Effects of rainfall variability on water quality of Yanze river in Kigali, Rwanda

Abstract

The effects of highly variable rainfall patterns on water quality of Yanze River in Kigali was investigated in this study. The study was divided into fieldwork and laboratory analysis where statistical analyses were used to establish the relationship between water quality and rainfall patterns. Anthropogenic activities along Yanze River were assessed by taking transect walks and conducting key informant interviews. Eight parameters were considered in this study and these include, turbidity, total suspended solids, conductivity, total dissolved solids, nitrate, phosphate, E-coli and total coliforms. This paper is focusing on the trend analysis of both rainfall and water quality of Yanze river for seven years (2012-2018). Trends of rainfall, water quantity and quality patterns of Yanze River were analysed using a non-parametric Mann–Kendall test and their relationship established using Pearson’s Product Moment correlation coefficient. The Mann-Kendall statistical test showed that the characteristics of water quality, rainfall and water levels of Yanze River varied across the years from 2012 to 2018. In particular, the MK test indicated that all the weather stations in Yanze region had a negative trend ranging between -0.137 to -0.233 implying a decrease in rainfall over the period of study. The trend analysis of Yanze water level generally shows a positive trend of 0.187. The results show increased water levels ranging between 0.55m and 0.9m in all seasons. For the trend analysis of water quality of Yanze, all eight parameters that were considered in this study showed a positive and erratic trend except nitrate. The results by Mann Kendall test indicated that e-coli, total coliforms, turbidity and total suspended solids showed the highest positive trend of 0.520, 0.50, 0.47 and 0.40 respectively. The results also showed the pattern of erratic rainfall for the past 7 years, characterized by increase of rainfall in the years 2012 to 2013, 2015 to 2016 and 2017 to 2018 and a decrease of rainfall from the year 2014 to 2015 and 2017 and the observed changes were affecting the water quality of Yanze River in the corresponding years. Correlation analysis using Pearson Product Moment between water quality and rainfall ranged from -0.10 to 0.47 while their coefficient of determination r2 varied between 0.04 % and 22.39%. It was noted that precipitation had an influence on the increase in concentration of water quality parameters especially turbidity, total suspended solids, total dissolved solids and electric conductivity. The r2 values in this study were low, which indicated that there could be other factors that could be contributing to the degradation of Yanze water quality such as land use and land cover, topography of the area as well as high population density coupled with poor human settlement