Multi-scale soil moisture measurements at the Gourma meso-scale site in Mali

This paper presents the ground soil moisture measurements performed over the socalled 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 2◦ in latitude between 15◦N and 17◦N. In 2007, it includes 10 soil moisture stations, of which 3 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 5 soil moisture stations are available (about 25km × 25km). 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 upscaling 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.