Thermodynamic constraints on effective energy and mass transfer and catchment function

Abstract. Understanding how water, energy and carbon are partitioned to primary production and effective precipitation is central to quantifying the limits on critical zone evolution. Recent work suggests quantifying energetic transfers to the critical zone in the form of effective precipitation and primary production provides a first order approximation of critical zone process and structural organization. However, explicit linkage of this effective energy and mass transfer (EEMT; W m⁻²) to critical zone state variables and well defined physical limits remains to be developed. The objective of this work was to place EEMT in the context of thermodynamic state variables of temperature and vapor pressure deficit, with explicit definition of EEMT physical limits using a global climate dataset. The relation of EEMT to empirical measures of catchment function was also examined using a subset of the Model Parameter Estimation Experiment (MOPEX) catchments. The data demonstrated three physical limits for EEMT: (i) an absolute vapor pressure deficit threshold of 1200 Pa above which EEMT is zero; (ii) a temperature dependent vapor pressure deficit limit following the saturated vapor pressure function up to a temperature of 292 K; and (iii) a minimum precipitation threshold required from EEMT production at temperatures greater than 292 K. Within these limits, EEMT scales directly with precipitation, with increasing conversion of the precipitation to EEMT with increasing temperature. The state-space framework derived here presents a simplified framework with well-defined physical limits that has the potential for directly integrating regional to pedon scale heterogeneity in effective energy and mass transfer relative to critical zone structure and function within a common thermodynamic framework.