Assessing the water balance in the Sahel: Impact of small scale rainfall variability on runoff: Part 1: Rainfall variability analysis

Abstract : The mesoscale variability of the Sahelian rainfall is analysed from a series of 30 high time resolution rainfall series covering 13 years and a 110 × 160 km2 area in the region of Niamey. It is shown that the convective scale variability is strongly influencing the spatial pattern of rainfields at larger time and spatial scales. This means that a proper assessment of the rainfall patterns at the mesoscale in the Sahel requires raingauge networks with a sufficient density to allow sampling this convective scale variability. This is usually not the case with operational networks whose density is in the order of 1–2 gauges per 10,000 km2. Computations carried out here show that the areal rainfall estimation error increases from 3% to 16% at the annual scale and from 21% to 113% at the event scale when the number of stations over a 100 × 100 km2 area decreases from 12 to 1. While being highly variable in space, the Sahelian rainfall is also highly intermittent in time. An analysis of the series of tipping bucket times leads to compute that 50% of the annual rain falls in less than 4 h with intensities larger than 35 mm/h. Areal rainfall statistics are compared to point rainfall statistics for event accumulated rainfall. The implication of these results for studying the influence of rainfall intermittency on runoff is discussed in a final section, as an introduction to a companion paper.