Lunchtalk by H.J. (Rick) Hogeboom

Title:On the influence of groundwater abstractions on Lake Naivasha’s water level.”

Abstract for the presentation:

Lake Naivasha in Kenya’s Rift Valley forms the scene for a wide variety of natural and human activities, amongst which its thriving horticulture industry. Starting around the 1980s, the lake provided water for irrigation of the flowers, but the last decade or so uptake is complemented by significant groundwater abstractions. Despite substantial research efforts, the understanding of the groundwater system is still frugal; hydrogeological build-up and parameterization, lake-aquifer interaction, the overall groundwater balance and the effect of groundwater abstractions on lake levels are largely unknown. The objective of this research was to explore the influence of groundwater abstractions on Lake Naivasha’s water level, by modeling groundwater flow around the lake. The Flower Business Park (FBP), located some 7 northwest of the lake, served as a test case. A steady state MODFLOW finite-difference model was developed to simulate exchange of water between the lake and its surrounding aquifer under natural conditions and under abstractions at FBP. The hypothesis that uncertainty concerning the conceptual model could result in multiple non-unique calibration parameterizations was tested by modeling the lakebed as being rather leaky and rather sealed. Results show flow patterns under natural conditions exhibit similar behavior in both parameterizations, i.e. laterally from the escarpments to the valley floor and axially from Lake Naivasha to the north (21-33%) and south (67-79%). Outflow from the lake occurs to the north and south, while inflow takes place from the east and west, with a net outflow into groundwater of around 55.0 . Flow patterns under abstractions at FBP are similar to those under natural conditions in most parts of the study area, except around FBP where a cone of depression is generated by the abstractions. Lake levels were lowered upon pumping by 0.7 – 7.5 . This reduction is attributed to the interruption of recharge from western mountain range Kinangop to the lake by FBP abstraction, viz. water pumped at FBP originates (for the largest part) from these higher ranges, rather than from the lake.