The UK Hydrological Outlook is based on a combination of observed data and projections into the future using models.
The models fall into two types. The first set of models is based on representing the processes that occur in the environment in computer codes. Within this set are meteorological models of the atmosphere and surface that start with observed data and run forwards to provide forecasts of rainfall and temperature. Hydrological models represent the land surface, rivers, vegetation, and sub-surface water. These models are again run forwards from the observed current situation using meteorological data. Two types of meteorological data are used. To provide one month and three months ahead outlooks the hydrological models are driven by data from meteorological forecasts. To look further into the future historical observed sequences of rainfall are used to drive the hydrological models. Hydrogeological models work in a similar way to hydrological models but with particular reference to the sub-surface movement of water in groundwaters.
The other type of model used to inform the Hydrological Outlook does not require meteorological data but instead looks into the future by identifying periods from the observed past that correspond to recent observations. Historical periods that demonstrate good correspondence with current conditions are termed analogues, and the observation of how the analogue sequences developed into the future are used to generate a projection from the currently observed state.