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UNESCO-IHE Institute for Water Education, Delft (2017)

Analysis of groundwater potential and interactions with surface water in the Sand River region, Mara Basin Kenya

Wekesa, Sospeter Simiyu

Titre : Analysis of groundwater potential and interactions with surface water in the Sand River region, Mara Basin Kenya

Auteur : Wekesa, Sospeter Simiyu

Université de soutenance : UNESCO-IHE Institute for Water Education, Delft

Grade : Master of Science (MS) 2017

Résumé partiel
Economic development in the arid and semi-arid areas such as the Sand River region in the Middle Mara basin Kenya is limited by water availability. Typical to dryland areas, surface flows in the Sand River are highly variable due to erratic rainfall patterns. Surface water harvesting infrastructures such as earth/water pans are also not suitable for such a water scarce region due to high rates of evapotranspiration exceeding precipitation. Groundwater is an alternative source to support local water demand for livelihood improvement. However, high water demand for domestic, small scale irrigation and high livestock population in particular cannot be met by the infrastructure to extract deeper groundwater which requires high investment as well as maintenance costs. The ephemeral Sand River constitutes a potential alternative groundwater source in such a region where little knowledge concerning deeper groundwater exists. Water stored in the sand deposits can be exploited to allow availability all year around. With very little known about the alluvial corridors, traditional techniques such sand scooping are used to extract water from the sand layers. The main purpose of this research is to demonstrate the unexploited groundwater potential in the Sand River region, and the flow characteristics and dynamics in the river including interactions with surface water. It furthers seeks to highlight the existing groundwater dependent ecosystem and hypothesize perceived impacts with development. In general, the research seeks to establish whether sustainable groundwater utilization in support of local demand can be achieved in the Sand River region. Techniques adopted in the study include geophysics (ERT) and probing, hydraulic conductivity and porosity estimation, groundwater level and surface flow measurements as well as hydro chemical and isotopic tracing of interaction between surface water and groundwater. Surveys and interviews have further enabled assessment of the current water resource availability versus the existing local demand. Spatial analysis in GIS and Remote Sensing tools have allowed qualitative assessment of recharge potential ; a vital component for groundwater potential assessment. Land use/vegetation cover, soil texture and slope are key primary factors necessary for qualitative quantification of recharge rates. Recharge potential in the sub-catchment varies from very high to low. However, a larger part of the sub-catchment lies in the medium and high recharge zones. Groundwater potential cannot be comprehensively assessed by solely considering recharge rates. Presence of springs, shallow wells and boreholes including slightly deeper sections of sand deposits along the river have given more insights into groundwater potential availability. Two main groundwater systems, deeper and shallow groundwater, exists in the study area with precipitation as the main input. Available groundwater resources support different sectors of demand such as livestock, domestic, small scale irrigation, institutional, tourism and commercial. Domestic and livestock water demands accounts for the major annual groundwater abstraction. However, comparing the two key sectors highlights livestock water demand as the highest, consuming > 80% of the total groundwater abstraction. Estimated annual demand is approximately 1.6 million m3. Annual diffuse percolation coarsely estimated based on global values is approximately 3 million m3. Much of the groundwater abstraction occurs in alluvial aquifer along the Sand River ; > 80 % and > 65% in Olderkesi and Naikarra respectively. This presents the Sand River as a critical alternative groundwater source in sustaining local livelihood.The Sand River is an ephemeral system characterised by highly variable and unpredictable flows due to erratic rainfall patterns. Gauge readings between July 2013 and January 2016 indicated that a single flood pulse was > 2.5 m. Other flood pulses recorded were less than 1.5 m. The inter-flood frequency in the river is approximately 4 months with a maximum flood duration of 15 days (2 weeks). In addition to rainfall characteristics, antecedent moisture in the sand deposits including the depth to water table determines the magnitude of flood from a particular event.

Sujets  : hydrogeochemistry ; groundwater ; surface water ; groundwater resources ; Kenya

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Page publiée le 4 mars 2020