Inferring scalar sources and sinks within canopies using forward and inverse methods

It is becoming clear that the description of water exchange over vegetated surfaces can benefit from a simultaneous consideration of heat and C02 exchanges, as all these exchange processes are intertwined at the most fundamental level. Over the last two decades, several approaches have been developed to infer scalar sources and sinks within canopies without resorting to gradient-diffusion theory. This study investigates recent developments in multi-layer methods to compute distributions and strengths of scalar sources and sinks within the canopy volume. Two types of model formulations are considered: 1) forward methods which require vertical foliage distribution along with canopy radiative, physiological, biochemical, and drag properties and 2) inverse methods which require measured mean scalar concentration distribution within the canopy. These approaches are able to reproduce measured turbulent fluxes above the canopy without relying on empirical relationship between turbulent scalar fluxes and mean concentration gradients. However, both approaches share the need for accurate description of the second moments of the velocity statistics inside the canopy.