Three different approaches are used for producing river flows for the UK Hydrological Outlook.
River flow analogy
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 flow at around 100 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.
River flow from historical climate
The Ensemble Streamflow Prediction (ESP) outlook, or “Outlook by historical climate”, is based on monthly ensembles of historical sequences of observed climate (rainfall and potential evapotranspiration) that form input to a hydrological model. The outputs are probabilistic simulations of the average river flow over the forecast period (1 to 12 months ahead), at each location. The simulations are generated by the GR4J conceptual rainfall-runoff model from INRAE (France), calibrated on observed flows.
This outlook is based entirely on historical sequences and therefore does not contain any knowledge of the state of the atmosphere and ocean. It is hence possible that some of the historical sequences used might be inconsistent with current large-scale atmospheric conditions and would therefore be unlikely to occur in the next few months.
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.
Detail of the modelling set up, and the skill of this method have been published in:
Harrigan, S, Prudhomme, C, Parry, S, Smith, K, and Tanguy, M. 2018. Benchmarking ensemble streamflow prediction skill in the UK. Hydrology and Earth Systems Sciences
River flow from rainfall forecast
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 UK, 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.