Combining scintillometer measurements and an aggregation scheme to estimate area-averaged latent heat flux during the AMMA experiment

Abstract

s u m m a r y This paper deals with the issue of using scintillometry in conjunction with a simple aggregation scheme to derive area-averaged sensible and latent heat fluxes over a small watershed in Niamey, Niger (Wank-ama catchment). Data collected in the context of the African Monsoon Multidisciplinary Analysis (AMMA) program has been used to test the proposed approach. For this purpose, a Large Aperture Scintillometer (LAS) was set up over heterogeneous surface transect of about 3.2 km spanning three vegetation types. The comparison between scintillometer-based estimates of area-averaged sensible heat fluxes and those measured by a network of the classical eddy covariance (EC) devices showed good agreement, with a relative error of about 20% (R 2 = 0.85, RMSD = 22 W m A2 , and SEE = 21.39 W m A2). This is a good result considering the contrast in terms of the footprint scales associated with the two devices, which is amplified by the heterogeneity of the catchment. The results also showed that using the LAS-derived values of area-average sensible heat flux in conjunction with a simple aggregation rule to estimate area-average available energy led to a reasonable prediction of area-averaged latent heat flux (R 2 = 0.75, RMSD = 64 W m A2 and SEE = 50 W m A2), when compared to those measured using the EC network. This is of interest since the LAS can potentially be used to validate estimates of surface fluxes, based on the meso-scale model, as well as those estimated using coarse-scale remote sensing-based algorithm. O 2009 Published by Elsevier B.V. Introduction The West African region has been undergoing tremendous changes over the last 30 years. The Sahel is prone to extremes in climate variability, with persistent drought which causes recurrent famine and humanitarian crises. It is thus of crucial importance to develop a clear understanding and accurate methods for predicting the combined effect of both human-and nature-induced changes on water resources so that decision makers can develop mitigating strategies based on solid science. In this context, several large-scale field experiments such as the Hydrologic and Atmospheric Pilot Experiment in the Sahel (Goutorbe et al., 1994) and the African Monsoon Multidisciplinary Analysis (AMMA: http://www.amma-international.org) were conducted over this region in recent years. These studies have substantially advanced the understanding of the interaction between regional climate and the hydrological cycle. Among the surface processes of prime importance in addressing the issue of the interaction of surface conditions and climate processes is the evapotranspiration or latent heat flux density (LE), which is essential in bringing together analyses of the physical, biological, and hydrological processes occurring at different space-time scales over continental surfaces. This same process describes the transport of water into the atmosphere from surfaces, through soil evaporation and plant transpiration. Additionally, understanding the spatial distribution of LE is essential for many environmental applications including water resources management, agriculture efficiency, global vegetation analysis, climate dynamics, and ecological applications. Consequently , the AMMA program put a strong emphasis on the challenging task of modeling and measurement of LE at different ranges of the space-time scale. The surface heterogeneity caused by the contrast in vegetation cover and type-and/or by precipitation generates a large spatial variability of surface fluxes. Therefore , estimating LE under such conditions is not an easy task.