ANALYSIS OF EARTH OBSERVATION TIME SERIES TO INVESTIGATE THE RELATION BETWEEN RAINFALL, VEGETATION DYNAMIC AND STREAMFLOW IN THE UELE’ BASIN (CENTRAL AFRICAN REPUBLIC) Daniela Stroppiana 1 , Mirco Boschetti 1 , Pietro A. Brivio 1 , Francesco Nutini 1,2 and Etienne Bartholomé 3 1 CNR-IREA, Institute of Electromagnetic Sensing of Environment, Via Bassini 15, Milano, Italy 2 DI.PRO.VE., Department of Crop Science, University of Milano, Via Celoria 2, Milano, Italy 3 EC-JRC, Global Environmental Monitoring Unit, via E. Fermi 1, Ispra, Varese, Italy ABSTRACT The hydrology of tropical forests play a key role in watershed processes such as soil erosion, streamflow and ground water recharge. However, tropical forests of Africa are least investigated due to the poor network for data acquisition. Earth Observations can fill this gap by providing consistent time series of data. We analyzed trends of rainfall, vegetation index and river water levels derived from satellite data for the Uele sub-basin and we pointed out that rainfall and river water levels are positively correlated only during the dry season when vegetation activities is low. The unexpected low correlation during the season of highest precipitations is due to the role of vegetation, which is characterized by a significant seasonality also in evergreen tropical forests. These results underline the importance of modeling the role of canopy in the interception and evapotranspiration of the available precipitation in order to provide reliable information on stream flow dynamics. Index Terms— River hydrology, Earth Observations, RFE, NDWI, radar altimeter 1. INTRODUCTION The role of forests on the hydrology of tropical ecosystems has attracted the attention of researchers in the last decades for the impacts that watershed processes (soil erosion rates, peak and low-flow levels, ground water recharge rates, and water quality) can have on human lives [1]. The research debate highlighted substantial ambiguities and gaps especially in research on the relation between evopotranspiration (ET), infiltration and the ‘low flow problem’ [2] and Africa and south Asia have been pointed as the least investigated regions of the globe. The African Monitoring of the Environment for Sustainable Development (AMESD; http://www.amesd.org/; last access august 2010) initiative established between the European Commission and the African Union Commission, has the objective of providing information on natural resources through Earth Observations (EOs) in a continent where human lives greatly depend on the environment. AMESD together with the Natural Resource Monitoring in Africa (NARMA) of the Geoland2 project are interested in the Central African thematic area and, in particular, have focused their attention on the use of satellite data for proper management of water resources of the Congo River basin. Two operational systems are deemed of particular importance: a “Low water alert system”. This service is dedicated to the navigation on the Ubangi, a trans- boundary river shared by the Central African Republic, the Republic of Congo and the Democratic Republic of Congo. The service aims to facilitate the interior navigation by providing relevant information to boatmen to enable proper loading of boats to pass the navigation steps on this inter- States corridor. This early warning system should provide alarm of possible low water condition several weeks in advance. However, the system relies on the available knowledge and information on the complex biophysical model behind the relationship between forest and the hydrological cycle. In this framework we analyze the information which could be brought by EO systems to the understanding and the description of the relation between streamflow, rainfall and forest for the Uele sub-basin of the Ubangi river. EO systems can in fact provide timely and consistent datasets for hydrological monitoring in very large and remote areas such as the Congo River basin where other sources of quantitative information are not available. 2. MATERIALS AND METHODS 2.1. Study area The study area is focused on the Uele basin which is part of the Congo river basin, the second largest river basin of the World after the Amazonian one. The Uele (basin surface is about 139.700 km2) is the most important tributary of the Ubangi, also spelled Oubangui, which in turn is the largest right-bank tributary of the Congo river (Fig. 1). Ubangi river together with Uele covers more than 2400 km and they contribute significantly to the transport network for river boats. Indeed, interior navigation is crucial for those countries sharing the Congo river basin since other transport networks (i.e. road and aerial) are not so developed and reliable. However, in the last 30 years the traffic along the river network severely decreased partly due to the decrease MultiTemp 2011 978-1-4577-1203-6/11/$26.00 ©2011 IEEE 177