Simulation of Abstractions for Water Allocation Planning In Sosiani Sub-Catchment in Nzoia River Basin Kenya

Abstract

Anthropogenic activities reduce availability of water resources by nature of,use, which alter flow regimes within surface water systems. In Sosiani catchment, information. on impact of the abstraction on river flow, security of supply and reserve flow enforcement at a hydrologically . daily timescale is scanty. However, water allocation hence abstraction continues to be carried out albeit without adequate knowledge of the spatio-temporal availability status of the resource in the catchment. This study evaluated the potential effects of surface water abstraction on flow regimes by comparing abstraction to streamflow on a daily basis in Sosiani catchment. The study aimed to simulate abstractions for water allocation planning in Sosiani catchment. Specific objectives of the study were to determine the water balance for Sosiani catchment in Nzoia River basin; simulate water abstraction in Sosiani catchment based on current water uses and to examine various water abstraction scenarios for effective water allocation and planning. A mix of empirical cross-sectional descriptive, experimental and evaluation research designs were adopted in the study. Purposive survey sampling technique was employed targeting all 124 surface water abstractors. A structured questionnaire was administered to abstractors to gather qualitative and quantitative primary data on water abstraction in the catchment. Hydrological and meteorological data was obtained from WRA and KMD respectively. HEC HMS SMA model was used to analyze water balance in Ellegirini, Endoroto and Sosiani Kapsaos sub-catchments in Sosiani catchment. Modified surface water balance method was used to simulate water abstraction while Water Evaluation and Planning System (WEAP) model was used to evaluate various scenarios indicating future water demands based on current water allocation practice. Falkenmark, IWMI and UN water scarcity indicators were used to assess the water stress levels. Nash-Sutcliffe coefficient and coefficient of determination were used to assess performance of HEC HMS model. The HEC HMS model well simulated total flow volumes as estimated residue in percentage of total observed flow was 0.2%, 14.05% and 27% for Endoroto, Ellegirini and· Sosiani Kapsaos sub-catchments respectively. Model validation results yielded 6% - 17% of the simulated flows as given by residual in percentage of total observed discharge and Nash Sachliffe that ranged from 58% to 74%. This depicted a satisfactory representation of hydrological characteristics of Sosiani. The current water withdrawal relative to available water is low «10%) as "depicted by 8.35%, 2.29% and 4.53% in Sosiani Kapsaos, Ellegirini and Endoroto sub-catchments in that order. This suggests thatthe catchment is not vulnerable to water scarcity. A projected 3.3% population growth by 2030 will increase water abstraction by 6.6%- 29.1%. By 2030, moderate water scarcity is expected in Endoroto and Sosiani Kapsaos subcatchments while little or no scarcity of water is expected in Ellegirini sub-catchment based on water withdrawal with respect to available water. A projected 10% increment in water withdrawal by 2030 indicates that Sosiani Kapsaos sub-catchment will have a deficit of 6%- 103% by volume in December, January, February and March; 6%-61% deficit in November and December in Endoroto sub-catchment while Ellegirini sub-catchment will experience no water deficits. The available flows in Sosiani catchment can adequately meet current net water demand/abstraction. However, enforcement of the reserve flow reduces the available flow especially during dry seasons to cause water deficits. These results are useful for assessing sustainability of the supply source to meet current and future water demands and for planning and sustainable management of the resource.