Title: Greenhouse gas emissions from rivers in Taita Hills, Kenya
Author: Sharon Gubamwoyo
Supervising Institution: IHE Delft - Institute for Water Education
Year: 2019
Abstract:
Head water streams (HWS) are thought to play a disproportionate role in inland aquatic greenhouse gas (GHG) emissions due to their close relationship with the watershed and the role of the benthic zone in biogeochemical processes responsible for GHG emissions. The contribution of headwater streams at catchment scales, however, has rarely been quantified. Furthermore, land use, in addition to stream order may also play a role in GHG emissions. Little is known about GHG emissions from tropical systems, including sub-Saharan Africa, where land use change is occurring rapidly, in part due to agricultural intensification. Furthermore, the seasonality, particularly with respect to rainfall, may be important, but has not been fully explored. Land use and rain events may create hot spots and hot moments of GHG emissions. Therefore, this study explored the GHG (CO2, CH4, and N2O) concentrations, fluxes and gas transfer velocities in the Taita Hills region in south eastern Kenya. Two different catchments were studied (Bura and Wundanyi) with a total of 48 sampling points, across 6 stream orders and different land use types, from forest, mixed, and agriculture. I further intensively surveyed two headwater sub-catchments in the main catchments mentioned above at 5 m intervals during a wet and dry period to understand how stream width is related to GHG flux, resulting in ~600 sites in the Bura catchment and ~800 sites in the Wundanyi catchment. These measurements were then used in a simple up-scaling calculation to sub-catchment level to compare the two catchments and moisture conditions. I found that stream order was more important than land use, especially for partial pressure for CO2 (pCO2), and flux (fCO2). fCO2 ranged from -16.82 -3595.02 mmol m2d-1 with first order streams contributing the highest flux rates. Neither land use nor stream order were statistically important for fCH4(ranged from -0.13 -17.23 mmol m-2d-1) nor fN2O (ranged from -0.038 -0.740 mmol m-2d-1). Relating water quality variables to GHG fluxes led us to hypothesize that the main biogeochemical controlling process were terrestrial carbon inputs for CO2, denitrification and methanogenesis for N2O and CH4 respectively.The intensive field surveys showed a modal stream width of 29 cm. The up-scaled estimates showed that the HWS were over-saturated with CO2, while CH4 and N2O were lower. The survey also indicated that different seasons (moisture conditions) have an impact on GHG emissions, and need to be explored more. Wundanyi catchment, the wetter of the two, increased by a magnitude of 2 from 17 kg of CO2equivalents (CO2e) in the first survey in wet conditions to 34 kg of CO2e in the second survey, which was much drier. Bura catchment also increased between the two surveys from 12 kg CO2e in the first survey to 20 kg CO2e in the second survey.