Groundwater and Lake Water Balance of Lake Naivasha Using 3-D Transient Groundwater Model.

Abstract

Integrated water resources management is necessary, particularly in a system where considerable interactions exist between ground and surface water resources. Integrated study requires reliable estimation on an overall basin water budget and reliable estimates on hydrologic fluctuations between ground water and surface water resources. The objective of this study is to construct and calibrate a 3-D transient groundwater model that simulates the long term groundwater and lake water balance of the Lake Naivasha basin and that could be utilized to evaluate the effects of changes in system flux over time. methodological design of this study starts with a field work Geodetic-GPS survey program in order to accurately measure height of the groundwater level and surface water levels. The data analysis involved a separate characterization of both surface and subsurface parameter. Time series data Including lake level, surface water-inflow, evaporation and precipitation were analyzed on a monthly basis. Pump test data was analysed for recently drilled boreholes. Recharge was estimated by relating monthly change groundwater level and average recharge measured in the area. Water abstraction data mainly from the irrigated commercial farms was analysed based on the irrigation area-depth relationship. The model developed using GMS software, covers an area of 1817 sq. km with two aquifer systems. The upper aquifer is unconfined and the lower aquifer is semi-confined. The upper aquifer is in hydraulic link with the lake. The model grid contains 104 rows, 120 columns with a uniformly horizontal spacing equal to 500 m. The lake bathymetry was represented by the lake package Triangular Networks (TIN) of GMS functionality. The model design spans over 79 years (1932-2010) with a total of 942 stress periods and a Single time step. In the modeling process the applications of the conceptual model approach of Modflow and Lake Package LAK3 was extensively explored. Model calibration was highly constrained by observing the measured and calculated aquifer and Lake level. The final calibrated model, implements the application of parameter estimation tools, PEST. The model matches the observed lake level with R2= 0.985, steady State and R2= = 0.905, transient State. Model sensitivity analysis result shows that the steady state model is highly sensitive to increasing and decreasing of recharge and highly sensitive to a decreasing than increasing in hydraulic conductivity. The transient model shows equal sensitivity with increasing and decreasing of the storativity but with a slow response. The long term lake water balance is calculated by Modflow using the stage-volume rating curve of Lake Package LAK3. The long term average storage volume is 8.4* 108 m3/month. The long term average fluxes in to the lake arc precipitation 7.72 *106 m3/month, surface inflow 19.36*106 m3/month and groundwater Inflow (Lake seepage-In) 1.1*106 m3/month. The long term average fluxes out of the lake are evaporation 21.41 *106 m3/ month, lake water abstraction 1.92 *106 m3/ month (equivalent to 5*106 m3/month over the past 30 years) and groundwater outflow (Lake seepage-Out) 5.5*106 m3/month. The lake water balances suggests t.hat the lake is not In equilibrium with the Inflow and outflow terms, a long term net lake level fall of 5Am resulted in a lake storage loss of 6.73*108 m3 over the period, 1932-2010. A long term groundwater budget is calculated reflecting all water flow In to and out of the regional aguifer. The inflow components include recharge 2.8*106 m3/month, river leakage-in 1.4*105 m3/month and Lake Seepage-in (groundwater outflow from the lake) 5.56*106 m3/month. The outflow components include well abstraction 7.5*105 m3/month (equivalent to 2*106 m3/month over the past 30 years), river leakage out 2*104 m3/month, Lake Seepage-out (groundwater inflow In to the lake) 1.1*106 m3/month and groundwater outflow through the head dependent boundaries 6.7*106 m3/month. The model water balance suggests that lake Naivasha basin is in equilibrium balance suggests that lake Naivasha basin is in equilibrium with a net outflow about 1% greater than the Inflow over the calibrated period of time (1932-2010)