Spatial and seasonal patterns of diurnal differences in ERS Scatterometer soil moisture data in the Volta Basin, West Africa

Soil moisture is the key variable in the hydrological cycle. In the Volta Basin, West Africa, where rainfed agriculture forms the main source of income for the majority of the population; productivity relies on available soil moisture or “green water”. Progress will depend on good management of green water, and will be strongly based on monitoring results. Data scarcity in the Volta Basin emphasizes the necessity for remotely sensed soil moisture estimates that allow for more stable monitoring techniques. New soil moisture satellites such as SMOS and MetOp provide improved technical means for soil moisture monitoring. In preparation for these new sensors, historical ERS Scatterometer data over the Volta Basin provided by the Global Soil Moisture Archive have been analysed. The basin area is subject to a natural 1000 km long moisture gradient from the southern tropical rain forest to the northern Sahelian grass savanna. The soil moisture fields generated from ERS Scatterometer data reflect the spatial and temporal climatic patterns well. Our study investigated a weak but consistent anomaly between the backscatter measurements acquired during morning and evening overpasses. Maps generated from the difference between morning and evening overpass data reveal spatial and seasonal patterns that differ from the moisture gradient driven by the local climatology. The observed diurnal differences are in some regions, in particular in the central and forested savanna areas of the Volta basin, in the order of 1 dB, or even somewhat higher. In addition the detected patterns shift temporally in accordance with the transition from wet to dry seasons. Regional and seasonal deviations from the natural moisture gradient are identified and possible explanations examined. It appears that water stress causes diurnal changes in forest canopy water content resulting in somewhat lower backscatter during evening than during the morning acquisitions, but other explanations such as azimuthal effects cannot yet be excluded.