Long-term observations of turbulent fluxes over heterogeneous vegetation using scintillometry and additional observations: A contribution to AMMA under Sudano-Sahelian climate

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Abstract

Based on a 3-year period of infrared scintillometry, soil and meteorological measurements, this study presents an analysis of the surface energy balance partitioning over a heterogeneous savannah, in the Sudano-Sahelian region. The site is located in Northern Benin, meso-site of the African Monsoon Multidisciplinary Analyses (AMMA) project. The 3-year period enables an analysis of several alternate dry and wet periods, as well as the intermediate dry-to-wet and wet-to-dry periods.

Infrared scintillometry, coupled with measurements of the available energy (net radiation minus ground heat flux) and a careful analysis of the aerodynamic properties of the scintillometer footprint, are employed to provide robust estimates of the turbulent (sensible and latent heat) fluxes over complex terrain, in terms of the topography and in terms of the spatially and temporally heterogeneous vegetation cover. A characterization of the uncertainties on each term of the energy balance is given at the scale of the scintillometer footprint. These uncertainties strongly depend on the season for the residual latent heat flux.

Results point out that the climate of the Sudano-Sahelian region is characterized by a strong seasonal cycle and inter-annual variability, related to changing atmospheric and land surface conditions. The evaporative fraction is found to be relatively constant during the wet period (0.67) and more variable during the dry and intermediate periods. In addition, sensible heat flux and net radiation are well correlated during the dry season. The diurnal cycle shows a predominance of evaporation during the wet season and sensible heat during the dry season. Results point a significant latent heat flux during the dry period, signature of persistent vegetation in the Sudano-Sahelian region. Finally, that data set at hourly time step would provide useful information for modelling and the parameterization of the associated processes for this region.

Highlights

► Infrared scintillometry with soil measurements are suitable tools for long-term actual evapotranspiration observations. ► Evapotranspiration is controlled by the net radiation in the wet season, and by the water availability in the dry season. ► Observations show a persistent latent heat flux during the dry season. ► This dataset is highly valuable for local catchment hydrology issues and land surface processes parameterization.

Introduction

West Africa is known to be a vulnerable area exposed to climate changes where large uncertainties remain, concerning rain amount tendencies and induced water resources. In particular, the impact of climate changes on the African surfaces and its feedback through their energy balance are still poorly understood (Boko et al., 2007), despite the fact that spatial and temporal energy partitioning variability is though to play a major role in the whole water and energy cycle of the West African monsoon (Charney et al., 1975, Zheng and Eltahir, 1997, Wang and Eltahir, 2000). From a hydrological point of view, Sahelian region had suffered, in the past decades, from dramatic/severe droughts, which had considerably changed land use and surface energy partitioning over a wide area. West African studies have then primarily focused on Sahelian surfaces, more exposed to climate variability (Wallace et al., 1991, Verhoef et al., 1996, Gash et al., 1997, Kabat et al., 1997, Lloyd et al., 1997). Kabat et al. (1997) and Gash et al. (1997) have speculated on the possible role of the vegetation gradient in the control of the monsoon. In this context, they asked the question of the energy partitioning depending on the vegetation types. This has been explored in the recent AMMA experiment (African Monsoon Multidisciplinary Analyses) in 2006 and documented by Ramier et al. (2009), Timouk et al. (2009) and Ezzahar et al. (2009).

The Sudano-Sahelian region, a large East-Western band (approximately between 7°N and 12°N), with an intermediate climate between the dry Sahel and the Guinean Coast, is also an important zone in the West African climatic context. In particular, the role of this South-North West African vegetation gradient on Monsoon cycle is not fully understood. In the Sudano-Sahelian region, annual rainfall amounts range from 800 mm to 1200 mm, which also confer to this region an important role of agricultural production. The uncertainty in rainfall tendency and the demographic growth caused by Sahelian migration is then susceptible to increase anthropic pressure on land use and water resources. The need for long term combined hydrological and earth surface studies becomes a priority to prepare water management policy.

Sudano-Sahelian region is composed of a wide range of vegetation cover, a mixture of cultivated areas and savannah. In this region, energy fluxes have been recently documented in Ivory Coast (Touré, 2004), in the Upper Volta (Schuettemeyer et al., 2006) and in Burkina-Faso (Bagayoko et al., 2007). More recently, in Burkina Faso, close to the Sahelian landscapes, a whole energy-biogeochemical study of a savannah has been undertaken by Bruemmer et al. (2008) and Groete et al. (2009). Bruemmer et al. (2008) published the first set of pluri-annual data for a Sudano-Sahelian savannah landscape undisturbed by human presence. These studies emphasize the need for more long-term observations of the energy balance partitioning in the West African context where temporal variability of energy fluxes is high. These savannah areas are deforested little by little and changed to crop fields. This adds another difficulty to study Sudano-Sahelian areas because of the patchiness of the land cover. This spatial heterogeneity makes the comparison between local data and satellite estimations or Soil Vegetation Atmosphere (SVAT) models outputs very questionable. Although the need of energy partitioning observations is required, Timouk et al. (2009) proposed an aggregation scheme from punctual data to document the energy partitioning over patchy areas, which have been used for SVAT validation (Boone et al., 2009).

Several authors have shown the relevance of the use of infrared scintillometer measurements to estimate sensible heat flux over patchy areas, and latent heat flux when combined with an estimation of the other terms of the energy budget (Meijninger et al., 2002b, Schuettemeyer et al., 2006, Hoedjes et al., 2007, Ezzahar et al., 2009). Most of these authors estimated the net radiation as a local measurement, which does not represent the whole surface where the energy budget should be done. Furthermore, the ground heat flux is often estimated as a proportion of the net radiation.

Recently, Guyot et al. (2009) proposed a similar methodology where they emphasized the necessity of using aggregated aerodynamical parameters, net radiation and ground heat flux at the scintillometer footprint scale. These estimations lead to a comprehensive energy budget, and therefore a robust estimation of the latent heat flux (LE) as a result of the energy balance over a composite Sudano-Sahelian landscape. However, Guyot et al. (2009) studied only a short period of three months of the dry season, period with few atmospheric and surface conditions variability. The study of multi-annual energy partitioning cycles requires taking into account the evolution of vegetation, in terms of surface reflectivity and surface aerodynamic properties.

In the present study, we propose to extent Guyot et al. (2009) methodology to take into account the temporal variability and spatial heterogeneity of the surface. It is applied to a Sudano-Sahelian landscape, part of the AMMA-CATCH network. The site, the experimental setup and the climatological context are presented in Section 2. The methodology includes temporal variability for albedo, roughness length and displacement height, which are known to be the most sensible parameters to derive sensible heat fluxes from scintillometric measurements. A footprint temporal variability is also included in the sensible heat flux calculation. This is detailed in Section 3. Then, Section 4 presents three years of measurements, encountering a strong seasonal and inter-annual variability of both atmospheric and surface conditions. Finally, a first daily, seasonal and annual analysis is proposed in Section 5 in the light of composite daily cycles and annual cycle of evaporative fraction. The major conclusions are reviewed in Section 6.

Section snippets

Experimental setup

The 12 km2 Ara watershed (Fig. 1) and its experimental setup have been presented extensively in Guyot et al. (2009).

The base map in Fig. 2 presents the vegetation distribution in the Ara catchment. This classification is derived from SPOT 4-HRVIR scenes acquired during 2005 and 2006 (I. Zin, personal communication). Vegetation has been classified as: bare soil/fallows/crops (32%), shrub savannah (61%) and woody savannah (7%). This distribution is similar to the distribution observed at the Donga

Turbulent heat fluxes with scintillometry

Guyot et al. (2009) presented an extensive study to derive sensible heat flux and latent heat flux from scintillometer measurements. One can refer to this paper to have a complete description of the methodology.

Because the underlying cover is changing under the scintillometer path, we extend the methodology from Guyot et al. (2009) in order to take into account the change of effective instrument height along the year. Latent heat flux is estimated through the energy budget made at the scale of

Results

The dataset presented in this study is of high interest for land-surface processes parameterization used in meteorological models, thus a focus is made on the uncertainties on each term of the energy balance. Each term is presented separately and discussed, and finally, the residual latent heat flux is presented as itself and as an evaporative fraction.

Interactions between components

In this section, we look at the energy budget partitioning during an annual cycle, as defined by the VPD criteria. The daily composite fluxes of the surface energy balance are plotted in Fig. 12 for the three periods (dry, intermediate and wet).

In order to characterize the co-evolution of <H> and <Rn> for the conditions identified by the VPD, statistical criteria are calculated on a circadian cycle (statistical criteria applied to 24 values for one day). Table 4 gives the average statistical

Conclusion

This paper presents an improved methodology for long-term observations using scintillometer in an energy budget. Time and spatial variations of land cover are taken into account in the estimation of the aggregated resultant latent heat flux. This methodology provides an estimation of turbulent (sensible and latent heat) fluxes over complex terrain (complex both in terms of the topography and in terms of the spatially and temporally heterogeneous vegetation cover). This study shows the

Acknowledgements

The authors wish to thank Moussa Doukouré for his help for the fieldwork, and Simon Allonganvinon for his close survey of the scintillometer and data collection. The authors also wish to thanks Isabella Zin for providing land cover maps, and Joris Pianezze for his collaboration as part as an internship at LTHE. The authors are particularly grateful to two anonymous reviewers for their critical examination of the manuscript and helpful suggestions. This work has been performed within the AMMA

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