Hydrological forecasts up to a few weeks or months ahead are, in many areas, dependent on accurate knowledge of the current storage of water in the landscape. For the Hydrological Outlook, a UK-wide hydrological model (Grid-to-Grid or G2G) is used to provide an estimate of the current total subsurface water storage (in both soil and groundwater) across Britain: i.e. how wet or dry the landscape is.
Based on this information, maps of ‘relative wetness’ or ‘relative dryness’ can be produced, indicating areas where subsurface water storage is particularly high or low (compared to long-term averages), and hence may be prone to flood or drought conditions in the coming days or weeks.
During periods of drought, the deficit in subsurface water storage can be used to calculate the required amount of additional rainfall over subsequent months that would enable subsurface water storage and river flows to return to average monthly values, and an estimation made of how likely it is that this amount of rainfall will occur.
Further details on the methods used to produce these maps.
For other sources of current condition information you might like: