Catchment Daily Series
The NRFA provides the catchment daily rainfall (CDR) series to aid the interpretation of river flow data.
Definition
A time series of the total rainfall averaged over the catchment in millimetres for each day.
Access
Catchment daily rainfall data is available to download on the NRFA Daily Flow Data pages for each gauging station (e.g. https://nrfa.ceh.ac.uk/data/station/meanflow/39001). Catchment daily rainfall data is available from 1961, or the start of the daily flow record if earlier.
Computation and Accuracy
The catchment averaged daily rainfall data have been derived from CEH-GEAR data (Tanguy et al., 2016), a 1km gridded rainfall dataset generated from all daily and monthly observed rainfall data available for the UK from the Met Office. The rainfall grids were produced using the natural neighbour interpolation method, including a normalisation step based on average annual rainfall (1961-1990). The estimated rainfall on a given day refers to the rainfall amount precipitated in 24 hours between 9am on the day until 9am on the following day. The CEH-GEAR dataset has been developed according to the guidance provided in British Standard Institution’s Guide to estimate areal rainfall (BS 7843-4:2012).
As an indicator of the quality of the estimates, the dataset also contains for each grid generated, the value of the distance to the closest raingauge used to interpolate rainfall at that grid point. The catchment average of this distance (in metres) is provided alongside the catchment averaged rainfall time series. It is important to check the average distance to the closest gauge alongside the rainfall as it indicates the degree of uncertainty associated with the data. The larger the distance, the more uncertain the rainfall total will be. This is particularly important for the period before 1961 when the raingauge network density was much lower, and this density varies significantly over the country.
More information on CEH-GEAR rainfall dataset can be found in Keller et al. (2015).
The catchment boundaries used for calculating catchment average daily rainfall are those available for download from the NRFA Catchment Information pages for each gauging station, and are described here. For the averaging process, a grid cell was considered within a catchment if more than 50% of its area was inside the catchment. No weighted average was applied on grid cells located at the edge of the catchment. This impact of this approach on total catchment average rainfall will be negligible for large catchments, but may be significant for smaller catchments, particularly in areas of high rainfall variability. The uncertainties within the underlying gridded rainfall are also larger in these areas, driven by the density of the raingauge network. This is currently an active area of research at CEH and improved catchment average techniques are being developed.
References
- Keller, V. D. J., Tanguy, M., Prosdocimi, I., Terry, J. A., Hitt, O., Cole, S. J., Fry, M., Morris, D. G., and Dixon, H. (2015). CEH-GEAR: 1 km resolution daily and monthly areal rainfall estimates for the UK for hydrological use, Earth Syst. Sci. Data Discuss., 8, 83-112. https://doi.org/10.5194/essdd-8-83-2015
- Tanguy, M.; Dixon, H.; Prosdocimi, I.; Morris, D. G.; Keller, V. D. J. (2016). Gridded estimates of daily and monthly areal rainfall for the United Kingdom (1890-2015) [CEH-GEAR]. NERC Environmental Information Data Centre. https://doi.org/10.5285/33604ea0-c238-4488-813d-0ad9ab7c51ca
Catchment Monthly Rainfall
The NRFA can also provide the catchment monthly rainfall (CMR) series, which is derived from the catchment daily rainfall series, on request.
