Coupled flow and salinity transport modelling in semi-arid environments: The Shashe River Valley, Botswana Peter Bauer a, * , Rudolf J. Held a , Stephanie Zimmermann a , Flenner Linn b , Wolfgang Kinzelbach a a Institute of Hydromechanics and Water Resources Management, ETH Zu ¨rich, Switzerland b Water Resources Consultants, Gaborone , Republic of Botswana Received 2 December 2003; revised 15 April 2005; accepted 29 April 2005 Abstract Numerical groundwater modelling is used as the base for sound aquifer system analysis and water resources assessment. In many cases, particularly in semi-arid and arid regions, groundwater flow is intricately linked to salinity transport. A case in point is the Shashe River Valley in Botswana. A freshwater aquifer located around an ephemeral stream is depleted by the combined effect of transpiration and pumping. Quantitative system analysis reveals that the amount of water taken by transpiration is far more than the quantities pumped for water supply. Furthermore, the salinity distribution in and around Shashe River Valley as well as its temporal dynamics can be satisfactorily reproduced if the transpiration is modelled as a function of groundwater salinity. The location and dynamics of the saltwater–freshwater interface are highly sensitive to the parameterization of evaporative and transpirative salt enrichment. An existing numerical code for coupled flow/transport simulations (SEAWAT) was adapted to this situation. Model results were checked against a large set of field data including water levels, water chemistry, isotope data and ground and airborne geophysical data. The resulting groundwater model was able to reproduce the long-term development of the freshwater lens located in Shashe River Valley as well as the decline in piezometric heads observed over the last decade. Furthermore, the old age of the saline water surrounding the central freshwater lens could be explained. q 2005 Elsevier B.V. All rights reserved. Keywords: Evapotranspiration; Salinity; Okavango Delta; Groundwater modelling; Aquifer management 1. Introduction In semi-arid and arid environments, hydrologic systems typically cannot be understood without analysing the transport of salinity and the feedback mechanisms between water flow and salinity trans- port. In this study, we focus on a river system in semi-arid Botswana, the Shashe River Valley. Journal of Hydrology 316 (2006) 163–183 www.elsevier.com/locate/jhydrol 0022-1694/$ - see front matter q 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jhydrol.2005.04.018 * Corresponding author. Address: Institute of Environment and Resources, Technical University of Denmark, Bygningstorvet, B115, DK-2800 KGS Lyngby, Denmark. Tel.: C45 4525 2171. E-mail address: pbg@er.dtu.dk (P. Bauer).