: S2D predictions of weather and climate can be derived both from statistical (or empirical) and dynamic models . The former approach is usually based on regional historic relationships between climate variables; most recent dynamic approaches use fully coupled ocean-atmosphere general circulation models (CGCMs). Some S2D forecasting systems, particularly the CGCM approaches, may include impact-relevant outputs directly (“online” approaches), for example via river flow models, soil moisture calculations, or estimates of vegetation productivity. Validation and skill assessment in these systems may also provide valuable information on the overall performance of the seasonal prediction system itself. For example, rivers integrate land hydrology over large geographic areas and are important sources of freshwater input to the oceans.