Fig. 1. Schematic of the canopy sublayer (CSL) domain bounded by the vadose zone and the atmospheric surface layer (ASL), where h is the canopy height. An elemental volume of thickness dz sufficiently large to include biological sources and sinks are shown in the canopy sublayer and the vadose zone. Inverse problems in both domains estimate such biological sources and sinks from the depth-time measured concentration, temperature, or soil moisture content.