Most UK regions record high excess winter rainfall

UK rainfall has generally been above average over the October–January period, which has implications for nitrogen management, according to AHDB.

Most UK regions are already in the highest excess winter rainfall (EWR) category, which is associated with increased nitrate leaching and reduced soil nitrogen supply (SNS).

With two months of the EWR period remaining (February–March), more areas are likely to enter higher EWR categories – news that will disappoint growers as they grapple with relatively high fertiliser prices.

EWR reflects the amount of rainfall the land receives after the soil profile becomes fully wetted in the autumn (field capacity) and before the end of drainage in the spring. It can be used to indicate how much nitrate has leached into drainage water.

AHDB uses Met Office data to map EWR across the UK (199 40 km by 40 km grid) using three categories:

• Low: less than 150 mm EWR
• Moderate: 150 to 250 mm EWR
• High: over 250 mm EWR

Two map types are created: for bare soil and cropped land. The latter maps (for winter wheat, winter barley and winter oilseed rape) account for water lost through the crop.

The EWR categories correspond to three look-up tables in section 4 of the AHDB Nutrient Management Guide (RB209), which can be used to estimate a field’s SNS index and help determine recommended nitrogen rates.

With many farmers applying nitrogen before April, AHDB has published interim data based on the first four months (October–January) of the full EWR period (October–March), to help them fine-tune nitrogen applications.

Emma Willis, who manages nutrient management research at AHDB, said: “Apart from December, the UK typically has seen above-average rainfall levels since October. Even for oilseed rape, which has relatively high evapotranspiration levels, around two-thirds of the UK area is already classified as having high EWR. As usual, excess winter rainfall generally increases away from the drier eastern England.

"February has got off to a relatively dry start. Although no widespread heavy spells of rain are in the current Met Office long-range forecast, bands of rain are expected. As a result, the final data, due in early April, is likely to show a very large UK area in the highest EWR category. This contrasts to last season (2021–22), which was relatively dry”.

To plan nitrogen applications, use the nitrogen calculator, follow RB209 guidance and access the EWR maps on the AHDB website: ahdb.org.uk/ewr

The two maps show interim (October 2022–January 2023) excess winter rainfall (EWR) data (bare soil and oilseed rape) presented over 199 data points, each point represents a 40x40 km square.