UNDERSTANDING WATER IN A DRY ENVIRONMENT INTERNATIONAL CONTRIBUTIONS TO HYDROGEOLOGY 23 INTERNATIONAL ASSOCIATION OF HYDROGEOLOGISTS Understanding Water in a Dry Environment Hydrological Processes in Arid and Semi-arid Zones Editor Ian Simmers Faculty of Earth and Life Sciences, Vrije Universiteit, Amsterdam A.A. BALKEMA PUBLISHERS / L ISSE /A BINGDON /E XTON (PA) /T OKYO Library of Congress Cataloging-in-Publication Data Applied for Front cover: The Manga Grasslands—the ‘lake district’ of the Sahel. Rainfall of 400–500 mm/year in this area of Holocene sand dunes in northern Nigeria (Borno State) is sufficient to sustain a series of groundwater-fed lakes and fragile ecosystems. Recharge rates are around 40 mm/year, as estimated using both the Cl mass balance (unsaturated zone profiles) and groundwater models. During the 20-year Sahel drought of the 1970s and 1980s, many of the lakes dried completely or left a series of swamps. Photograph by W. Mike Edmunds, British Geological Survey, UK. Copyright # 2003 Swets & Zeitlinger B.V., Lisse, The Netherlands All rights reserved. No part of this publication or the 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 publishers. Although all care is taken to ensure the integrity and quality of this publication and the inform ation herein, no responsi bility is assum ed by the publishers nor the autho r for any damage to property or persons as a result of operation or use of this publicat ion and/or the informat ion conta ined herein. Published by: A.A. Balkema Publisher, A member of Swets & Zeitlinger Publishers www.balkema.ima.nl and www.szp.swets.nl. ISBN 90 5809 618 1 Financially supported by UNESCO as a contribution to the International Hydrological Programme IHP-V, Project 5.1 This edition published in the Taylor & Francis e-Library, 2005. “To purchase your own copy of this or any of Taylor & Francis or Routledge’s collection of thousands of eBooks please go to www.eBookstore.tandf.co.uk.” ISBN 0-203-97130-2 Master e-book ISBN (Print Edition)