The critical zone includes the coupled earth surface systems of vegetation, regolith and groundwater that are essential to life on the planet. Understanding the interactions among physical, chemical and biological processes that govern the critical zone is a major challenge to earth system sciences. We developed a thermodynamic framework that characterizes the critical zone as a system open to energy and mass fluxes forced by radiant, geochemical, and elevational gradients. We derive an integrated energy and mass balance demonstrating the importance of solar radiation, water, carbon, and denudation mass fluxes to the critical zone. Within this framework we use rates of effective energy and mass transfer [EEMT; W/m2] to quantify the relevant flux-gradient relations. Synthesis of data across global, watershed and pedon scales demonstrates that variation in energetics associated with primary production (EBIO) and effective precipitation (EPPT) explains substantial variance in critical zone structure and function. Across all spatial scales we observe threshold behavior in systems that transition to primary production predominance of the energy flux term (EBIO/EEMT = FBIO > 0.5). At the global scale, we observe significant relationships between EEMT, FBIO and ecosystem and soil taxonomic unit distribution with the transition to dominance of EEMT by EPPT near an EEMT of 70 MJ/m2/yr. At the watershed scale we derived the EEMT term from empirical data for an 89 watershed subset from the USA MOPEX dataset. Empirical measures of EPPT were derived from watershed water balance, whereas EBIO was derived using satellite derived measures of net primary production (MODIS, MOD17 data product). These data demonstrate unique patterns of water and carbon partitioning relative to traditional hydrologic measures of water availability, and a threshold cutoff in monthly EEMT production at vapor pressure deficits above 1,000 Pa. Furthermore, application of the EEMT model to watershed scale data representing critical zone function indicated significant empirical relationships among EEMT, FBIO, watershed Si solute efflux, denudation, and modeled soil depth. At the pedon scale, significant relationships were observed between EEMT and measures of soil development across a series of environmental gradients in the Sierra Nevada of California. Furthermore, we have applied thermal techniques to quantify pedogenic energy storage and consumption in the Sierra Nevada soils. Patterns of subsurface net energy storage correspond to modeled EEMT and suggest a direct linkage between energy and mass flux and system energy storage. The proposed framework provides a first order approximation of critical zone processes that may be coupled with physical and numerical models to better constrain critical zone evolution. This approach is currently being explored in detail in the Santa Catalina Mountain and Jemez River Basin Critical Zone Observatories in southern Arizona and northern New Mexico, USA.
Rasmussen, C., Troch, P.A., Brooks, P.D., Pelletier, J.D., Chorover, J. (2010): Application of thermodynamics to quantify the energetics of pedogenesis and critical zone evolution. AGU Fall Meeting (Invited) Abstract H52B-03..