Greenhouse gas dynamics of the Pokfulam reservoir catchment in Hong Kong

Abstract

Inland waters, such as streams and reservoirs, have been recognized to be major players in the global carbon cycle and climate system owing to their capabilities in receiving a significant amount of terrestrially derived carbon and emitting a significant amount of greenhouse gas (GHG) to the atmosphere through biochemical reactions that occur in the water column. However, similar studies have rarely been conducted in Hong Kong and the public has not paid much attention to this issue. In order to study the GHG emissions potential from inland waters in Hong Kong more in detail, a year-long measurement of common water quality parameters and GHG dynamics has been carried out in the Pokfulam Reservoir catchment. The GHG fluxes measured by deploying a floating chamber over the water surface demonstrated that the reservoir catchment is an overall source of carbon dioxide (CO2), although it can occasionally act as a sink for atmospheric CO2 when exposed to intense solar radiation. Meanwhile, the reservoir catchment also emits considerable amounts of methane (CH4), and it causes even greater climate change impacts than CO2. The study of groundwater-fed headwater stream within the reservoir catchment found that the headwater is very significantly supersaturated with CO2, and the evasive loss of CO2 was extremely rapid along the reach as more than 70% of CO2 has lost just over a distance of <10 meters. This highlighted the necessity of probing into the GHG dynamics of such headwater systems to obtain accurate estimates of GHG emissions within a given reservoir catchment. With the deployment of a CO2 sensor, the CO2 level in the water column was found to be experiencing significant diel variabilities at both lentic and lotic waters in the Pokfulam Reservoir catchment, emphasizing the importance of conducting nocturnal measurements to evaluate the role of a water body playing in the carbon cycle more accurately. Although our study revealed that the reservoirs in Hong Kong contribute to GHG emissions and influence the climate system, the quantification of GHG emissions and associated climate impacts are not a mandatory aspect of the environmental impact assessment (EIA) procedures in the territory. This reflected the necessity for the EIA process to take into account the climate change impacts of major projects such as the construction of reservoirs. It is believed that a more comprehensive assessment of GHG emissions from Hong Kong inland waters would permit a more reliable estimate of the GHG emissions inventory of the territory and facilitate the formulation of climate-related policies.