A model of potential carbon dioxide efflux from surface water across England and Wales using headwater stream survey data and landscape predictors

Abstract

Measurements of CO2 partial pressures (pCO2) in small headwater streams are useful for predicting potential CO2 efflux because they provide a single concentration representing a mixture from different hydrological pathways and sources in the catchment. We developed a model to predict pCO2 in headwater streams from measurements undertaken on snapshot samples collected from more than 3000 channels across the landscape of England and Wales. We used a subset of streams with upstream catchment areas (CA) of less than 8 km2 because below this scale threshold pCO2 was independent of CA. A series of catchment characteristics were found to be statistically significant predictors of pCO2, including three geomorphic variables (mean altitude, mean catchment slope and relief) and four groups of dominant catchment land cover classes (arable, improved grassland, suburban and all other classes). We accounted for year-round, temporal variation in our model of headwater pCO2 by including weekly or monthly analyses of samples from three headwater catchments with different land use and geomorphic features. Our model accounted for 24% of the spatial and temporal variation in pCO2. We combined predictions from the pCO2 model (on a 1 km grid) and monthly runoff volumes (litres) on 0.5° resolution grid across England and Wales to compute potential C fluxes to the atmosphere. Our model predicts an annual average potential C flux of 65.4 kt C across England and Wales (based on free C concentrations), with lower and upper 95% confidence values of 56.1 and 77.2 kt C, respectively.