|Wilson, T -|
|Meyers, T -|
|Kochendorfer, John -|
|Heuer, M -|
Submitted to: Agricultural and Forest Meteorology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: March 19, 2012
Publication Date: N/A
Interpretive Summary: Understanding the water use (evapotranspiration) and carbon uptake by forest systems is critical to making accurate assessment of the regional, continental, and global water and carbon cycles. Often neglected in forest models is the impact of residue cover (forest litter) on the overall energy balance. Forest residue serves as both a insulator – inhibiting transmission of heat into the soil surface, and as a sponge, altering infiltration of precipitation into the soil. The Atmosphere-Land Exchange (ALEX) model simulating the soil-plant-atmosphere carbon, water and energy exchanges was modified to include a surface residue component. Inclusion of the residue sub-model significantly improved model estimates of total water use and carbon uptake at a forested site in Oak Ridge, Tennessee. This sub-model will be incorporated in future applications of the ALEX model over forested sites, and may be of benefit in natural rangeland systems as well. More accurate estimates of water use and carbon sequestration of forest and rangeland ecosystems will improve soil and water conservation strategies being developed by the Forest Service, NRCS and other state and federal agencies concerned with environmental conservation.
Technical Abstract: The Atmosphere-Land Exchange Surface Energy (ALEX) balance model is an analytical formulation of the energy and mass transport within the soil and the vegetation canopy used for simulating energy, evapotranspiration, and CO2 fluxes in a wide range of vegetation environments. The objective of this study was to evaluate the ability of ALEX to simulate the effect of soil-surface leaf litter residue on soil heat conduction (G), sensible heat (H), evapotranspiration (ET) (or latent heat (LE) when expressed as rate of energy loss) and CO2 fluxes in a deciduous forest. The model was evaluated in a deciduous forest in Oak Ridge, Tennessee where about 550 g m-2 of dry weight of slow decomposing leaf litter is produced annually during the fall season. Incorporating an explicit formulation of water and energy exchanges within the residue layer in ALEX improved the performance of the model against eddy covariance and G measurements. The discrepancies between model simulations made with and without leaf litter residue were largest during the spring and fall, when soil contributions dominated the energy budget of the forest. During these periods, particularly during the spring, without the inclusion of the residue layer the model over-predicted LE, G, soil temperature and soil moisture, and under-predicted H. The model showed no differences in simulating above-canopy net radiation (RN), with a slight difference in the above-canopy CO2 flux. The largest model improvement for residue effects was in the simulation of G, with the slope of the regression line between predicted and measured values reduced from 2.28 for the model without residue effects to 1.07 when the residue effect was considered.