Dear Sara, Louise,
indeed skill in forecasting streamflow is varies (a lot) with lead time, time of the year and river basin. And the forecast quality for discharge can be better than that for precipitation. For Spain we know from previous work (NOT this EHYPE model, but statistical models) that streamflow forecast quality is quite good for at least Central, Western Spain (Douro, Tejo, Guadiana) for winter and early spring (DJFM). Also our own model shows potential skill in this period and basins, but also in july/august more towards your area (Mediterranean basins).
Another issue is the used metric for the skill. The maps presented are masked out based on CPRS, this is a measure across all flow percentiles and therefore perhaps a bit strict. Other skill scores that measure the performance of forecasting only for high flow or low flows, such as ROCS may give less conservative indications of forecast quality. Refering the previous examples: in winter high flows are better predicted than low flows, in summer the other way around: low flows are better predicted than high flows. Moreover, more extreme anomalies are often better forecasted than less extreme ones.
May be Louise can comment a bit more already on winter skill and skill for high or low flows for the EHYPE model.