Why Do Large-Scale Land Surface Models Produce a Low Ratio of Transpiration to Evapotranspiration?

Li Ling Chang, Ravindra Dwivedi, John F. Knowles, Yuan Hao Fang, Guo-Yue Niu, Jon Pelletier, Craig Rasmussen, Matej Durcik, Greg A Barron-Gafford, Thomas Meixner

Research output: Contribution to journalArticle

16 Scopus citations

Abstract

Most land surface models (LSMs) used in Earth System Models produce a lower ratio of transpiration (T) to evapotranspiration (ET) than field observations, degrading the credibility of Earth System Model-projected ecosystem responses and feedbacks to climate change. To interpret this model deficiency, we conducted a pair of model experiments using a three-dimensional, process-based ecohydrological model in a subhumid, mountainous catchment. One experiment (CTRL) describes lateral water flow, topographic shading, leaf dynamics, and water vapor diffusion in the soil, while the other (LSM like) does not explicitly describe these processes to mimic a conventional LSM using artificially flattened terrain. Averaged over the catchment, CTRL produced a higher T/ET ratio (72%) than LSM like (55%) and agreed better with an independent estimate (79.79 ± 27%) based on rainfall and stream water isotopes. To discern the exact causes, we conducted additional model experiments, each reverting only one process described in CTRL to that of LSM like. These experiments revealed that the enhanced T/ET ratio was mostly caused by lateral water flow and water vapor diffusion within the soil. In particular, terrain-driven lateral water flows spread out soil moisture to a wider range along hillslopes with an optimum subrange from the middle to upper slopes, where evaporation (E) was more suppressed by the drier surface than T due to plant uptake of deep soil water, thereby enhancing T/ET. A more elaborate representation of water vapor diffusion from a dynamically changing evaporating surface to the height of the surface roughness length reduced E and increased the T/ET ratio.

Original languageEnglish (US)
JournalJournal of Geophysical Research: Atmospheres
DOIs
StateAccepted/In press - Jan 1 2018

Keywords

  • complex mountain terrain
  • ET partitioning
  • land surface models (LSMs)
  • lateral surface and subsurface flows
  • soil surface evaporation
  • three-dimensional process-based ecohydrological model

ASJC Scopus subject areas

  • Geophysics
  • Forestry
  • Oceanography
  • Aquatic Science
  • Ecology
  • Water Science and Technology
  • Soil Science
  • Geochemistry and Petrology
  • Earth-Surface Processes
  • Atmospheric Science
  • Earth and Planetary Sciences (miscellaneous)
  • Space and Planetary Science
  • Palaeontology

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