Effects of Land Use/Land Cover Change and Rainfall Variability on Hydrological Characteristics of River Ruiru Watershed, Kiambu County, Kenya

Date

2021

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Kenyatta University

Abstract

Watersheds and their water resources are highly vulnerable to land use/land cover changes and seasonal rainfall variability as they directly influence basin hydrological characteristics in terms of water quantity and quality. This study assessed the effects of land use/land cover change and rainfall variability on hydrological characteristics of River Ruiru watershed in Kiambu County. The study integrated the use of remote sensing, Geographic Information System (GIS), water quality sampling, hydrological modelling and statistical methods to collect and analyse the collected data. Results of land use/land cover change analysis indicated that built-up areas, annual crops and perennial crops (tea and coffee) increased by 3.068%, 35.848% and 11.493% respectively between 1976 and 2017. However, it was observed that perennial crops increased gradually between 1976 and 1995 but declined by 1.94% between 1995 and 2017. Grassland, shrubland and forestland declined by 7.48%, 13.25% and 29.79% respectively between 1976 and 2017. Consequently, Soil and Water Assessment Tool (SWAT) model simulation results indicated that the land use/land cover changes that occurred in River Ruiru watershed between 1984 and 2017 had effects on the quantity of river discharge and water quality for drinking purposes. Surface runoff (SURQ), lateral flow (LATQ) and groundwater contribution to discharge (GWQ) increased from 30.25mm/yr, 8.48mm/yr and 9.95mm/yr to 181.25mm/yr, 11.44mm/yr and 10.66mm/yr respectively. Moreover, the nitrates in surface runoff (NO3SURQ), nitrates in lateral runoff (NO3LATQ), Organic Nitrogen (N.ORG) and Organic Phosphorus (P.ORG) increased from 0.05kgN/ha/yr, 0.06kgN/ha/yr, 5.77kgN/ha/yr and 0.87kgP/ha/yr to 1.47kgN/ha/yr, 0.19kgN/ha/yr, 70.60kgN/ha/yr and 8.86kgP/ha/yr respectively. In additions, findings from the study indicated that temporal rainfall variability had effects on the quantity of river discharge and water quality of River Ruiru. Based on regression analysis, the correlation between the observed and predicted value of dependent variable indicated an association of R=0.972 between temporal rainfall variability and quantity of river discharge hence showing a strong positive linear relationship. Similarly, Pearson’s correlation analysis results showed that temporal rainfall variability was strongly and positively correlated to NO3, N.ORG and P.ORG with R=.695, p<.001; R=.781, p<.001 and R=-.780, p<.001. Furthermore, results from a paired sample t-test indicated that pH, turbidity, Dissolved Oxygen (DO), electrical conductivity and Total Dissolved Solids (TDS) had higher mean during wet season (M=8.40, M=79.00, M=51.20, M=87.00 and M=54.20) than in dry season (M=6.80, M=11.60, M=43.40, M=73.00, and M=45.40). The difference for these water quality parameters between dry and wet seasons was significant (P=0.003, P=0.034, P=0.005, P=0.013 and P=0.014). Moreover, findings from the study indicated that DO, TDS, electrical conductivity, total phosphorus and total nitrogen values were within World Health Organization (WHO) and National Environment Management Authority (NEMA) recommended values. However, the results for the total coliforms and E. coli indicated that River Ruiru was severely polluted with faecal organisms. In conclusion, land use and land cover changes and temporal rainfall variability have affected the quantity of river discharge and water quality in River Ruiru watershed. As such, remedial actions to address the hydrological effects of land use/land cover change and temporal rainfall variability both by national and county governments are required.

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