much less variable than the annual fluxes (Fy) (from 4.9 to 7.5 mg DOC.L^-1 and 5.4 to 39.7 kg DOC.ha^-1.y^-1). PLSR suggests that Cy variations are affected by antecedent drought conditions, while Fy variations are controlled by winter wetness.

Seasons A, B and C export respectively between 0.3 and 17.2%, 64.3 and 95.7%, 3.5 and 23% of the annual DOM. Within each season, mean DOC concentrations decrease with time, especially when water fluxes increase. However, it remains above 2.1 mg.L^-1 even during exceptionally wet winters. These results support the hypotheses of different sources located in the riparian zone and upslope zone, some of them subjected to quick exhaustion by flowing water, the others more stable, and both reconstituted during dryer periods.