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Hydrology and Earth System Sciences An interactive open-access journal of the European Geosciences Union
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Preprints
https://doi.org/10.5194/hess-2019-598
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/hess-2019-598
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

  04 Dec 2019

04 Dec 2019

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A revised version of this preprint is currently under review for the journal HESS.

Multi-variable, multi-configuration testing of ORCHIDEE land surface model water flux and storage estimates across semi-arid sites in the southwestern US

Natasha MacBean1, Russell L. Scott2, Joel A. Biederman2, Catherine Ottlé3, Nicolas Vuichard3, Agnès Ducharne4, Thomas Kolb5, Sabina Dore6, Marcy Litvak7, and David J. P. Moore8 Natasha MacBean et al.
  • 1Department of Geography, Indiana University, Bloomington, IN 47405, USA
  • 2Southwest Watershed Research Center, United States Agricultural Department, Agricultural Research Service, Tucson, AZ 85719, USA
  • 3Laboratoire des Sciences du Climat et de l'Environnement, LSCE/IPSL, CEA-CNRS-UVSQ, UniversitéParis-Saclay, Gif-sur-Yvette, F-91191, France
  • 4UMR METIS, Sorbonne Université, CNRS, EPHE, Paris, F-75005, France
  • 5School of Forestry, Northern Arizona University, Flagstaff, AZ, 86011, USA
  • 6Hydrofocus, Inc., Davis, CA, 95618, USA
  • 7Department of Biology, University of New Mexico, Albuquerque, NM, 87131, USA
  • 8School of Natural Resources and the Environment, University of Arizona, Tucson, AZ, 85721, USA

Abstract. Plant activity in semi-arid ecosystems is largely controlled by pulses of precipitation, making them particularly vulnerable to increased aridity expected with climate change. Simple bucket-model hydrology schemes in land surface models (LSMs) have had limited ability in accurately capturing semi-arid water stores and fluxes. Recent, more complex, LSM hydrology models have not been widely evaluated against semi-arid ecosystem in situ data. We hypothesize that the failure of older LSM versions to represent evapotranspiration, ET, in arid lands is because simple bucket models do not capture realistic fluctuations in upper layer soil moisture. We therefore predict that including a discretized soil hydrology scheme based on a mechanistic description of moisture diffusion will result in an improvement in model ET when compared to data because the temporal variability of upper layer soil moisture content better corresponds to that of precipitation inputs. To test this prediction, we compared ORCHIDEE LSM simulations from (1) a simple conceptual 2-layer bucket scheme with fixed hydrological parameters; and (2) a 11-layer discretized mechanistic scheme of moisture diffusion in unsaturated soil based on Richards equations against daily and monthly soil moisture and ET observations, together with data-derived transpiration / evaporation, T / ET, ratios, from six semi-arid grass, shrub and forest sites in the southwestern USA. The 11-layer scheme also has modified calculations of surface runoff, bare soil evaporation, and water limitation to be compatible with the more complex hydrology configuration. To diagnose remaining discrepancies in the 11-layer model, we tested two further configurations: (i) the addition of a term that captures bare soil evaporation resistance to dry soil; and (ii) reduced bare soil fraction. We found that the more mechanistic 11-layer model results better representation of the daily and monthly ET observations. We show that is likely because of improved simulation of soil moisture in the upper layers of soil (top 5 cm). Some discrepancies between observed and modelled soil moisture and ET may allow us to prioritize future model development. Adding a soil resistance term generally decreased simulated E and increased soil moisture content, thus increasing T and T / ET ratios and reducing the negative T / ET model-data bias. By reducing the bare soil fraction in the model, we illustrated that modelled leaf T is too low at sparsely vegetated sites. We conclude that a discretized soil hydrology scheme and associated developments improves estimates of ET by allowing the model to more closely match the pulse precipitation dynamics of these semi-arid ecosystems; however, the partitioning of T from bare soil evaporation is not solved by this modification alone.

Natasha MacBean et al.

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Natasha MacBean et al.

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