Thierry Lebel

Rainfall monitoring during HAPEX-Sahel. 1. General rainfall conditions and climatology

The HAPEX-Sahel experiment took place in the midst of the most severe drought that has ever plagued the region since rainfall records have been available in the Sahel. The aim of this paper is to describe the rainfall conditions that were observed during HAPEX-Sahel by a network of 100 recording raingauges, and to analyse them in the perspective of the long-term statistics at the Niamey station. Globally it is found that the average rainfall over the HAPEX-Sahel study area, as well as over Niger as a whole, during the experiment has been moderately (1991 and 1992) to markedly dry (1990 and 1993), indicating that the drought of the past 20 years has not ended. More detailed statistics point to the high intermittency of the Sahelian rainfall, both in space (each year the ratio between the maximum and minimum recorded seasonal rainfall was of the order of 2) and in time (half the annual rain falls in 5 h). Particular attention is paid to the climatology of the rainy events. The point event rainfall is nearly exponentially distributed with a mean of about 14 mm, while the average areal event rainfall over the HAPEX-Sahel 1° × 1° square is of the order of 10.5 mm. The probability of zero rainfall is thus close to 14. Rain rates are often heavy, with half the annual rain falling at rain rates higher than 35 mm h−1 and one third of it falling at rain rates higher than 50 mm h−1. The year to year statistics of both the event rainfall and the rain rates vary little, indicating that most of the inter-annual rainfall variability is due to the variation of the number of rainfall events, rather than to variations in rainfall intensities or in the mean event rainfall.

Hydrology of the HAPEX-Sahel Central Super-Site: surface water drainage and aquifer recharge through the pool systems

Abstract The hydrology of the Sahel is characterized by the degradation of the drainage network, resulting in the lack of large watersheds over which the spatial integration of the hydrological processes could be studied. The main hydrological units are small endoreic areas, measuring a few hectares to a few square kilometres and the surface runoff is collected into pools. A detailed investigation of the role of these pools in the hydrology of the HAPEX-Sahel Central Super-Site was carried out from 1991 to 1993. The first results of this investigation are presented. A typology in three classes of the endoreic systems (valley bottoms; sinks; plateaux) is proposed. The behaviour of one representative pool in each class is analysed, showing that the partition between evaporation and deep infiltration depends on the level of filling of the pools. The bottom of the pool is clogged by clay deposits, which prevent infiltration. Above a threshold varying between 1 and 2 m most of the water stored in the pool after runoff infiltrates, contributing to the recharge of the aquifers. On a seasonal basis, deep infiltration accounts for less than 50% of the water collected by the plateau pool, and more than 80% for the valley bottom pools. Almost all the water running off to the sink pools infiltrates rapidly and deeply into the ground. The valley pools (both valley bottoms and sinks) appear to be the major contributors to the recharge of the upper aquifer. The proportion of the HAPEX-Sahel Central Super-Site water balance that is taken by the deep infiltration from the pools varies greatly depending on the temporal distribution of rainfall. Whereas similar seasonal rainfalls were recorded in 1991 and 1992, it is estimated that 5% of the water precipitated over the valley pool watershed infiltrated towards the aquifer in 1991 and 20% in 1992. This difference is explained by a very irregular time distribution of precipitation in 1992, most of the major rainfall events being observed over a short period during the intensive observation period. In conclusion some preliminary figures are given regarding the importance of recharge from the pools as compared with in situ recharge.

Rainfall monitoring during HAPEX-Sahel. 2. Point and areal estimation at the event and seasonal scales

The water and energy balance models that will be used in HAPEX-Sahel to analyze the data collected during the experiment are strongly conditioned by the rainfall estimation accuracy over the areas of interest. Following the description of the rainfall conditions that prevailed during HAPEX-Sahel and the computation of statistics characterising the point rainfall processfor the Sahel, presented in a companion paper, it is examined here how accurate are the areal rainfall estimates provided by the EPSAT-Niger network at the event and seasonal scales. Using a geostatistical framework, it is shown that it is possible to infer a climatological variogram that represents the average variability of the event rainfields, within the limits imposed by the resolution and sampling window of the network. Average event rainfall estimation errors are derived. The density of the EPSAT-Niger mesoscale network (one station for 150 km2) allows the estimation of the event areal rainfall over the 1° × 1° square with an average uncertainty around 5%. On the supersites (100–750 km2) the average uncertainty can be as large as 20%, depending on the site and rainfall considered. This uncertainty is reduced by two-thirds, or more, when using the denser network designed specifically to cover the supersites. It is then demonstrated how the variability within the event rainfields and the space-distribution of the rainy events combine to determine the spatial structure of a rainfield resulting from the accumulation of several event rainfelds. Over the HAPEX-Sahel study area, the within-the-event-rainfield variability is dominant over the variability resulting from the distribution of the events in space. Consequently the correlation length does not vary much when shifting from the event to the seasonal scale. At an unsampled location the seasonal rainfall error magnitude is on the order of the average error at the event scale, divided by the square root of the number of events recorded during the season.

Typology of Rainfall Fields to Improve Rainfall Estimation in the Sahel by the Area Threshold Method

The stratification of rainfall fields to improve specific rainfall models is a subject that has received relatively little attention in the literature. It is shown here that objective classification techniques, based on the intensities and spatial distribution of the rainfall fields, can produce meaningful results in terms of the area threshold method (ATM) model and climatology. Four approaches for rainfall classification, using rain gauge data, are proposed in order to improve the average areal rainfall estimation in the Sahel by the ATM model. Two of them are based on the structural behavior of the rainy area (area where it rains above a given threshold) function against a threshold. Based on this function, a new parameter, called the under profile area (UPA), has been proposed for the classification of rainfall fields. The groups obtained from the method based on this parameter are characterized by different average spatial structures. A significant improvement on the ATM model is observed by considering classification based on the UPA parameter. An average reduction of 34% of the root-mean-square error is observed in a validation term. This improvement is a direct consequence of the fact that the optimal thresholds are different from one group to another, which is an important point when considering the impact of classification on the ATM model. 1. Why a Typology of Rainfall Fields?