[Global Meteorology 2012; 1:e1] [page 1] Coupling soil-vegetation- atmosphere-transfer model with energy balance model for estimating energy and water vapor fluxes over an olive grove in a semi-arid region Jamal Ezzahar, 1,2 Salah Er-Raki, 3 Hamid Marah, 1 Said Khabba, 2 Noureddine Amenzou, 1 Ghani Chehbouni 4 1 Water and Climate Unity, CNESTEN, Rabat, Morocco; 2 Department of Physics, Faculty of Sciences Semlalia, Marrakech, Morocco; 3 LP2M2E, Équipe d’Instrumentation, Métrologie et Procédés (IMP), Department of Physics, Faculty of Sciences and Technology, Marrakech, Morocco; 4 CESBIO – UMR CNRS-CNES-IRD-UPS, Toulouse, France Abstract Simple soil-vegetation-transfer (SVAT) and energy balance models were used to estimate the surface turbulent fluxes (i.e. sensible and latent heat fluxes) over a complex olive grove using thermal infra-red surface temperature (TIRST). This approach used a dual source SVAT model to calculate the sensible heat flux- es from radiometric surface temperature. These fluxes were then used together with the estimates of the available energy also derived from TIRST to estimate the latent heat flux by applying the first law of thermodynamics i.e. the conservation of energy principle. The data used to validate this approach were collected over an irrigated olive grove site located in central Morocco near Marrakech. Mass and energy fluxes, as well as micrometeorological parameters, were continuously measured dur- ing the year 2003. The comparison between estimated and measured daily sensible heat fluxes yielded an acceptable agreement in spite of the complexity of the study surface with a correlation coefficient (R²=0.86) and root mean square error (RMSE) of 28 Wm –2 . For the latent heat fluxes, the statistical result for the comparison between estimated and measured daily values showed a larger scatter than that revealed for the sensible heat fluxes (R²=0.75; RMSE=31.42 Wm –2 ). However, the correspondence is to be considered acceptable given the difficulty in estimating latent heat flux over such a complex field. Therefore, it can be concluded that, in spite of the simplici- ty of the proposed approach, it can be consid- ered a suitable tool for estimating the turbu- lent fluxes using TIRST over complex surfaces. Introduction An accurate determination of regional tur- bulent fluxes of sensible heat (H) and latent heat (LE) at the land-atmosphere interface are required in a wide variety of applications. Agricultural and forestry research uses these convective fluxes to predict crop growth. Meteorologists need accurate estimates of latent and sensible heat surface fluxes for weather predictions. Biologists aim to have a better understanding of how ecosystems func- tion. Climatologists need estimates of how the land surface and its vegetation may influence climate, and hydrologists use estimates for water balance simulations on different scales. With the high cost of measuring turbulent fluxes, a strong emphasis has been directed toward understanding the processes governing the exchange of water and energy between the land surface and the atmosphere in order to model turbulent fluxes at the different ranges of the space-time scale. The model parameter- ization of the interaction between a land sur- face and the atmosphere is known as a soil- vegetation-atmosphere transfer scheme (SVAT). Numerous SVAT schemes of varying complexity have been developed in recent years. These SVAT schemes have been used in conjunction with thermal infra-red data (i.e. radiometric surface temperature) to estimate accurate turbulent fluxes. 1-7 Most of these investigations consist of estimating sensible heat flux, net radiation and soil heat flux from thermal infra-red data and calculating latent heat flux as a residual term of the energy bal- ance equation by applying the simplified expression of the first law of thermodynamics. The latter is referred to as the conservation of energy principle, meaning that energy can nei- ther be created nor destroyed, but rather trans- formed into various forms. Estimating reliable values of sensible heat flux represents the most problematic aspect of this approach. In such model parameteriza- tions, the soil-vegetation system is treated as a single source of heat exchange with the over- lying atmosphere. These kinds of SVAT are classified as one-layer models. The heat flux is related to the difference between the radio- metric temperature and the air temperature at a reference height. This approximation is still acceptable over homogenous surfaces where the vegetation is very dense and short or over bare soil. However, in most cases, the land- scape is under partial vegetation canopy so that both soil and vegetation components con- tribute to the sensitive heat exchange. 8 Therefore, extension of the one-layer models to sparsely vegetated surfaces can produce sig- nificant errors in predicted heat fluxes. 9 In this context, much effort has been made to investi- gate the parameterization of heat transfer and to improve the accuracy in estimating heat fluxes over different land surface cover by developing models which explicitly treat the energy exchanges between the soil, vegetation and the overlying atmosphere. 9 Lhomme et al. have considered that the soil-vegetation sys- tem can be approximated with a two-layer model where the energy fluxes are partitioned between the soil and vegetation. They found that the estimated heat fluxes from thermal infra-red data over the millet crop using a two- layer model showed good results compared to the use of a one-layer model which over-esti- mates the sensible heat flux. In this specific study, we will investigate the applicability of the two-layer model developed by Lhomme et al. 9 over a more complicated sur- face. The complexities are related to the sparse- ness of the vegetation, the heterogeneity of the soil characteristics and, most importantly, the heterogeneity in terms of soil moisture induced by the irrigation method of flood irrigation, which has an irregular pattern in space and time. Additionally, the validation of the model will be performed over an extended period (measurements taken over approximately one year) in order to test its consistency for accu- rately estimating heat fluxes under all weather conditions. Data used in these investigations were collected within the framework of the SUDMED project, 10 which was carried out in southern Mediterranean regions (Marrakech, Morocco) to assess the spatio-temporal variabil- Global Meteorology 2012; volume 1:e1 Correspondence: Jamal Ezzahar, Water and Climate Unity, CNESTEN, Rabat, Morocco. E-mail: ezzahar@cnesten.org.ma Acknowledgments: this research was conducted within the framework of the SUDMED project, the EU funded IRRIMED (www.irrimed.org), and RAF 5058 projects. We are grateful to the International Joint Laboratory (TREMA) for its financial and technical support. Contributions: all authors agree to the contents and submission of the paper. Key words: sensible heat flux, latent heat flux, soil-vegetation-transfer model, thermal infra-red data, available energy, olive grove, semi-arid region. Received for publication: 15 June 2011. Revision received: 23 March 2012. Accepted for publication: 28 March 2012. 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