Numerical Modeling of Soil Gas Mobility in Shallow Unsaturated Agricultural Soils

ABSTRACT: In 2016, greenhouse gases (GHG) released from agricultural soils were estimated to be between 19 and 24 megatonnes of carbon dioxide equivalent with an increasing temporal trend. The processes controlling soil gas mobility in the vadose zone are poorly understood and significantly influence GHG emissions during a seasonal cycle. In this research, conceptual models of the fate of GHG in the vadose zone are presented and investigated through numerical modeling tools. By employing HydroGeoSphere (HGS), the soil gas emissions are estimated by tracking transient variations in soil water storage, assuming that soil pores are either occupied by gas or by water. Variations in soil water saturation can be converted to variations in soil gas saturation and subsequently, GHG emissions. Field data collected at one of Agriculture Canada's WEBS research stations near Ottawa, Ontario were used to inform the numerical experiments. One and three dimensional simulation domains were used to investigate the influence of soil type, macropore fraction, ground surface relief, soil heterogeneity, tile depth and number, and storm intensity on the mobility and emission behaviour of soil GHG. A 10-day conceptual storm simulation and longer-term simulations utilizing field precipitation and evapotranspiration data are used to investigate the transient nature of soil GHG mobility. The results suggest that permeable soils tend to have the highest potential for GHG emission and that macropores may significantly facilitate soil degassing by enhancing the drainage capacity and lowering the initial water saturation of soils. Reductions in tile spacing and depth promote soil drainage and consequently act to reduce soil degassing processes. In addition, evapotranspiration appears to increase the soil gas emission, however, the influence of tile drains and evapotranspiration is strongly influenced by the soil water saturation conditions.