Three different approaches are used for producing river flows for the Hydrological Outlook UK. They are -
Descriptions of each can be found below.
The historical flow analogues approach to seasonal river flow forecasting is based on selecting the previously observed sequences of flows that are the most similar to the recently observed past.
The assumption is that this similarity will carry on in the coming few months. New one-month and three-month forecasts are made each month using monthly river flows at 93 stations in the National Hydrological Monitoring Programme. These stations have at least 30 years of data in the period from January 1883 onwards.
The bulk of the forecasts are persistence forecasts, which are made when these outperform the historical analogues approach. They are particularly useful for slowly responding catchments with large underground water storage in aquifers.
These river flow outlooks are based on ensembles of historical sequences of observed climate (rainfall and temperature) being input to a rainfall-runoff model. The outputs are probabilistic forecasts of the average river flows occurring over the forecast horizon (i.e. over the next, say, three months), at each location.
To help with the interpretation of the results, the outlook is compared with the expected (or reference) distribution for each location, time of the year and N-month accumulation period. To avoid possible bias in the hydrological modelling to influence the interpretation of the results, the reference distribution is derived from simulated (and not gauged) flows using observed historical climate as input data. This is done for each start date and N-month accumulation period.
An experimental modelling tool for national hydrological outlooks has been developed which combines a hydrological model (Grid-to-Grid or “G2G”) estimate of total subsurface water storage (in both soil and groundwater) and across Britain with a range of seasonal rainfall forecasts provided by the Met Office to provide estimates of area-wide hydrological conditions up to a few months ahead.
For many areas, hydrological “forecasts” up to a few weeks or months ahead are dependent on accurate knowledge of the current storage of water in the landscape. This information provides the hydrological initial condition, or “initial state”, from which future simulations will depart following changes in boundary conditions, consisting primarily of the weather and water consumption. For the Hydrological Outlook, the G2G estimate of subsurface water storage across the UK can provide an initial condition of subsurface water in storage across the UK, derived using the most recent observations of rainfall and potential evaporation. This hydrological initial condition then provides a starting point from which estimates of water storage and river flows for one to three months ahead can be produced as perturbations from the initial state, driven by Met Office rainfall forecasts.
During periods of drought, the link can made between a deficit in subsurface water storage and a requirement for additional rainfall over subsequent months to enable subsurface water storage and river flow to return to mean monthly values. The methodology can also provide an indication of locations where sub-surface water storage is particularly high and may be prone to flooding in the coming days/weeks.