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Title Understanding the Daily Cycle of Evapotranspiration: A Method to Quantify the Influence of Forcings and Feedbacks
Type Publication
Abstract:

A method to analyze the daily cycle of evapotranspiration over land is presented. It quantifies the influence of external forcings, such as radiation and advection, and of internal feedbacks induced by boundary layer, surface layer, and land surface processes on evapotranspiration. It consists of a budget equation for evapotranspiration that is derived by combining a time derivative of the Penman–Monteith equation with a mixed-layer model for the convective boundary layer. Measurements and model results for days at two contrasting locations are analyzed using the method: midlatitudes (Cabauw, Netherlands) and semiarid (Niamey, Niger). The analysis shows that the time evolution of evapotranspiration is a complex interplay of forcings and feedbacks. Although evapotranspiration is initiated by radiation, it is significantly regulated by the atmospheric boundary layer and the land surface throughout the day. In both cases boundary layer feedbacks enhance the evapotranspiration up to 20 W m−2 h−1. However, in the case of Niamey this is offset by the land surface feedbacks since the soil drying reaches −30 W m−2 h−1. Remarkably, surface layer feedbacks are of negligible importance in a fully coupled system. Analysis of the boundary layer feedbacks hints at the existence of two regimes in this feedback depending on atmospheric temperature, with a gradual transition region in between the two. In the low-temperature regime specific humidity variations induced by evapotranspiration and dry-air entrainment have a strong impact on the evapotranspiration. In the high-temperature regime the impact of humidity variations is less pronounced and the effects of boundary layer feedbacks are mostly determined by temperature variations.

Available from http://journals.ametsoc.org/doi/abs/10.1175/2010JHM1272.1
Author
VILA GUERAU DE ARELLANO Jordi
C. C. van Heerwaarden, A. Gounou, F. Guichard, F. Couvreux
Reference
Journal Journal of Hydrometeorology
Volume 11
Pages 1405-1422
Year 2010
Times cited 0
Institute country Netherlands
Type of science
  • Atmospheric dynamics (includes thermodynamics)
  • Soil-Atmosphere interactions
Field of science
  • Troposphere
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Added Sept. 29, 2010, 00:00
Last update May 3, 2016, 14:50
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File name vanheerwaarden_etal_2010pdf_20100929111634.pdf
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