Dry end to RB209 excess winter rainfall period

In this blog, Emma Willis explains how relatively dry conditions over the last two months have largely balanced the wet start to the excess winter rainfall (EWR) period.

Nitrogen is a vital nutrient that can be tricky to manage. Too little can stunt crop development, too much can increase environmental risks (e.g. nutrient run-off risks) and ultimately waste fertiliser.

This is why EWR data is key, as it can be used to help account for potential nitrate losses in nutrient management plans for both arable crops and grassland.

EWR is 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. 

The EWR period is quite long, often starting in October and finishing around the end of March, with local weather and site conditions determining the precise period.

We use Met Office data to map EWR across the UK (199 40 km by 40 km grid), from the start of October to the end of March, using three categories:

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

The categories can be used to identify the most appropriate soil nitrogen supply (SNS) tables in the AHDB Nutrient Management Guide (RB209). These tables can help you calculate crop fertiliser requirements (as part of the field assessment method).

The online maps also account for evaporation from winter crops (wheat, barley and oilseed rape) and grass (which we have included for the first time).

As you might expect, results are similar between cereals and grass, with a slightly greater drying effect in the latter (but not quite as much as in winter oilseed rape).

According to the Met Office, five of the ten wettest years for the UK occurred in this century, so we have also revised the long-term average data in the online maps (from 1981–2010 to 1991–2020).

The long-term average data for bare soil now has no ‘low’ categories and just 28 ‘moderate’ categories, with the rest classified as ‘high’ EWR (171).

At the start of April, we issued the final EWR update for the full 2024/25 winter period, which reveals a game of two halves: a wet start and a dry finish.

In fact, most of the UK was already classified as having high EWR in the interim results, which were published in February (at a critical time for nutrient management planning).

Since then, drier conditions have dominated.

March was one of the driest months on record for some parts of the UK. For example, Suffolk and Norfolk experienced their second-driest Marches on record (only March 1929 was drier) and many parts of the country had low rainfall:

• England – 25% of the average rainfall
• Northern Ireland – 32% of the average rainfall
• Scotland – 64% of the average rainfall
• Wales – the driest March on record since 1944

The upshot is that most of the categories were ‘fixed’ in the interim data, with relatively little change recorded on the final EWR maps.

As the dry spells largely balanced out the wet spells, most sites received the same EWR category as in a ‘typical’ year.

There is some regional variation. For example, some sites around The Midlands are typically ‘moderate’ but have moved to the ‘high’ category this year.

Consistent with long-term trends, the driest areas are mainly clustered towards the eastern coastline, which closely mirrors the key wheat production areas.

The winter oilseed rape map shows regional variation the clearest, because of its higher evapotranspiration. This year, for this crop, there is an equal split between ‘low’ and ‘moderate’ categories (30 sites in each), with all other sites assigned the ‘high’ category (139 sites).

With its temperate climate, wet winters are a characteristic of the UK. However, this year’s EWR data is a better result than last year, which saw extreme rainfall throughout the winter.

At present, there are some variable crops across the UK and nitrogen plans will need careful refining, especially to adjust fertiliser rates to reflect expected yields.

The timing of nitrogen applications is crucial to ensure it is available to the crop when it needs it the most – which is right now (during periods of rapid growth).

To plan and refine nitrogen applications, use the nitrogen calculator, follow RB209 guidance and access the EWR maps.