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J. Friesen, W. Hessel, R. Beck, K. Scipal, W. Wagner, N. Van De Giesen:
"Spatial and seasonal patterns of diurnal differences in ERS scatterometer soil moisture data in the Volta Basin, West Africa";
Talk: IUGG 2007, Perugia; 2007-07; in: "Proceedings of the Symposium HS3007 at IUGG2007", IAHS Publication 316, (2007), 47 - 55.

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.

remote sensing; soil moisture; West Africa