Technical Note: A comparison of model and empirical measures of catchment-scale effective energy and mass transfer

C Rasmussen,E L Gallo

doi:10.5194/hess-17-3389-2013

Abstract

Abstract. Recent work suggests that a coupled effective energy and mass transfer (EEMT) term, which includes the energy associated with effective precipitation and primary production, may serve as a robust prediction parameter of critical zone structure and function. However, the models used to estimate EEMT have been solely based on long-term climatological data with little validation using direct empirical measures of energy, water, and carbon balances. Here we compare catchment-scale EEMT estimates generated using two distinct approaches: (1) EEMT modeled using the established methodology based on estimates of monthly effective precipitation and net primary production derived from climatological data, and (2) empirical catchment-scale EEMT estimated using data from 86 catchments of the Model Parameter Estimation Experiment (MOPEX) and MOD17A3 annual net primary production (NPP) product derived from Moderate Resolution Imaging Spectroradiometer (MODIS). Results indicated positive and significant linear correspondence (R2 = 0.75; P < 0.001) between model and empirical measures with an average root mean square error (RMSE) of 4.86 MJ m−2 yr−1. Modeled EEMT values were consistently greater than empirical measures of EEMT. Empirical catchment estimates of the energy associated with effective precipitation (EPPT) were calculated using a mass balance approach that accounts for water losses to quick surface runoff not accounted for in the climatologically modeled EPPT. Similarly, local controls on primary production such as solar radiation and nutrient limitation were not explicitly included in the climatologically based estimates of energy associated with primary production (EBIO), whereas these were captured in the remotely sensed MODIS NPP data. These differences likely explain the greater estimate of modeled EEMT relative to the empirical measures. There was significant positive correlation between catchment aridity and the fraction of EEMT partitioned into EBIO (FBIO), with an increase in FBIO as a fraction of the total as aridity increases and percentage of catchment woody plant cover decreases. In summary, the data indicated strong correspondence between model and empirical measures of EEMT with limited bias that agree well with other empirical measures of catchment energy and water partitioning and plant cover.

Highlights

A major challenge to the Earth sciences is understanding how energy, water, carbon, and sediment cycles interact to control process, function, and evolution of the critical zone, or the zone surface that extends from the top of the vegetative canopy down to and including groundwater (NRC, 2001)
Recent studies indicate strong correlation of critical zone properties to a flux term referred to as effective energy and mass transfer (EEMT). This term represents the energy and mass transferred to the critical zone in the form of water in excess of evapotranspiration and biological production; EEMT provides a measure of the energy available to perform work on the subsurface
Where EET is energy and mass flux associated with evapotranspiration, EPPT heat energy associated with effective precipitation energy and mass transfer, EBIO net primary productivity energy and mass transfer, EELEV potential energy associated with gravity-driven transport of sediment, EGEO geochemical potential of chemical weathering, and Ei any other external energy and mass input such as dust, anthropogenic inputs, or the heat exchange between soil and the atmosphere

Summary

Introduction

A major challenge to the Earth sciences is understanding how energy, water, carbon, and sediment cycles interact to control process, function, and evolution of the critical zone, or the zone surface that extends from the top of the vegetative canopy down to and including groundwater (NRC, 2001). Recent studies indicate strong correlation of critical zone properties to a flux term referred to as effective energy and mass transfer (EEMT). This term represents the energy and mass transferred to the critical zone in the form of water in excess of evapotranspiration and biological production; EEMT provides a measure of the energy available to perform work on the subsurface. Previous work demonstrates strong correlation of EEMT to measures of critical zone structure and function including regolith depth, chemical depletion and denudation rates, soil development and taxonomic classification, and ecosystem respiration (Pelletier and Rasmussen, 2009a; Rasmussen et al, 2005, 2011; Rasmussen and Tabor, 2007), which has been used as a predictive parameter in numerical models of Published by Copernicus Publications on behalf of the European Geosciences Union. To date, the application and derivation of EEMT has been purely driven with long-term average climate data, with no comparison of model estimates to empirical measures of EEMT to confirm model accuracy

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