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Whatever the weather

The impact of changing weather patterns on water quality

TResearch Autumn Winter 2021Teagasc’s Agricultural Catchments Programme has investigated the impact of changing weather patterns on water quality in three river catchments over a 10-year period.

Between 2010 and 2019, Teagasc and Ulster University researchers studied water quality, water quantity and weather data in three contrasting agricultural catchments: Ballycanew and Castledockrell in County Wexford, and Timoleague in County Cork. Their aim was to find out how changing weather patterns affected the quality of water.

The summer effect

Over the 10-year period, soil temperature in both Ballycanew and Castledockrell increased between June and September. In Ballycanew, the number of times that soil temperature exceeded 17°C three days in a row also steadily increased. 

Edward Burgess, Agricultural Catchment Specialist, says: “Soil nitrogen mineralisation (the conversion of organic nitrogen into inorganic nitrogen) is closely linked to soil temperature. If warmer summers are followed by more rain in the winter, soil nitrogen is more vulnerable to leaching (the loss of nutrients) into rivers.”

Ballycanew is not generally considered vulnerable to nitrate loss as the soil has a high clay content with poor drainage. This results in anaerobic conditions that favour denitrification (a process that converts nitrate to nitrogen gas).

River nitrate levels are typically around 2.6mg per litre, but following the summer drought in 2018, daily average concentrations reached 13.6mg per litre when the rain arrived in September.

An extra 6.5kg per hectare of nitrate above average for the month of November was recorded leaving the catchment. This added over half the average yearly load of nitrate in one month.

Rainfall intensity

The researchers found that the amount of yearly rainfall didn’t change over the 10-year period, but there was an increase in the number of days with heavy rain and high air temperatures. Both of these things significantly influence the mobilisation of nutrients – like nitrogen and phosphorus – into rivers.

“All three catchments were more likely to experience large amounts of rain – exceeding 25mm per day – in the October to December period,” says Edward. “And the number of these rainfall events increased in the Wexford catchments for the month of December.

“Long-term data from Ireland’s national meteorological service Met Éireann have also shown a gradual increase in the likelihood of large rainfall days.”

Such large winter rain events – during which soil is more likely to be full of moisture – will cause surface runoff (water that flows over the land). This then dissolves and leads to the loss of phosphorus from the soil.

“Even well-drained soils, such as in Castledockrell, can become full of moisture in winter months,” says Edward. “Heavy rain then leads to more surface runoff and soil erosion, in particular from river banks and exposed soil.” 

Source impact

Drier summers result in rivers with lower flows, and pollutants from identifiable sources become more obvious because of less dilution.

The researchers found a small regular source in one of the catchments during the 2018 summer drought, where the levels of phosphorus spiked every day to a level almost 10 times the ecological quality standard.

Cases like this are more likely to occur in small streams during summer droughts, and will have a significant impact on aquatic ecology. It’s unlikely this would have been identified without river nutrient and flow data being collected by the Agricultural Catchments Programme every 10 minutes. 

Implications for policy

Since 2009 national agricultural goods production has increased on average by over €300 million each year. This is closely linked to the expansion of the national dairy herd following the removal of milk quotas.

Over the same period the Environmental Protection Agency has reported a decline in water quality, with agriculture being identified as a major source of nutrients. The coinciding nature of increased agricultural production and declining water quality understandably suggests that both are linked. 

“Actions and regulations to improve water quality could focus only on limiting the source of nutrients,” says Edward, “but recent weather events – like major storms and the hot, dry summer of 2018 – also influence the release and transport of nutrients to our watercourses.

“The differing soil textures and chemistries between the three catchments we studied have resulted in a contrasting response to changing weather patterns. This means that, if agri-environmental measures are to be effective, they must include an awareness of changing and extreme weather patterns influencing the link between land use and water quality.”


The Agricultural Catchments Programme is fully funded by the Department of Agriculture, Food and the Marine. We thank the farmers managing land.


Per-Erik Mellander, Senior Research Officer Agricultural Catchments Programme, Teagasc, Johnstown Castle, Co. Wexford

Edward Burgess, Agricultural Catchments Specialist, Teagasc, Johnstown Castle, Co. Wexford


Phil Jordan, Professor of Catchment Science Ulster University, Coleraine, Co. Derry


Nutrient loss in the closed period

Phosphorus management