For example, for the discharge simulation at Victoria Falls during calibration period Harrison and Whittington (2002) obtained a correlation coefficient R2 of 0.61, which is lower than the results presented here with R2 of 0.88. Similarly, Winsemius et al. (2006) report for their two models a Nash–Sutcliffe Efficiency NSE of 0.72 and 0.82 respectively, whereas we obtained a slightly higher performance with NSE of 0.88. Note that Winsemius et al. did only see more apply their model to the upper Zambezi and did not focus on impact modelling. Unfortunately, the other impact modelling studies of the

whole Zambezi basin ( Hoekstra, 2003, Yamba et al., 2011 and Beck and Bernauer, 2011) do not report performance statistics. However, we believe that the model simulations presented here are among the most accurate – if not best – models for simulation of Zambezi discharge currently available. The exact reason for the higher model performance as compared to previous studies remains unclear. It may be related to improved input data (GPCC), calibration method, consideration of wetlands and river routing. The latter two are important for simulation of timing of Zambezi discharge (Cohen-Liechti et al., 2014) and would cause serious modelling problems if not explicitly considered, with the risk of corrupting parameter values to obtain simulations that are “right for the wrong reason” (Refsgaard and GDC 941 Henriksen, 2004). The higher performance

is most likely not related to the structure of the water balance model (see Fig. 4, left), as here the applied models are all very similar in the various studies. The evaluation of historic discharge conditions (see Fig. 5 and Fig. 6) also shows the considerable impact of the large reservoirs and the problem of reservoir operation; where (ad hoc?) release decisions at upstream reservoirs complicate simulation of downstream discharge. Different sets of operation rules would have to be applied to different time periods, but instead fixed operation rules – as effective during the 2000s – were imposed on the model. Therefore, simulations in the downstream sections (e.g. at Tete) frequently show

deviations to observations. Due to the above mentioned peculiarities of Zambezi discharge in downstream sections, we focussed PLEKHM2 on the simulation results averaged over the land-surface – thereby excluding the confounding impacts of reservoirs – to learn more about the hydrology in the context of the seasonal water balance (see Fig. 9). The hydrology in the Zambezi basin is characterized by representing a water limited system – as opposed to energy limited. Already under historic climate the potential evapotranspiration cannot be met by the actual evapotranspiration (see Fig. 9), simply because there is not enough water stored in the soil due to insufficient annual precipitation amounts. Therefore, any increases in temperature – and consequently increases in potential evapotranspiration – have a small impact on discharge.