PhD defence: Tanja Denager

Tanja Denager defends her thesis,

Analysis of water and energy balance components at field scale using data-driven and modelling approaches

Supervisors:
Professor Karsten Høgh Jensen, IGN
Associate Professor Majken C. L. Zibar, IGN
Senior Researcher Torben O. Sonnenborg, GEUS

Assessment committee:
Professor Harrie-Jan Hendricks-Franssen, Research Center Julich – Germany
Senior Researcher Ebba Dellwik, DTU Wind Energy – Denmark
Associate Professor Søren Jessen (chair), IGN

Summary:
As the land surface provides the link between the atmospheric climate and the hydrologic cycle, the land surface energy and water balances are closely related. The latent heat flux/evapotranspiration is a key variable in both equations. The eddy covariance method is one of the most established measurement techniques for estimating the flux of latent heat for evapotranspiration and the flux of sensible heat. However, the imbalance between the land surface energy input and output is a well-known fact, and therefore the eddy covariance method potentially underestimates actual evapotranspiration.
All weather and climate models inherently demand a complete closure of the land surface energy balance at each modelling time step. Thereby, numerical models, in contrast to eddy covariance observations, closes the land surface energy balance. Eddy covariance observations are essential for parameterizing and validating land surface models in climate simulations, and any systematic errors in the eddy covariance observations will propagate directly into the climate models.
This PhD project is a part of the Danish Hydrological Observatory, HOBE. Over the project period, HOBE established a unique high quality dataset of all major water balance terms (precipitation, evapotranspiration, groundwater recharge and soil moisture) and energy balance terms (net radiation components, latent heat flux, sensible heat flux and ground heat flux).
The main findings in this PhD study are that both a data-driven and a modelling approach demonstrate that the eddy covariance method provides accurate estimates of actual evapotranspiration, and that other processes, potentially the estimation of sensible heat, may contribute to the lack of energy conservation at land surface when using eddy covariance estimates.
Further, the results of this PhD underlines the necessity of re-parameterization of land surface models using available observations of energy and hydrologic variables to obtain an optimal parameter set. The results from this study has contributed to improvement of model characterization of water and energy fluxes, especially when eddy covariance flux station data is available.

A digital version of the PhD thesis can be obtained from the PhD secretary Mikala Heckscher at mikala@ign.ku.dk