Holocene palaeohydrology, groundwater and climate change in the lake basins of the Central Kenya Rift

Abstract

The Central Kenya Rift contains small soda lakes such as Nakuru, Elmenteita and Bogoria, freshwater Lake Naivasha, and the partly (spatially) freshwater Lake Baringo. The hydrology of this area is controlled mainly by climate, tectonically controlled morphological and volcanic barriers, faults, and local water-table variations. Much of the area relies on groundwater for human and industrial use, though there are widespread quality issues particularly in relation to fluoride. Despite the huge demand for the resource, little is known about the highly complex groundwater systems; lacking monitoring data, an assessment is developed on the basis of regional geological, hydrogeological and hydrochemical analyses. Significant hydrological changes have taken place in the region over the last 10 000 years as a result of global, regional and local changes, but the impacts on groundwater resources are still largely unknown. The IPCC projects a 10-15% increase of rainfall in the area, but it may not necessarily result in a proportional increase in groundwater recharge. High groundwater recharge periods appear to be anchored on a decadal cycle.Key words Holocene; hydrology; groundwater; climate change; Central Kenya Rift; lake basins Paléohydrologie Holocène, eaux souterraines et changement climatique dans les bassins lacustres du Rift du Centre du Kenya Résumé Le Rift du Centre du Kenya contient de petits lacs salés tels que les lacs Nakuru, Elmenteita et Bogoria, le Lac Naivasha d'eau douce, et le Lac Baringo partiellement (spatialement) d'eau douce. L'hydrologie de cette zone est principalement contrôlée par le climat, des barrières volcaniques et morphologiques sous contraintes tectoniques, des failles, et des variations piézométriques locales. La plupart de la région compte sur les eaux souterraines pour satisfaire les besoins domestiques et industriels, bien que les problèmes de qualité soient largement présents en particulier en termes de fluorures. En dépit des besoins immenses vis-à-vis de la ressource, la connaissance des systèmes hydrogéologiques hautement complexes est faible. Manquant de données de suivi, une évaluation est développée sur la base d'analyses régionales géologiques, hydrogéologiques et hydrochimiques. Des changements hydrologiques significatifs ont eu lieu dans la région au cours des 10 000 dernières années suite à des changements globaux, régionaux et locaux, mais les impacts sur les ressources en eaux souterraines sont largement inconnus. Le GIEC prévoit une augmentation de 10-15% de la pluviosité dans la région, mais qui ne résultera pas nécessairement en une augmentation proportionnelle de la recharge de nappe. Des périodes de forte recharge en eaux souterraines apparaissent être inscrites dans un cycle décennal.

Climate Change and Sustainable Development of Water: Sub-Saharan Africa

“…Inter-annual variability of rainfall is linked with inter-annual variability of surface water (runoff) such that wetter periods observe greater volumes of stream flow surface water and drier periods observe reduced volumes of surface waters. Relationships between reduced rainfall and declining stream flows have been observed for streams in the Central Kenyan Rift, Volta River Basin, and Niger River Basin (Olago et al, 2009;Mahe 2009). Relationships have also between observed between reduced rainfall, declining stream flows, reduction of recharge to groundwater, and subsequent decline of groundwater levels.…”

“…A variety of methods have been used to evaluate the effect of temperature increases on climate and hydrology, resulting in a broad range of projections. Some of these projections include: increase of annual rainfall and subsequent runoff; decrease of annual rainfall and subsequent runoff; increased evaporative demand; higher evapotranspiration combined with shorter rainfall seasons resulting in persistent soil moisture defecits; soil moisture defecits leading to reduced recharge to groundwater; increase of intense rainfall events; increase of evapotranspiration may negate some of the effects of increased rainfall; increase of arid and semi-arid lands by up to 8%; increases and decreases to recharge of groundwater, varying regionally by up to 53% (Challinor et al, 2007;Olago et al, 2009;Christensen et al, 2007;Milly, 2005;Milham et al, 2009). A regional example is increase of recharge for Sahel and decrease of groundwater recharge to south-west Africa (Kundzewicz and Döll, 2009).…”

“…Contributing to the non-linearity of the relationship between rainfall and recharge, some studies suggest that a certain threshold of rainfall (annual or event-driven) must be reached before any recharge to groundwater occurs. The large inter-annual variability of rainfall, thus, may lead to large inter-annual variability of recharge (Eilers et al, 2007;Edmunds, 2009;Olago et al, 2009). Insignificant recharge is thought to occur when rainfall is generally less than 200 mm yr -1 , though some recharge during individual intense events less than that amount (Scanlon, et al, 2006;Edmunds 2009).…”

“…Insignificant recharge is thought to occur when rainfall is generally less than 200 mm yr -1 , though some recharge during individual intense events less than that amount (Scanlon, et al, 2006;Edmunds 2009). One study observed recharge taking place approximately every 10 years for a duration of a 3-year pluvial event (Olago et al, 2009). Individual intense events, in general, may be contributing more to recharge than the sum of all daily rainfall events (Taylor and Howard, 1996;Eilers et al, 2007;Owor et al, 2009).…”

Impact of Climate Change on Groundwater Recharge in a Semi-Arid Region of Northern India

“…They also observed that such episodic recharge would interrupt multiannual recessions in groundwater levels and would maintain the water security of the groundwater dependent communities in this region. Olago et al (2009) studied the impact of climate change on ground water in the lake basins of Central Kenya Rift. They observed that the IPCC projected rainfall increase of 10-15% might not necessarily result in a proportional increase in groundwater recharge.…”