Quantifying the thermodynamic entropy budget of the land surface: is this useful?

Abstract. As a system is moved away from a state of thermodynamic equilibrium, spatial and temporal heterogeneity is induced. A possible methodology to assess these impacts is to examine the thermodynamic entropy budget and assess the role of entropy production and transfer between the surface and the atmosphere. Here, we adopted this thermodynamic framework to examine the implications of changing vegetation fractional cover on land surface energy exchange processes using the NOAH land surface model and eddy covariance observations. Simulations that varied the relative fraction of vegetation were used to calculate the resultant entropy budget as a function of fraction of vegetation. Results showed that increasing vegetation fraction increases entropy production by the land surface while decreasing the overall entropy budget (the rate of change in entropy at the surface). This is accomplished largely via simultaneous increase in the entropy production associated with the absorption of solar radiation and a decline in the Bowen ratio (ratio of sensible to latent heat flux), which leads to increasing the entropy export associated with the latent heat flux during the daylight hours and dominated by entropy transfer associated with sensible heat and soil heat fluxes during the nighttime hours. Eddy covariance observations also show that the entropy production has a consistent sensitivity to land cover, while the overall entropy budget appears most related to the net radiation at the surface, however with a large variance. This implies that quantifying the thermodynamic entropy budget and entropy production is a useful metric for assessing biosphere-atmosphere-hydrosphere system interactions.