Storm pulses of dissolved CO2 in a forested headwater Amazonian stream explored using hydrograph separation
Johnson, University of British Columbia, email@example.com
Couto, Universidade Federal de Mato Grosso, firstname.lastname@example.org
Lehmann, Cornell University, CL273@cornell.edu
Riha, Cornell University, email@example.com
Weiler, University of British Columbia, firstname.lastname@example.org
Event water vs. pre-event water contributions to storm hydrographs in the humid tropics have received much less attention than in temperate regions. In research in a forested headwater catchment of the Brazilian Amazon, we collected data in situ using a multiparameter sonde (pH, DO, EC and temperature) and an infrared gas analyzer (dissolved CO2) in addition to hydrometric measurements. We then analyzed 14 storms (11 rainy-season and 3 dry-season storms) for dissolved CO2 dynamics and event water contributions to storm flow. An increase in streamwater CO2 concentration was observed on the falling limb of the storm hydrographs (referred to here as a CO2 pulse), indicating contributions from flowpath(s) with higher CO2 concentrations that lag the storm peak. We applied the transfer function hydrograph separation model (TRANSEP) using specific conductivity as a conservative tracer, and found that pre-event water as a fraction of storm hydrographs was 0.79±0.03 (mean±1SE for n=14 storms). The pre-event water fraction decreased linearly with event size (r= -0.59, p=0.02). The pre-event water fraction was also negatively related to the magnitude of CO2 pulses observed (r= -0.97, p<0.0001). The relationship between CO2 pulses and pre-event water contributions to storm flow suggests a rapid interaction between CO2 in the soil atmosphere and infiltrating storm precipitation, which contributes dissolved CO2 to storm flow as event water late in the events. During most storms, a decline in dissolved CO2 concentrations in streamwater was observed on the rising limb, indicating event water contributions early in the events from quickflow pathways that are low in CO2 including direct precipitation/throughfall and overland flow. These dissolved CO2 trends are superimposed on a background of CO2 concentrations derived from pre-event emergent groundwater that were more than 100 times greater than CO2 concentrations in precipitation, though this CO2 largely outgasses to the atmosphere within the headwater reach.