Streamflow Simulation and Environmental Flow Assessment in the Umba River, Kenya

Abstract

The Umba River basin is a transboundary river catchment shared between Kenya and Tanzania. There are plans to construct two dams in the catchment to meet the water demands for irrigation and water supply. Hence, there is growing concern on the alteration of the river flow that can significantly affect the functioning of the riverine ecosystems. The integration of hydrologic, hydraulic, and ecological data is necessary to understand the flow characteristics and to assess the environmental flows of the river. However, sufficient streamflow observations are not available due to missing flow records and limited observation periods in the river. The overall objective of the study was, therefore, to conduct an environmental flow assessment for the lower reach of the river using observed and simulated streamflow data. A rainfall-runoff model is simulated using Hydrologic Modelling System (HMS) to determine continuous time series of daily streamflow for the last 30 years. The statistical relationships between the historical flow regime and the Umba River ecosystems are evaluated by the use of Ecosystem Functions Model (EFM). A hydraulic model was setup using River Analysis System (RAS) to simulate the streamflow in the lower 45 km Umba River reach. Results from the hydraulic model were used to investigate the relationships between the flow characteristics and the natural environmental flows of the river. Different flow requirements including low flows (0.20 m3/s to 0.70 m3/s), base flows (0.7 m3/s to 4.0 m3/s), pulses (4 m3/s to 20 m3/s), high flows (20 m3/s to 50 m3/s), and flood events (50 m3/s to 120 m3/s) are identified. These flow types need to flow in different months of the year to preserve the riverine ecosystems and maintain their services. For effective management of the river flow, consideration of environmental flows and continuous collection of data on hydrology, hydraulics, and ecology are recommended.