The drought in the African Sahel, persisting since 1968, emphasizes the vulnerability of local water resources to climatic variations and land use changes in this semiarid environment. Following the Hapex–Sahel experiment (1992–1994), research aims at understanding and modeling the impact on the hydrological cycle of these changing climatic and environmental conditions over a 1500-km2 area east of Niamey, Niger. The hydrological landscape consists of a myriad of small endoreic catchments feeding temporary or permanent pools from which water percolates to an unconfined aquifer. As the first step in a local-to-regional scale approach to the water resource recharge in the area, a physically based rainfall–runoff model is developed for a typical, 1.9-km2 endoreic system named Wankama, which is monitored continuously since 1992. This first of two companion papers describes the key elements of the hydrological landscape, the experimental setup with which the Wankama catchment and pool are monitored, the r.water.fea distributed hydrological model and its application to the construction of the Wankama catchment model. Because of the difficulties of long-term field experimentation in the region, not all observed events can be considered equally well recorded; in particular, detection of exogenous water inflow occurrences that may alter Wankama runoff estimates is needed. The paper describes the careful, model-aided data analysis performed in order to select the observation set with which the model can most reliably be tuned and operated. In this rainfall–runoff data analysis phase, the model is used prior to any calibration. Model calibration and verification, including output uncertainty, are the subject of the second paper [J. Hydrol. (2003)]. All steps of analysis and modeling were performed on the 1992–1998 data only, before the 1999–2000 data became available, and are tested a posteriori against the latter.