Rainfall geochemistry in the Sahel region of northern Nigeria

Abstract

Chemical data and stable isotope (δ¹⁸O, δ²H) results are presented for monsoon rains for several years during the 1990s from northern Nigeria in the Sahel region of Africa. The isotopic data from Garin Alkali (Nigeria) are related by a line $\text{δ}{}^2\text{H=6.33}\text{δ}{}^{18}\text{O+9.9}$ with a weighted mean value of −3.6‰ for δ¹⁸O. The heaviest rains have the lightest isotopic compositions; the lighter rains’ enrichment is as a result of convection. The mean 1992 concentrations of Cl in rain ranged from 1.3 to 2.8 mg l⁻¹ for the two stations in Nigeria. The early rains have higher Cl than the later events although Cl accumulations are in general directly related to rainfall amount. The Br/Cl ratios of all rains are enriched above marine values, which may in part be attributed to a preferential concentration of Br in smaller size particles, although more likely, is related to release from the biomass as the air masses pass over vegetated areas. The high Br/Cl ratios rule out dust from halite sources during the monsoon. The ratios of Na and Cl are similar to those in sea water, although all other elements (especially Ca, SO₄, NO₃ and K) are enriched relative to marine aerosols and indicate continental sources. The element ratios (Ca/SO₄); K/Mg; K/Na) are remarkably similar to those in ash leachates from tropical vegetation and this is proposed as the main solute source in the present day monsoon rains, reinforcing the evidence of Br/Cl ratios. The chemical results show the considerable terrestrial influence and are in line with isotopic evidence, which demonstrates considerable modification by convective circulation and continental influence as the monsoon air masses track northwards over the Sahel.

Introduction

The object of this paper is to report investigations of the chemistry of rainfall in an area of northern Nigeria lying within the Sahel region, made over a period of five years in the 1990s, in particular with reference to within-season variation, inter-annual variation, marine vs terrestrial origins of solutes, and to compare these with isotopic results and with data from the surrounding area in Nigeria and Niger. These results form part of a wider integrated study of climate and environmental change in northern Nigeria (Edmunds et al., 1999) and are directed mainly at water resource investigations, which require improved knowledge of rainfall recharge and solute inputs for water balance assessment. The results have direct relevance to methods of recharge estimation in semi-arid areas and studies of recharge history (Cook et al., 1992), as well as to investigations of groundwater quality and use, since evaporated rainfall is often the most important source of solutes to aquifers.

It is generally accepted that the Sahel region has been experiencing one of the worst droughts on record (see for example, Nicholson, 1985). Hulme (1992) has found rainfall in the Sahel to be significantly lower in the period 1961–90 compared to 1931–60. Carter (1994) has reported a decline in mean annual rainfall from 456 mm for the period 1940–70 to 314 mm between 1971–91, a reduction of 31%. In the present study the mean annual rainfall from 1992–97 is 389 mm, a decline of 15% when compared to the 1940–70 records. Possible causes outlined by Hulme (1992) for the changes in rainfall in the Sahel are those related to land use degradation, changes in the global ocean circulation associated with patterns of sea-surface temperatures (SSTs), and those related to the changing composition of the global atmosphere.

The isotope chemistry of rainfall in the Sahel region is extremely variable, both geographically and temporally, responding to atmospheric circulation patterns (Joseph et al., 1992). The source of precipitation for the Sahara-Sahel, including Nigeria, is the Gulf of Guinea (Taupin et al., 2000). However re-evaporated water from the continent is an important source of water vapour as shown by the lack of continental effect and also a large deuterium excess at the beginning and end of the rainy season (Taupin et al (1997), Taupin et al (1995)). Although temperature is an important factor controlling the variation in the isotopic content of rainfall, it has been shown (e.g. Fontes, 1976) that there is often no clear relationship between temperatures measured on the ground and the isotopic composition of tropical rains, indicating that other processes must be involved. As storms develop, convection leads to low condensation temperatures at the height of the vertical cloud development (Fontes et al., 1993; Taupin et al., 1995). Thus rains in north-central Nigeria and elsewhere at the peak of the season in August are the most depleted in the heavier isotopes (Mbonu and Travi, 1994; Taupin et al., 1997). The amount of rainfall in a storm event can also affect the isotopic signature. The relative humidity of the air column is also very important. Rains of less than 5 mm have been shown to be heavier as a result of evaporation leading to loss of the lighter isotopes as rain droplets fall through drier air (Gat, 1980). Thus the extreme climatic and meteorological situations found in tropical monsoon regions can produce very different isotopic signatures for individual rain episodes.

Compared with isotopic studies the chemistry of rain in the Sahara-Sahel region is comparatively unknown. Geographical and temporal variations in the major, minor and trace element chemistry of precipitation have been studied for one year's rainfall in Senegal (Travi et al., 1987). The controlling factors were identified as being the different origins of the atmospheric vapour generating the monsoon rains; in this coastal area the solutes are supplied mainly by marine aerosols.

The Nigerian sites are situated in the northeast of the country, close to the border with Niger and the sites in Niger are east of the capital Niamey (see Fig. 1). The rainfall distribution over this region and indeed over western Africa is determined by the position of the meteorological equator and its two associated structures, the Inter Tropical Front (ITF) and the Inter Tropical Convergence Zone (ITCZ). The ITCZ at the present day rarely exerts its influence over continental areas north of latitude 12°N, the southern boundary of the Sahel. Thus Sahelian rainfall depends almost exclusively on the position and structure of the ITF, and is mostly of convective origin, either from isolated cumulo-nimbus or from cloud formations, often evolving in the form of squall lines which move in a general east to west direction across the Sahel belt (Lebel et al., 1992).