Dynamics of transmission losses in a large arid stream channel Jens Lange * Institute of Hydrology, University of Freiburg, Fahnenbergplatz, 79098 Freiburg, Germany Received 17 October 2003; revised 9 September 2004; accepted 13 September 2004 Abstract The temporal dynamics of transmission losses are illustrated during single events within a 150 km long arid channel reach of the Kuiseb River, Namib Desert, Namibia. This is facilitated not using a traditional water balance study but by applying a mathematical flow routing scheme with parameters not calibrated with runoff information. Transmission losses are deliberately excluded from the model so that actual transmission losses can be identified as the component of model simulations that plots significantly higher than the measured discharges at the downstream end of the reach. Results indicate that transmission losses concentrate during high discharge peaks and are minor during small to medium flows. This behaviour is attributed mainly to enhanced water losses in flooded overbank areas. Although the exact percentage of groundwater recharge from these waters cannot be quantified, the present study implies that single high magnitude flows are more important than frequent small to medium flows for providing recharge and thereby sustaining the riparian vegetation of large desert streams. q 2004 Elsevier B.V. All rights reserved. Keywords: Large ephemeral streams; Transmission losses; Arid environments; Flow routing; Riparian vegetation; Kuiseb River; Namib Desert 1. Introduction In arid regions surface waters are rare and unequally distributed over space and time owing to infrequent rainfall and high evaporation. Where groundwater is available all year long it becomes the only dependable water resource. Only when groundwater abstractions correspond to long term recharge under present climatic conditions, is a stable hydrological aquifer balance achieved. Hence reliable recharge estimates are the most important premise for sustainable groundwater management. An overview of types, processes and estimation of recharge in dry climates is given by De Vries and Simmers (2002). They conclude that quantitative estimates from all principal recharge mechanisms, i.e. direct, localised and indirect recharge (Lerner et al., 1990), remain fraught with uncertainty and leave open many questions. However, it is widely acknowledged that indirect recharge by transmission losses (i.e. infiltra- tion through the beds of ephemeral water courses) increases with aridity (e.g. Lloyd, 1986; De Vries and Simmers, 2002) making it a key factor for sustainable freshwater management in truly arid regions. Although the significance of transmission losses has been known for many years (e.g. Dubief, 1953; Journal of Hydrology 306 (2005) 112–126 www.elsevier.com/locate/jhydrol 0022-1694/$ - see front matter q 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jhydrol.2004.09.016 * Tel.: C49 761 203 3546; fax: C49 761 203 3594. E-mail address: jens.lange@hydrology.uni-freiburg.de.