The implications of lag times between nitrate leaching losses and riverine loads for water quality policy

Understanding the lag time between land management and impacts on riverine nitrate-nitrogen (N) loads is critical to understand when action to mitigate nitrate-N leaching losses from the soil profile may start improving water quality. These lags occur due to leaching of nitrate-N through the subsurface (soil and groundwater). Actions to mitigate nitrate-N losses have been mandated in New Zealand policy to start showing improvements in water quality within five years. We estimated annual rates of nitrate-N leaching and annual nitrate-N loads for 77 river catchments from 1990 to 2018. Lag times between these losses and riverine loads were determined for 34 catchments but could not be determined in other catchments because they exhibited little change in nitrate-N leaching losses or loads. Lag times varied from 1 to 12 years according to factors like catchment size (Strahler stream order and altitude) and slope. For eight catchments where additional isotope and modelling data were available, the mean transit time for surface water at baseflow to pass through the catchment was on average 2.1 years less than, and never greater than, the mean lag time for nitrate-N, inferring our lag time estimates were robust. The median lag time for nitrate-N across the 34 catchments was 4.5 years, meaning that nearly half of these catchments wouldn't exhibit decreases in nitrate-N because of practice change within the five years outlined in policy.