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GROUNDWATER RECHARGE ESTIMATION AND WATER RESOURCES ASSESSMENT IN A TROPICAL CRYSTALLINE BASEMENT AQUIFER Groundwater Recharge Estimation and Water Resources Assessment in a Tropical Crystalline Basement Aquifer DISSERTATION Submitted in fulfilment of the requirements of the Board for Doctorates of Delft University of Technology and of the Academic Board of the UNESCO-IHE Institute for Water Education for the Degree of DOCTOR to be defended in public on Thursday, June 29, 2006 at 10:00 hours in Delft, the Netherlands by Nyasha Lawrence NYAGWAMBO born in Mutasa, Zimbabwe Master of Science, UNESCO-IHE This dissertation has been approved by the promotor Prof. dr. ir. H.H.G. Savenije TU Delft/UNESCO-IHE Delft, The Netherlands Members of the Awarding Committee: Chairman Rector Magnificus TU Delft, The Netherlands Co-chairman Director UNESCO-IHE, The Netherlands Prof. dr. S. Uhlenbrook UNESCO-IHE, The Netherlands Prof.dr. J. J. de Vries Free University, The Netherlands Prof.dr.ir. N. van de Giesen TU Delft, The Netherlands Prof.ir. T. Olsthoorn TU Delft, The Netherlands Dr. J. Sakupwanya Zambesi Water Authority Copyright © Nyasha Lawrence Nyagwambo, 2006 All rights reserved. No part of this publication or information contained herein may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, by photocopying, recording or otherwise, without written prior permission from the publisher. Although care is taken to ensure the integrity and quality of this publication and the information therein, no responsibility is assumed by the publishers not the author for any damage to property or persons as a result of operation or use of this publication and/or the information contained herein. Published by A.A. Balkema Publishers, a member of Taylor & Francis Group plc www.balkema.nl and www.tandf.co.uk ISBN 0 415 41692 2 / 978-0-415-41692-4 (Taylor & Francis Group) To my family, Anna, Panashe, Ariko and The Three. To my Dad who says all the right things, my Mum who does most of the correct ones, and my sisters who cheer me all the way. ii Groundwater Recharge Estimation and Water Resources Assessment In A Tropical Crystalline Basement Aquifer. ACKNOWLEDGEMENTS First and foremost I would like to thank my Promoter, Prof. Dr. Ir. Huub Savenije who kept on urging me on when the chips were down and invested so much of his time when my efforts seemed so hopeless. My special thanks too to Ir. Jan Nonner who sacrificed a lot of his time both in the Netherlands and in Zimbabwe to get this work were it is today. Then there was one very special person Marieke de Groen who kept on reminding me that the work could be done. Your encouragement was not in vain. I would like to thank my colleagues at the University of Zimbabwe Civil Engineering Department for the encouragement and support. Evans Kaseke who always showed there was light at the end of the tunnel (even when it looked like a train!), David Katale whose discussions in other subjects gave me time off, Dr Bekithemba Gumbo who always reminded me of the “big picture”, Dr Pieter van der Zaag, Dr Nhapi, Alexander Mhizha and of cause Dr Vassileva whose faith in me never wavered. Of cause the fieldwork would never have been carried out without Mssrs Chawira, Mbidzo and Mabika and the Chikara community’s help. Thanks too to Maoneyi for calling me Dr even before the first draft was written, at least you kept the end in sight. To Thomas Natsa, Dr Patrick Moriarty and Dr Richard Owen those initial discussions were very insightful, thank you so much. Most importantly, I would like to thank Anna and Panashe for letting me go away for all those months, my sisters for encouraging me all the way, and my parents for praying all the time. And to my superiors at IWSD, Eng Mudege and Mrs Neseni thanks for the encouragement and support when the load looked too heavy. i Groundwater Recharge Estimation and Water Resources Assessment In A Tropical Crystalline Basement Aquifer. PREFACE This study started in 1999. When I was asked to apply for a PhD study under the auspices of the Capacity Building Program for Zimbabwe and the Southern Africa Region I had no clue about groundwater let alone groundwater recharge as a science. This fact latter created problems for me during the course of the study. I was an engineer who branched to water resources management in my MSc study. So to the engineers the subject of groundwater recharge was like funky music to a monk – they may like it but not openly. I was embarking on a study that was neither hydrology no management, neither engineering nor science. In short I was disowned by all but then I had to do a PhD! I suppose this is what they call a Catch 22 situation. But off I went. I wrote a proposal which I still suspect very few people read. Some commented with scorn and the hydrogeologists were so scared I wanted to work with them that they bolted at my sight. I felt lost and for a moment wanted to quit. But my Promoter kept probing. He too did not particularly like my topic but as usual he saw the future and urged me on. So I decided to do some work on recharge not because of its scientific appeal but because I believed, I am not so sure now, that it touches so many people in the third world, my village relatives included. I always got fascinated by groundwater experts when they talk about sophisticated methods that are needed to tell people how much groundwater is available. Equally perplexing to me was the fact that so much is invested in determining accurately something so uncertain and imprecise as natural recharge. Surely the uncertainty in the final value did not warrant the investment. There must be simpler ways of assessing groundwater availability before resorting to experts and their complicated and costly methods. Surely if water witching has as much a chance of determining where groundwater is as the most sophisticated geophysical methods simpler ways must exist to give an indication of available groundwater resources. Is there any correct answer? So I decided to answer two questions. How does the different recharge estimation methods relate to each other and cannot a simpler way, based only on physical and rainfall properties, be found to give a preliminary estimate of groundwater recharge? The quest to answer this question is what follows in this dissertation. But the road was fraught with roadblocks. My catchment had no data and a monitoring network had to be established. The logistics of such an approach chewing much valuable time from the real work. The equipment that was installed often failed at crucial periods such as the water gauge for the hydrological year 2000/2001. Finances were never adequate to put in place a comprehensive monitoring regime. Short cuts were taken and in some cases quality was compromised. Then there was my inexperience as a researcher and family catastrophe as two members of my family passed away. In the end the study was a journey against the odds and its conclusion more a triumph of determination over adversity than a scientific accomplishment. The output from this study is merely a beginning since the proposed model and its approach need refinement and to be tested in different regions as well. ii Groundwater Recharge Estimation and Water Resources Assessment In A Tropical Crystalline Basement Aquifer. SUMMARY Hydrogeologists and Priests have one thing in common: they talk of things we cannot see but must believe in to live. Groundwater is vital, important and fascinating. Its occurrence has long been associated with sacredness. Groundwater recharge is perplexing and the recharge estimation methods are nothing more than an attempt by man to unmask this sacredness. While most methods give reasonable long-term annual average estimates very few if any at all offer guidance on short-term monthly recharge. In crystalline basement aquifers (CBAs) the problem is compounded by the high storage to flux ratio, which results in groundwater resources that are highly seasonal and high intra-annual and inter-annual variability. This study proposes a simple method for assessing the potential available monthly and annual recharge in a small CBA catchment taking into consideration the rainfall and physical characteristics of the catchment. By studying the geology, land use and rainfall and applying different techniques for estimating recharge it has been deduced that occurrence of groundwater recharge, and hence the potential for recharge, is controlled by the rainfall regime and evaporative fluxes whilst the geology and vegetation characteristics of a catchment influence the mechanism and magnitude of actual recharge. Three methods have been used to estimate groundwater recharge in a small catchment (180 Mm2), the chloride mass balance (CMB), the daily catchment water balance (WB) and the water table fluctuation (WTF) methods. The study concluded that though the methods yielded the same range of recharge, between 8% and 15% of annual rainfall, all methods show a high spatial variability with coefficients of variation of up to 65% indicating that no single point measurements of recharge is a good indicator of regional recharge. The study also showed that all recharge estimation methods used in the study had the weakness of over reliance on one critical parameter such as the chloride deposition for the CBM method and specific yield for the WTF method. The WB method, at a daily time step, was influenced heavily by the spatial distribution of rainfall. The use of groundwater models such as MODFLOW was found to be of limited value in assessing recharge in CBAs due to the high heterogeneity in the geological properties of the aquifer.

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