Impact modelling of water resources development and climate scenarios on Zambezi River discharge

• Hydrological modelling of 1.4 Mio km2 to simulate Zambezi discharge.
• We demonstrate the high sensitivity of discharge to small variations in rainfall.
• We quantify historic and future impacts of reservoirs and large scale irrigation.
• Diverging climate change scenarios result in large uncertainty for future discharge.
• Existing reservoirs have bigger impact than expected impact due to climate change.

Study regionThe Zambezi River basin (1.4 × 106 km2) in southern Africa, which is shared by eight countries and includes two of the World's largest reservoirs.Study focusImpacts on future water resources in the Zambezi basin are studied, based on World Bank projections that include large scale irrigation and new hydropower plants. Also the impacts of climate change scenarios are analysed. Modelling challenges are the large basin area, data scarcity and complex hydrology. We use recent GPCC rainfall data to force a rainfall-runoff model linked to a reservoir model for the Zambezi basin. The simulations are evaluated with 60 years of observed discharge and reservoir water level data and applied to assess the impacts on historical and future discharges.New hydrological insights for the regionComparisons between historical and future scenarios show that the biggest changes have already occurred. Construction of Kariba and CahoraBassa dams in the mid 1900s altered the seasonality and flow duration curves. Future irrigation development will cause decreases of a similar magnitude to those caused by current reservoir evaporation losses. The discharge is highly sensitive to small precipitation changes and the two climate models used give different signs for future precipitation change, suggestive of large uncertainty. The river basin model and database are available as anopen-online Decision Support System to facilitate impact assessments of additional climate or development scenarios.