Hydrological consequences of proposed interventions in the Tana basin, Kenya - A water balance study

Abstract

The Tana River is the largest river in Kenya, with a large part flowing through a (semi-)arid region. The Tana delta is one of Africa’s most important wetlands, generating a large variety of ecosystem services. However, the natural system is under pressure from anthropogenic developments. The aim of this study is to provide improved understanding of the hydrological consequences of anthropogenic interventions on the hydrological system in the Tana basin, with specific focus on water supply to the Tana delta. Following the research aim, this master’s thesis provides a quantitative scenario-based analysis of the Kenyan Tana Basin water balance using the global hydrological model PCR-GLOBWB. The main research question that is addressed in this thesis is: How will the river discharge of the Tana River in the Tana delta be affected by climate change and anthropogenic developments in the region? The interventions are translated into four scenarios: (1) High Grand Falls Dam (HGFD), (2) HGFD plus planned large-scale irrigation schemes, (3) water demand from Nairobi and Lamu Port, and (4) a combination of all scenarios. The results of these interventions are compared to the status quo, i.e. the current land use, with only the climate as changing factor (which is also included in all scenarios). PCR-GLOBWB was not completely successful in predicting the discharge. In Garsen the validation results are: LNSE = -2.96, R2 = 0.45, RMSE = 123.85 m3/s. To remove the model bias a normalized correction method has been applied (after calibration). After this correction the validation results in Garsen are: LNSE = -0.10, R2 = 0.41, RMSE = 34.14 m3/s. The effects of the scenarios are assessed by determining the change in Q10 and Q90 for the full year and the rainy and dry seasons, respectively for the near, mid- and far future. In conclusion, climate change is expected to lead to an increase in discharge during the rainy season, while there is no strong effect expected in the dry season. With all interventions combined the all-year mean Q10 decreases by 2.60%, 15.16% and 15.65%, in the near, mid- and far future, respectively, while the all-year mean Q90 increases by 10.07% and 3.38% and decreases by 2.85%, respectively.