The use of scintillometry for validating aggregation schemes over heterogeneous grids

Abstract : A number of studies have been devoted to derive the diurnal course of regional evapotranspiration (ET) especially in semi-arid areas where the assessment of this term is of crucial importance for water resources management. One approach to derive regional evapotranspiration is based on the use of aggregation schemes in conjunction with energy-balance or land-surface models. However, the effectiveness of this approach cannot be fully assessed without a comparison between the model's flux simulations and the ground truth observations. In the present study, the issue of using scintillometry for validating spatial and temporal aggregation schemes over heterogeneous grids has been investigated. Data collected within the SUDMED project over the oliveyard of Agdal which was located near the city of Marrakech, Morocco, have been used to test the aggregation schemes. The Agdal oliveyard was made up of two contrasted fields, or patches. Even though the two sites appear relatively homogeneous, they differ strongly in terms of soil moisture status and vegetation percent cover. The higher soil moisture in the northern site creates heterogeneity at the scale of the entire olive yard (i.e. at grid-scale). Firstly, the diurnal course of the grid-scale evapotranspiration (< ETsim >(SA)) estimated from spatial aggregation scheme is compared to that derived from scintillometry (< ETLAS >). < ETsim >(SA) is obtained as the residual term of the energy balance providing the estimates of the available energy (AE = R-n - G), where R-n and G are the net radiation and the soil heat flux, respectively, and sensible heat flux. The latter is estimated by using a simple two-layer model developed by Lhomme et al. (1994). The root mean square difference (RMSD) and the correlation coefficient (R-2) between < ETsim >(SA) and < ETLAS > were about 46 W m(-2) and 0.78, respectively. Secondly, we compared the diurnal course of grid-scale evapotranspiration (< ETsim >(TA)) estimated from the temporal aggregation scheme with the < ETLAS >. < ETsim >(TA) is obtained by extrapolating the instantaneous values of the available energy and the evaporative fraction (EF = ET/AE) estimated at the satellite overpass to daily ones. The instantaneous values of AE and EF have been derived using remotely sensed surface temperature measured using a ground-based infrared thermometer combined with ancillary micrometeorological data such as wind speed, incoming and outgoing solar radiation, and temperature and humidity of the air. The RMSD and the R 2 were about 43 W m(-2) and 0.7, respectively. Despite the complexity of the site induced by the strong heterogeneity in the soil moisture which is related to the employed irrigation method (flood irrigation), and the consequences in terms of the footprint of the instruments. the obtained statistical results showed that both aggregation schemes performed successfully with regard to estimates of the evapotranspiration over heterogeneous grids. Finally, to further assess the performance of the developed approach, a second dataset collected in northern Mexico has been also used. The result shows that the approach provides acceptable values of aggregated evapotranspiration. Consequently, scintillometry can potentially be used in the development and the validation of aggregation approaches to improve the representation of surface heterogeneity land-surface-atmosphere models operating at large scales.
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Agricultural and Forest Meteorology, Elsevier Masson, 2009, 149 (12), pp.2098-2109. 〈10.1016/j.agrformet.2009.09.004〉
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J. Ezzahar, A. Chehbouni. The use of scintillometry for validating aggregation schemes over heterogeneous grids. Agricultural and Forest Meteorology, Elsevier Masson, 2009, 149 (12), pp.2098-2109. 〈10.1016/j.agrformet.2009.09.004〉. 〈ird-00610456〉

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