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Summary: Studying surface–atmosphere feedback is often limited by the accuracy of the land surface observations (particularly soil moisture estimates) or the performance of the land surface models. To further our understanding of soil moisture effects on land–atmosphere fluxes, improvements in soil moisture mapping over large regions are necessary. The aim of this study was to obtain accurate soil moisture mapping over a 120 by 100km2 area in West Africa using the considerable amount of measurements available from local and regional scales, recorded during the African monsoon multidisciplinary analysis (AMMA) experiment. The modelling strategy was based on the use of a land surface model (LSM), employed to provide high-resolution soil moisture mapping over the studied area. A microwave emission model was then used to simulate associated microwave brightness temperatures (TB) to compare with the Advanced microwave scanning radiometer (AMSR) at the same spatial (25 by 20km2) and temporal resolution (daily). Discrepancies between observed and simulated TB were analysed and used to calibrate the LSM and the microwave emission models to match the specific hydrology and soil microwave behaviour of the studied area. Nevertheless, a positive bias of the near-surface soil moisture remained and the land surface model was still unable to reproduce the rapid dynamic of the near-surface soil moisture observed at the local and regional scales in this climatic context. To solve this problem, a secondary surface soil layer was added to match in situ soil moisture measurements as well as satellite microwave measurements. Additionally, the choice of the soil permittivity model was found to be of prior importance in order to perform suitable microwave brightness temperatures. Finally, a soil moisture retrieval algorithm based on AMSR and meteosat second generation (MSG) measurements was proposed in order to improve the quality of the soil moisture estimates over the studied area (the root mean square error decreases from 5.4 % vol. to 2.8 % vol). [Copyright &y& Elsevier]

Summary: This paper presents the ground soil moisture measurements performed over the so-called Gourma meso-scale site in Mali, Sahel, in the context of the African Monsoon Multidisciplinary Analysis (AMMA) project. The Gourma meso-scale soil moisture network is part of a complete land surface processes observing and modelling strategy and is associated to vegetation and meteorological field measurements as well as soil moisture remote sensing. It is spanning in latitude between N and N. In 2007, it includes 10 soil moisture stations, of which three stations also have meteorological and flux measurements. A relevant spatial sampling strategy is proposed to characterise soil moisture at different scales including local, kilometer, super-site and meso-scales. In addition to the local stations network, transect measurements were performed on different coarse textured (sand to sandy-loam) sites, using portable impedance probes. They indicate mean value and standard deviation (STD) of the surface soil moisture (SSM) at the kilometer scale. This paper presents the data set and illustrates soil moisture spatial and temporal features over the Sahelian Gourma meso-scale site for 2005–2006. Up-scaling relation of SSM is investigated from (i) local to kilometer scale and (ii) from local to the super site scale. It is shown to be stable in space and time (2005–2006) for different coarse textured sites. For the Agoufou local site, the up-scaling relation captures SSM dynamics at the kilometer scale with a 0.9% accuracy in volumetric soil moisture. At the multi-site scale, an unique up-scaling relation is shown to be able to represent kilometer SSM for the coarse textured soils of the meso-scale site with an accuracy of 2.2% (volumetric). Spatial stability of the ground soil moisture stations network is also addressed by the Mean Relative Difference (MRD) approach for the Agoufou super site where five soil moisture stations are available (about ). This allows the identification of the most representative ground soil moisture station which is shown to be an accurate indicator with low variance and bias of the soil moisture dynamics at the scale of the super site. Intensive local measurements, together with a robust up-scaling relation make the Gourma soil moisture network suitable for a large range of applications including remote sensing and land surface modelling at different spatial scales. [Copyright &y& Elsevier]

Summary: The West African monsoon interacts strongly with the land surface, yet knowledge of these interactions is severely limited by the lack of observations of surface energy fluxes. Within the framework of the AMMA project, three eddy covariance flux stations were installed to sample the three main surface types near Hombori (Mali) in the central Sahel at 15.3°N, and a fourth station was installed near Bamba in the northern Sahel at 17.1°N to sample semi-desert conditions. Observed land types near Hombori comprised a grassland growing on sandy soil (near the village of Agoufou), a flooded forest in a clay-soil depression (Kelma), and a bare rocky soil (Eguerit). The energy balance closure at the grassland site was satisfactory, but less so at the flooded forest site. Surface water heat storage during the flood and advection probably were responsible for most of the imbalance. The daily sensible heat flux (H) was fairly constant throughout the year at Bamba and Eguerit, with only a slight increase during the monsoon season corresponding to increased net radiation. By contrast, the seasonal cycle of the grassland site was marked, with H decreasing during the monsoon season from 70Wm−2 in May to 20Wm−2 in August. The flooded woodland exhibited the strongest contrast between the dry and wet seasons, with daily sensible heat flux close to zero during the flood. During the peak monsoon season, the two vegetated sites had the highest net radiation and the lowest sensible heat flux, as a consequence of the strong evapotranspiration rates caused by both high soil moisture availability and high leaf area index. Lateral fluxes of water were found to be strong drivers of inter-site sensible and latent heat fluxes variability, with water leaving bare rocky soils as surface runoff and ending in the clay depressions (e.g., Kelma), whereas the sandy soils were locally endorheic, with most of the rainfall being rapidly returned to the atmosphere. An attempt was made to scale the sensible heat flux up to the scale of the AMMA northern super-site (60km×60km), following a simple scaling scheme, which accounted for the contrasting surface types and water regimes. The super-site average sensible heat flux proved to be close to the grassland sensible heat flux, in part because grassland occupies 55% of the area. A strong spatial variability was caused by the difference in water regime and vegetation type, at a scale large enough to potentially influence the atmospheric properties such as the boundary layer. [Copyright &y& Elsevier]

Summary: The Gourma site in Mali is one of the three instrumented meso-scale sites deployed in West-Africa as part of the African Monsoon Multi-disciplinary Analysis (AMMA) project. Located both in the Sahelian zone sensu stricto, and in the Saharo–Sahelian transition zone, the Gourma meso-scale window is the northernmost site of the AMMA-CATCH observatory reached by the West African Monsoon. The experimental strategy includes deployment of a variety of instruments, from local to meso-scale, dedicated to monitoring and documentation of the major variables characterizing the climate forcing, and the spatio-temporal variability of surface processes and state variables such as vegetation mass, leaf area index (LAI), soil moisture and surface fluxes. This paper describes the Gourma site, its associated instrumental network and the research activities that have been carried out since 1984. In the AMMA project, emphasis is put on the relations between climate, vegetation and surface fluxes. However, the Gourma site is also important for development and validation of satellite products, mainly due to the existence of large and relatively homogeneous surfaces. The social dimension of the water resource uses and governance is also briefly analyzed, relying on field enquiry and interviews. The climate of the Gourma region is semi-arid, daytime air temperatures are always high and annual rainfall amounts exhibit strong inter-annual and seasonal variations. Measurements sites organized along a north–south transect reveal sharp gradients in surface albedo, net radiation, vegetation production, and distribution of plant functional types. However, at any point along the gradient, surface energy budget, soil moisture and vegetation growth contrast between two main types of soil surfaces and hydrologic systems. On the one hand, sandy soils with high water infiltration rates and limited run-off support almost continuous herbaceous vegetation with scattered woody plants. On the other hand, water infiltration is poor on shallow soils, and vegetation is sparse and discontinuous, with more concentrated run-off that ends in pools or low lands within structured endorheic watersheds. Land surface in the Gourma is characterized by rapid response to climate variability, strong intra-seasonal, seasonal and inter-annual variations in vegetation growth, soil moisture and energy balance. Despite the multi-decadal drought, which still persists, ponds and lakes have increased, the grass cover has largely recovered, and there are signs of increased tree cover at least in the low lands. [Copyright &y& Elsevier]