Hydrochemistry and source of high fluoride in groundwater of the Nairobi area,Kenya / Hydrochimie et origine des fortes concentrations en fluorure dans l'eau souterraine de la région de Nairobi,au Kenya

Abstract

This study aims to identify the hydrogeochemical processes influencing the high fluoride concentrations in groundwater of the Nairobi area, Kenya. For this purpose 16 groundwater samples were collected and analysed. Fluoride concentrations above the WHO standard are found in the downstream areas. The high F^? concentrations are correlated with high sodium and pH and low Ca²⁺ concentrations. Weathering of sodium-rich alkaline igneous rocks causes a pH increase resulting in an increase in HCO₃ ^? and CO₃ ^2- by dissolution of CO₂. Groundwater becomes oversaturated compared to calcite and calcite precipitation occurs, leading to a decrease in Ca²⁺. This causes a sub-saturation with respect to fluorite and dissolution of fluorite increases the F^? concentration. These reactions were modelled using the PHREEQC model and the results showed a good agreement with the measured groundwater quality, indicating that the proposed reactions are plausible for explaining the observed concentrations in groundwater.     

Remotely sensing the spatial and temporal land cover changes in Eastern Mau forest reserve and Lake Nakuru drainage basin,Kenya

This study aimed at characterizing land cover dynamics for four decades in Eastern Mau forest and Lake Nakuru basin, Kenya. The specific objectives were to: (i) identify and map the major land cover types in 1973, 1985, 2000 and 2011; (ii) detect and determine the magnitude, rates and nature of the land cover changes that had occurred between these dates, and; (iii) establish the spatial and temporal distribution of these changes. Land cover types were discriminated through partitioning, hybrid classification and spatial reclassification of multi-temporal Landsat imagery. The land cover products were then validated and overlaid in post-classification comparison to detect the changes between 1973 and 2011. The accuracies of the land cover maps for 1973, 1985, 2000 and 2011 were 88%, 95%, 80% and 89% respectively. Six land cover classes, namely forests-shrublands, grasslands, croplands, built-up lands, bare lands and water bodies, were mapped. Forests-shrublands dominated in 1973, 1985 and 2000 covering about 1067 km², 893 km² and 797 km² respectively, but were surpassed by croplands (953 km²) in 2011. Bare lands occupied the least area that varied between 2 km² and 7 km² during this period. Overall, forests-shrublands and grasslands decreased by 428 km² and 258 km² at the annual average rates of 1% each, whereas croplands and built-up lands expanded by 660 km² and 24 km² at the annual rates of 6% and 16% respectively. The key hotspots of these changes were distributed in all directions of the study area, but at different times. Therefore, policies that integrate restoration and conservation of natural ecosystems with enhancement of agricultural productivity are strongly recommended. This will ensure environmental sustainability and socio-economic well-being in the area. Future research needs to assess the impacts of the land cover changes on ecosystem services and to project the future patterns of land cover changes.     

利用MOD13Q1产品监测肯尼亚2001-2010年荒漠化动态

使用MODIS MOD13Q1-NDVI数据,通过NDVI累计计算,得到肯尼亚2001年和2010年植被生长状况较好时期的NDVI影像.基于像元二分模型,估算植被盖度.利用植被盖度进行荒漠化评价.结果表明,肯尼亚近10年来生态环境总体趋于改善,荒漠化发展态势并不明显,但荒漠化问题仍然严重.不同程度的荒漠化土地变化趋势不尽相同,轻度荒漠化和重度荒漠化转出面积大于转入面积,呈现减少趋势,非荒漠化和中度荒漠化变化相反,各省区的荒漠化土地变化趋势也不相同.在小范围地区内,使用Landsat-5 TM影像进行验证,发现与MODIS数据评价结果相似,但更能详尽的描述重点区域的荒漠化问题.     

Random walk, zonation and the food searching strategy of Terebralia palustris (Mollusca, Potamididae) in Kenya

Terebralia palustris is a common mud-whelk present at a particularly high density in all Indo-West Pacific mangroves. Young snails feed on nothing but mud while larger specimens are able to feed on fallen leaves too. In Kenya (Mida Creek) under the canopy, competition for mangrove leaves can be very high due to the high density of Sesarmidae crabs. On open exposed muddy platforms, no Sesarmidae occur but the leaf density is very low because the leaves are only randomly present as they are deposited and removed twice a day by the tide. However, the snail density is always very high, raising the question as to whether the snails use a special searching strategy to optimize their resource finding rather than a purely random movement. By analyzing the snails' movements on a uniform area at different levels and comparing them with simulated random paths, we could show that the snails' movements are not purely random. The distribution of different size classes of T. palustris in Mida Creek was known to be quite odd: the same simulation approach suggests that the zonation asymmetry could reasonably be due to the stochastic recruitment of juveniles in space and time and maintained by a substantial long-lasting spatial inertia.     

Variations of the Effective Temperature Index (ET) in Kenya

Using the Effective Temperature Index (ET) as a measure of climatic comfort conditions in Kenya, this study analyses its spatial, seasonal and diurnal variations. This index is computed using data collected from 57 weather stations spread throughout the country. Isarithmic mapping technique and the conditioning square method are used in this analysis. The results indicate that there exists in Kenya marked spatial, seasonal and diurnal variations of climatic comfort conditions. A classification of the country using this index and the Terjung (1968) system of climatic classification yields six climatic comfort zones. This type of physio-climatic studies has significant implications for Kenya in aspects such as building, clothing, recreation and sporting. This revised version was published online in July 2006 with corrections to the Cover Date.     

Kenya’s Secondary Cities Growth Strategy at a Crossroads: Which Way Forward?

Kenya has been promoting equitable urban and regional development since the 1970s despite the lack of a clearly formulated national urban policy or an urban and regional development policy. A key element of the country’s equitable urban and regional development effort is the promotion of secondary cities that would relieve population pressure in the countryside, help to better integrate the country’s rural and urban economies, help to reduce congestion and improve the quality of life in the metropolitan cities of Nairobi and Mombasa, and help increase the modernization spin-off which urban centers provide to the surrounding rural areas. Using recent census and economic survey data, this paper examines the current state of Kenya’s secondary cities in the context of its urban and regional development strategies. The paper finds that: (1) the country’s urban and regional development strategies have failed to work as planned largely because of insufficient devolution of power and fiscal responsibility to municipal and other local government units, (2) the country’s secondary cities are faced with immense challenges that undermine their ability to live up to expectations, (3) some of these cities have significantly grown economically over the last four decades despite immense challenges, and (4) Nairobi’s dominance of Kenya’s economy continues because of policies that unwittingly concentrate investments there. The paper concludes with strategies that could enhance the country’s urban and regional development programs and, in the process, aid the development of its secondary cities.     

Drivers of farmer satisfaction with small-scale irrigation systems

Climate change impacts the quantity and seasonality of rainfall, threatening Africans' ability to maintain satisfactory yields on rain-fed farms. On the semi-arid northwestern slopes of Mount Kenya, irrigation systems structured around Community Water Projects (CWPs) provide households with piped water, which helps mitigate drought impacts and enhances resilience to increasingly variable rainfall patterns. The vision of these irrigation systems is to equitably distribute water resources among CWP members both for household consumption and to maintain or improve agricultural yields. However, not all households receive equal quantities of water from their CWPs, and little is known about the impact of water provisioning on farmers' opinions or perceptions of CWP performance.In this study, we relate respondents' satisfaction with their CWPs to the quantity of water delivered. Not surprisingly, farmers who received low quantities of water from their CWPs expressed above-average rates of concern regarding drought. In contrast, satisfaction with one's CWP was unrelated to the absolute quantity of water received (log liters/minute), but it was strongly associated with relative measures of water delivery: respondents who received less water than other members of the same water project experienced high rates of dissatisfaction, as did respondents who received less water than in the past. These results suggest that Kenyan farmers may feel particularly dissatisfied—and, perhaps, particularly driven to demand improved water governance—when they perceive inadequate water delivery relative to some reference group.Because relative water quantity dictates satisfaction even after controlling for factors that can be resolved internally at the CWP level (by, for example, striving for uniformity of water flows across and within households), water project managers are limited to some degree in their ability to improve satisfaction through better governance. That said, governance actions that prevent declines in flows over time (e.g. maintaining infrastructure to reduce leaks) could ostensibly lead to meaningful improvements in member satisfaction.     

Farming as a livelihood source for the urban poor of Nakuru, Kenya

Farming among urban dwellers in Sub-Saharan Africa is a common phenomenon. The present study, carried out in a medium-sized town in Kenya, not only confirmed this but also showed that farming by urban dwellers in the rural areas was even more important for these households’ livelihoods than farming in town. However, those who could benefit most from farming, i.e. the urban poor, appeared to be underrepresented among urban farming households and those urban poor who did farm, either in urban or in rural areas, performed worse than the farming non-poor. The importance of farming as a livelihood source is illustrated by the fact that in years with very little or no harvest due to drought, many of the poor farming households faced food shortages.