Understanding the Impact of Human Interventions on the Hydrology of Nile Basin Headwaters, the Case of Upper Tekeze Catchments Tesfay Gebretsadkan Gebremicael UNDERSTANDING THE IMPACT OF HUMAN INTERVENTIONS ON THE HYDROLOGY OF NILE BASIN HEADWATERS, THE CASE OF UPPER TEKEZE CATCHMENTS Tesfay Gebretsadkan Gebremicael UNDERSTANDING THE IMPACT OF HUMAN INTERVENTIONS ON THE HYDROLOGY OF NILE BASIN HEADWATERS, THE CASE OF UPPER TEKEZE CATCHMENTS DISSERTATION Submitted in fulfillment of the requirements of the Board for Doctorates of Delft University of Technology and of the Academic Board of IHE Delft Institute for Water Education for the Degree of DOCTOR to be defended in public on Tuesday, 8 October 2019, 15:00 hours in Delft, the Netherlands by Tesfay Gebretsadkan GEBREMICAEL Master of Science in Water Resources Management IHE Delft Institute for Water Education, Delft, The Netherlands Born in Adwa, Ethiopia This dissertation has been approved by the promotor and copromotor Composition of the Doctoral Committee: Rector Magnificus TUDelft Chairman Rector IHE Delft Vice-Chairman Prof. dr. ir. P. van der Zaag IHE Delft and Delft University of Technology, promotor Dr. Y.A. Mohamed IHE Delft, copromotor Independent members: Prof. dr. ir. H.H.G. Savenije Delft University of Technology Prof. dr. W. Bewket Alemayehu Addis Ababa University, Ethiopia Prof. dr. G.P.W. Jewitt IHE Delft and Delft University of Technology Prof. dr. ir. R. Uijlenhoet Wageningen University & Research Prof. dr. ir. N.C. van de Giesen Delft University of Technology, reserve member This research was conducted under the auspices of the Graduate School for Socio-Economic and Natural Sciences of the Environment (SENSE) CRC Press/Balkema is an imprint of the Taylor & Francis Group, an informa business © 2019, Tesfay Gebretsadkan Gebremicael Although all care is taken to ensure the integrity and quality of this publication and information herein, no responsibility is assumed by the publishers, the author nor IHE Delft for any damage to property or persons as a result of operation or use of this publication and/or the information contained herein. A pdf version of this work will be made available as Open Access via http://repository.tudelft.nl/ihe. This version is licensed under the Creative Commons Attribution-Non Commercial 4.0 International License, http://creativecommons.org/licenses/by-nc/4.0/ Published by: CRC Press/Balkema Schipholweg 107C, 2316 XC, Leiden, The Netherlands [email protected] www.crcpress.com – www.taylorandfrancis.com ISBN: 978-0-367-42508-1 ACKNOWLEDGEMENTS Writing this thesis has been fascinating and extremely rewarding. First and for most, I would like to thank God for his protection and giving me a strength to complete this work. Without his blessings, this achievement would not have been possible. I would like to thank all who in one way or another contributed in the completion of this thesis. I am gratefully to acknowledge to my esteemed promoter Prof. dr. ir. Pieter van der Zaag for giving me the chance to study under his supervision. I greatly appreciate all the scientific guidance starting from my master thesis to the four years PhD study. I respectfully thank you for the trust, the insightful discussion, offering valuable advice, for your support during the whole period of the study, and especially for the timely response of my e-mails, critical comments, corrections and suggestions during writing articles. I will not forget those entertaining and helpful discussions with you at each step of my study. Without your continuous encouragements and supervision, I would not have completed this thesis. I have great pleasure in acknowledging my gratitude to my co-promoter Dr. Yasir Abbas Mohamed for his regular guidance, critical comments, corrections, suggestions and reviewing and shaping of the thesis to its current form. From Dr. Yasir, I learnt, not only the vast experience and knowledge in this field of catchment hydrology, but also how to write scientific paper in short and precise way. Without your encouragement and constant guidance, I would not have finished this thesis within four years. I also acknowledge my local supervisor Dr. Eyasu Yazew for his guidance, sharing ideas and the fun that made life easier during my field work in Ethiopia. I am sincerely grateful to the Netherlands Fellowships Programme (NFP) for their financial support in the form of a scholarship to undertake this study. I am also so thankful to the Tigray Agricultural Research Institute for the financial support and providing me vehicles during my field work. I would like also thank Ms. Jolanda Boots, PhD fellowship officer at IHE Delft for her continuous support in arranging air tickets during traveling for field work and overall handling financial issues during my study. I gratefully acknowledge data and information provided by the Ethiopian Meteorological Agency, and the Ethiopian Ministry of Water Resources, Irrigation and Electricity. I also thank my family and friends who encouraged me, supporting and allowing me to be away for part of my study. I would most thank my wife Tirhas Abraha for all incredible sacrifice during my first year of the PhD journey. You were alone during your pregnancy of our daughter, Heran. I will not forget what you’ve made on my behalf during your pregnancy and delivering of our daughter. No matter what, I will love you forever, Titiye; no matter how much we argue, or how much I upset you, I’ll love you unconditionally till the day I die. My son Nathan and my daughter Heran also deserve my appreciation for your patience and grief while both your mother and I were away. Your sacrifice for eight months without mother and father will be with me like a handprint on my heart. Last, but not least I am deeply grateful to my mother, Kidan v Welderufael, and my father, Gebretsadkan Gebremicael, who have prayed for me from the first day of my life. Tesfay Gebretsadkan Gebremicael Delft, The Netherlands October, 2019 vi SUMMARY The Tekeze-Atbara river basin, one of the largest tributaries of the Nile basin, and shared between Ethiopia and Sudan, is crucial for economic development and environmental sustainability in the region. The upstream part of this river basin, and in particular its headwaters comprising the Upper Tekeze basin, is currently the focus of the Ethiopian government for economic development in the semi-arid areas of Northern Ethiopia. The government has committed itself to an ambitious plan to the eradicate food deficit of the region by utilizing water resources of the basin for irrigation and hydropower developments. Nevertheless, land degradation, water scarcity, and inefficient utilization of the available water resources are bottlenecks for achieving this ambitious goal. Availability of water resources for economic development in the region has been influenced by various natural and anthropogenic factors. Therefore, understanding variability and drivers for change of the hydrology of the Upper Tekeze basin and its implications on water availability is vital for enhanced water resource management in the region. The Upper Tekeze basin (~45,000 km2) is characterized not only by severe land degradation and moisture stress but is also known for its recent successful integrated catchment management experience. It is claimed that catchment interventions have caused enhanced water availability at different locations in the upper parts of the basin. But it is not scientifically known how these human-induced environmental changes affect the hydrological processes and what the downstream implications are. Previous studies focused at either experimental plots or very small watersheds, from which it is difficult to extrapolate and infer basin-wide implications. Therefore, this study aims at better understanding the impact of anthropogenic catchment management dynamics on the hydrological processes at different spatio-temporal scales and their implication to downstream flows. This is achieved by combined use of primary and secondary information, remote sensing data, laboratory analysis and assessment using hydrological models. Satellite rainfall products are an important source of rainfall information in data scarce catchments such as the Upper Tekeze basin. First, the performance of eight widely used satellite-based rainfall estimates (TRMM, CHIRPS, RFEv2, ARC2, PERSIANN, GPCP, CMap and CMorph), were evaluated against 34 ground measurements to identify which products are suitable for the Upper Tekeze basin. Comparison of these products with the observed rainfall were conducted using different statistical indices at different spatial and temporal domains. The result shows that the rainfall data of CHIRPS outperformed all other products at all temporal and spatial scales. Next, estimates from RFEv2, 3B42v7, and PERSIANN products were closest to measurements at rain gauges for the daily, monthly and seasonal time scales, and both at point and spatial scales. The performance of all products improved as the temporal scale increased to monthly and seasonal time step at all spatial scales. Compared to low altitudes, the Percentage Bias (PBias) at high altitude increased by 35% whilst the correlation coefficient (r) vii dropped by 28%. CHIRPS and 3B42v7 products showed best agreement in mountainous terrains. CMorph and 3B42v7 consistently overestimated rainfall relative to all rain gauges during the pixel-to-point rainfall comparison approach and at lowland areas during the areal averaged rainfall comparison. The other six products