Streamflow and Salt Flux in Seasonal Rivers: The Case of the Semi-Arid Tiva River Basin, Kenya

Abstract

This paper presents the results of a study on the influence of streamflow variability on salinity, total dissolved solids (TDS) and conductivity in a semi-arid Tiva River Basin in Kenya. Measurements of salinity, TDS, conductivity and river discharges were undertaken in sampling stations by applying standard hydrologic methods. The mean and maximum river discharges for the sub-basins of the river ranged from 11 to 33 m³s^-1 and from 118 to 210 m³s^-1, respectively, with the peak river discharge at the main Tiva  branch being 270 m³s^-1.  The study shows that there is a significant relationship between the variability of streamflow and the variability of salinity, conductivity and TDS in the river. The relationships between streamflow and salinity were best represented by power functions rather than linear regression functions. The relationships were negative so that the levels of salinity, conductivity and TDS decreased with an increase in river discharge.  Salinity, TDS and conductivity were also inversely correlated to turbidity. An increase in turbidity corresponded to a decrease in conductivity and hence salinity and TDS. The low conductivity at high streamflow conditions were attributed to the dilution effect of increased volume and also by the presence of high proportion of non-conductance organic and inorganic materials such sand and clay.  The highest TDS, conductivity and salinity values were measured during the low flow (baseflow) conditions and the concentrations were lowest during high streamflow conditions. It is postulated that the high concentrations were a result of high evapotranspiration and seepage of subterranean water from bank storage and groundwater aquifers, and that low concentrations were a result of dilution and flushing effect of high streamflow. Inter-subbasin variations in the levels of salinity were attributed to differences in landuses, lengths and sizes of the sub-basins. The effects of upstream irrigation were evident in one of the main sub-basins-Mwitasyano river where the highest salinity levels (max: 3.4%; mean 1.1%) were measured. The total salt flux from Upper Tiva river basin was estimated to be 100,344 tonnes.yr^-1 with basin salt production rate of 27.87 tons.m^-2.yr^-1. This salt flux was attributed to the nature of the basement complex metamorphic rocks (e.g. Kankur limestone) through which the river drains. The influence of irrigation upstream was noted to be important in the Mwitasyano sub-basin that contributed 61% of the total salt load. The study emphasizes the need for water resources and agricultural development programmes in the semi arid Tiva River Basin to promote sustainable irrigation and landuse practices. It is suggested that construction of water reservoirs in the Tiva basin would help in controlling salinity levels in the river.

Keywords: Tiva River; Salinity; Total Dissolved Solids (TDS); Streamflow; Salt fluxes; Kenya