Irrigation in Pictures

Nitrate takes less than five years to travel from farm to river, on average

The decisions farmers make today to reduce excess nutrients will be reflected in water quality improvements in our rivers within five years, on average, finds new research on “lag times” from Our Land and Water. Article by Annabel McAleer, Senior Communications Advisor, Our Land and Water.

The decisions farmers make today to reduce excess nutrients will be reflected in water quality improvements in our rivers within five years, on average, finds new research from Our Land and Water.

The research looked at the “lag time” between farm management decisions to reduce nitrogen loss, and the resulting improvement in river health.

The average time for nitrate loads in rivers to reflect onfarm changes was four and a half years, calculated using data for 77 catchments from 1990 to 2018. Lag times varied from one year to over 12 years, with water in larger rivers and more steeply sloped catchments taking longer to reflect upstream land management changes.

Farmers can be confident their responsible actions on land will eventually be reflected in our rivers, said Professor Richard McDowell, lead author of the paper and chief scientist at Our Land and Water, but we should expect nitrate loads in some catchments to reflect past farm inputs for some years.

“New Zealand’s ‘team of five million’ has become familiar with lag times due to the COVID19 pandemic,” said Professor McDowell. “Just as we’ve seen COVID19 case numbers rise in the first weeks of our Level 4 lockdowns, before dropping, we may also see water quality in some of our rivers continue to decline for a while longer, despite strong and effective action being taken on farms.”

“If people who farm work as a team, hold their nerve, and continue to take strong action to improve our rivers, water quality will reflect these efforts within five years in many catchments,” said Professor McDowell.

“People in industry bodies, catchment groups, and farmer cooperatives can help by ensuring that people who are working to improve water quality have realistic expectations for the time frame in which we’ll see improvements, so they don’t get discouraged by lag times.”

The research suggests that because small catchments and subcatchments have shorter lag times, action by people in regional councils to expand and improve monitoring

Map of New Zealand showing the location, catchment size and estimated lag time between nitrate–N leaching losses and nitrate–N load for catchments of: (a) < 450, (b) 451–1800, (c) 1801–5000, and (d) > 5000 km2. Red dots refer to the location of the sampling site. Note that some sites are subsumed within larger catchments of the same lag time class. Maps were created using R (ver. 4.0.3) packages ‘sf’86 and ‘tmap’87; New Zealand boundary data sourced from StatsNZ; catchment boundaries sourced from the River Environment Classification database.

This research was published in the Nature Journal Scientific Reports in August 2021 and funded by the Our Land and Water National Science Challenge and MBIE.

MORE INFORMATION:

• McDowell, R.W., Simpson, Z.P., Ausseil, A.G. et al. The implications of lag times between nitrate leaching losses and riverine loads for water quality policy.

Sci Rep 11, 16450 (2021). • Our Land and Water (2021). Monitoring the Effectiveness of Actions to Improve

Freshwater. Guidance prepared by Our Land and Water (Toitu - te Whenua, Toiora te Wai) National Science Challenge, New Zealand.